Ebook undisturbed rest-v1 [res_tful apis]

© 2015. All Rights Reserved.
Undisturbed REST
First-look Edition
May 2015, PDF
ISBN 978-1-329-11656-6
To learn more, visit us online at http://www.mulesoft.com/restbook
Published by MuleSoft, 77 Geary Street, Suite 400, San Francisco, CA 94108

A Note from the Author
First and foremost, I want to thank you—the reader—for taking time out of your busy
schedule to read this book. Undisturbed REST is not designed to be a tell-all book that
covers every line of code in regards to building your API, but rather a book that takes
you back to the basics and focuses on how to not just build an API, but build a long-
lived API that your users will love. An API that is carefully created to be extendable and
flexible, and will save your users time, money and energy in the long run.
As this is the very first edition of this book, I would ask that if you find mistakes you are
both willing to forgive them and willing to let me know so that they can be corrected.
This feedback will be very much appreciated and can be directed to
me@mikestowe.com with the subject “Errata.”
Please also understand that while I would love to answer any questions, this may not
be feasible from a time standpoint. But I would encourage you all to post freely in the
MuleSoft forums (under API), and I, a member of my team or someone from our
community will be glad to help out.
I would also like to thank Dr. Roy Fielding for being extremely gracious in letting me
include his original doctoral dissertation diagrams in this book to demonstrate how the
different constraints of REST work together.
I’d also like to thank Arnaud Lauret (@apihandyman) and Jason Harmon (@jharmn) for
taking the time to review this book prior to publication. As well as a very big thank you
to Phil Hunter who spent countless hours copy-editing, and Chris Payne (@chrisypayne)
who meticulously designed the book’s amazing cover.
Last but not least, I want to thank MuleSoft for giving me time to write this book, and
for making it freely available. I have always believed in the power of community, and
been thankful for all of the valuable lessons I have learned from it over the years. I want
to also thank my many mentors, my family, and my friends for their patience, love, and
support.
Please enjoy this version of Undisturbed REST, a Guide to Designing the Perfect API—a
book that is dedicated both to you and the amazing developer community as a whole.
– Mike Stowe (@mikegstowe)

Table of Contents
1. What is an API
Web APIs
REST
1
3
6
2. Planning Your API
Questions to Ask
15
16
3. Designing the Spec
Versioning
Spec-Driven Development
Choosing a Spec
25
26
30
35
4. Using RAML
Query Parameters
Responses
ResourceTypes and Traits
39
43
44
45
5. Prototyping and Agile Testing
Getting Developers Involved
Getting Feedback
49
53
56
6. Authorization and Authentication
Generating tokens
OAuth2
59
62
64

OAuth and Security
Adding OAuth to RAML
68
71
7. Designing Your Resources
Nouns
Content-types
Versioning
Caching
73
74
76
83
88
8. Designing Your Methods
Items vs Collections
HTTP Methods
91
92
93
9. Handling Responses
HTTP Status Codes
Errors
101
102
105
10. Adding Hypermedia
HATEOAS
Hypermedia Specs
Hypermedia Challenges
117
121
124
136
11. Managing with a Proxy
API Access
Throttling
SLA Tiers
Security
139
140
142
144
147

12. Documenting and Sharing Your API
API Console
API Notebook
Support Communities
SDKs and Client Libraries
153
166
170
173
174
13. A Final Thought 179
Appendix: More API Resources 183
Appendix: Is Your API Ready? 187

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What is an API?
In the simplest of terms, API is the acronym for Application Programming
Interface, which is a software intermediary that allows two applications to
talk to each other. In fact, each time you check the weather on your phone,
use the Facebook app or send an instant message, you are using an API.
Every time you use one of these applications, the application on your phone
is connecting to the Internet and sending data to a server. The server then
retrieves that data, interprets it, performs the necessary actions and sends
it back to your phone. The application then interprets that data and
presents you with the information you wanted in a human, readable format.
What an API really does, however, is provide a layer of security. Because
you are making succinct and explicit calls, your phone’s data is never fully
exposed to the server, and likewise the server is never fully exposed to your
phone. Instead, each communicates with small packets of data, sharing
only that which is necessary—kind of like you ordering food from a drive-

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through window. You tell the server what you would like to eat, they tell you
what they need in return and then, in the end, you get your meal.
Many Types of APIs
There are many types of APIs. For example, you may have heard of Java
APIs, or interfaces within classes that let objects talk to each other in the
Java programming language. Along with program-centric APIs, there are
also Web APIs such as the Simple Object Access Protocol (SOAP), Remote
Procedure Call (RPC), and perhaps the most popular—at least in name—
Representational State Transfer (REST).
While the function of an API may be fairly straightforward and simple, the
process of choosing which type to build, understanding why that type of
API is best for your application, and then designing it to work effectively has
proven to be far more difficult.
One of the greatest challenges of building an API is building one that will
last. This is especially true for Web APIs, where you are creating both a
contract between you and your users and a programming contract between
your server and the client.
In this book, we’ll take a look at some of the different types of APIs, but
then we’ll switch gears and focus on building a REST API as defined by Dr.
Roy Fielding. We’ll cover important principles that are often ignored—and
while some of these may seem a little painful or like they just create more
work, you’ll find that by adhering to these best practices you will not only
create a better API, but save a lot of time and money doing so.
So without further ado, let’s get started.

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Web APIs
A Web API, otherwise known as a Web Service, provides an interface for
Web applications, or applications that need to connect to each other via the
Internet to communicate. To date, there are over 13,000 public APIs that
can be used to do everything from checking traffic and weather, to
updating your social media status and sending Tweets, to even making
payments.
In addition to the 13,000 public APIs, there are hundreds of thousands
more private Web APIs, or APIs that are not available for consumption by
the general public, that are used by companies to extend their capabilities
and services across a broad scope of use-cases, including multiple devices.
One of the most common forms of a private cross-device Web APIs would
be an API written for a mobile application that lets the company transmit
data to the app on your phone.
Since 2005, the use of Web APIs has exploded exponentially, and multiple
Web formats and standards have been created as technology has
advanced:

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Early on, one of the most popular enterprise formats for APIs was SOAP.
With the emergence of JavaScript Object Notation (JSON), we saw more
reliance on HTTP and the growth of JSON-RPC APIs, while REST has
grown in popularity and quickly become the de facto standard for general
Web APIs today.
SOAP
SOAP was designed back in 1998 by Dave Winer, Don Box, Bob Atkinson
and Mohsen Al-Ghosein for Microsoft Corporation. It was designed to offer
a new protocol and messaging framework for the communication of
applications over the Web. While SOAP can be used across different
protocols, it requires a SOAP client to build and receive the different
requests, and relies heavily on the Web Service Definition Language (WSDL)
and XML:
<?xml
version="1.0"?>

<soap:Envelope

xmlns:soap="http://www.w3.org/2001/12/soap-‐envelope"

soap:encodingStyle="http://www.w3.org/2001/12/soap-‐
encoding">

<soap:Body
xmlns:m="http://www.example.com/weather">

<m:GetWeather>

<m:ZipCode>94108</m:ZipCode>

</m:GetWeather>

</soap:Body>

</soap:Envelope>

Early on, SOAP did not have the strongest support in all languages, and it
often became a tedious task for developers to integrate SOAP using the
Web Service Definition Language. However, SOAP calls can retain state,
something that REST is not designed to do.

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XML-RPC
On the other hand, Remote Procedure Calls, or RPC APIs, are much
quicker and easier to implement than SOAP. XML-RPC was the basis for
SOAP, although many continued to use it in its most generic form, making
simple calls over HTTP with the data formatted as XML.
However, like SOAP, RPC calls are tightly coupled and require the user to
not only know the procedure name, but often the order of parameters as
well. This means that developers would have to spend extensive amounts
of time going through documentation to utilize an XML-RPC API, and
keeping documentation in sync with the API was of utmost importance, as
otherwise a developer’s attempts at integrating it would be futile.
JSON-RPC
Introduced in 2002, the JavaScript Object Notation was developed by State
Software, Inc. and made most famous by Douglas Crawford. The format
was originally designed to take advantage of JavaScript’s ability to act as a
messaging system between the client and the browser (think AJAX).
JSON was then developed to provide a simple, concise format that could
also capture state and data types, allowing for quick deserialization.
Yahoo started taking advantage of JSON in 2005, quickly followed by
Google in 2006. Since then JSON has enjoyed rapid adoption and wide
language support, becoming the format of choice for most developers.
You can see the simplicity that JSON brought to data formatting as
compared to the SOAP/ XML format above:
{“zipCode”
:
“94108”}

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However, while JSON presented a marked improvement over XML, the
downsides of an RPC API still exist with JSON-RPC APIs, including tightly
coupled URIs. Just the same, JSON-APIs have been widely adopted and
used by companies such as MailChimp, although they are often mislabeled
as “RESTful.”
REST
Now the most popular choice for API development, REST or RESTful APIs
were designed to take advantage of existing protocols. While REST can be
used over nearly any protocol, it typically takes advantage of HTTP when
used for Web APIs. This means that developers do not need to install
libraries or additional software in order to take advantage of a REST API.
As defined by Dr. Roy Fielding in his 2000 Doctorate Dissertation, REST
also provides an incredible layer of flexibility. Since data is not tied to
methods and resources, REST has the ability to handle multiple types of
calls, return different data formats and even change structurally with the
correct implementation of hypermedia.

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As you can see in this chart, each type of API offers different strengths and
weaknesses. REST, however, provides a substantial amount of freedom
and flexibility, letting you build an API that meets your needs while also
meeting the needs of very diverse customers.
Unlike SOAP, REST is not constrained to XML, but instead can return XML,
JSON, YAML or any other format depending on what the client requests.
And unlike RPC, users aren’t required to know procedure names or specific
parameters in a specific order.
But you also lose the ability to maintain state in REST, such as within
sessions, and it can be more difficult for newer developers to use. It’s also
important to understand what makes a REST API RESTful, and why these
constraints exist before building your API. After all, if you do not understand
why something is designed in the manner it is, you are more likely to
disregard certain elements and veer off course, often times hindering your
efforts without even realizing it.
Understanding REST
Surprisingly, while most APIs claim to be RESTful, they fall short of the
requirements and constraints asserted by Dr. Roy Fielding. One of the most
commonly missed constraints of REST is the utilization of hypermedia as
the engine of application state, or HATEOAS, but we’ll talk about that in
another chapter.
There are six key constraints to REST that you should be aware of when
deciding whether or not this is the right API type for you.

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Client-Server
The client-server constraint operates on the concept that the client and the
server should be separate from each other and allowed to evolve
individually. In other words, I should be able to make changes to my mobile
application without impacting either the data structure or the database
design on the server. At the same time, I should be able to modify the
database or make changes to my server application without impacting the
mobile client. This creates a separation of concerns, letting each
application grow and scale independently of the other and allowing your
organization to grow quickly and efficiently.
Stateless
REST APIs are stateless, meaning that calls can be made independently of
one another, and each call contains all of the data necessary to complete
itself successfully. A REST API should not rely on data being stored on the
server or sessions to determine what to do with a call, but rather solely rely
on the data that is provided in that call itself.
This can be confusing, especially when you hear about using hypermedia
as the state of the application (Wait—I thought REST was stateless?)..Don’t
worry, we’ll talk about this later, but the important takeaway here is that
sessions or identifying information is not being stored on the server when
making calls. Instead, each call has the necessary data, such as the API
key, access token, user ID, etc. This also helps increase the API’s reliability

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by having all of the data necessary to make the call, instead of relying on a
series of calls with server state to create an object, which may result in
partial fails. Instead, in order to reduce memory requirements and keep
your application as scalable as possible, a RESTful API requires that any
state is stored on the client—not on the server.
Cache
Because a stateless API can increase request overhead by handling large
loads of incoming and outbound calls, a REST API should be designed to
encourage the storage of cacheable data. This means that when data is
cacheable, the response should indicate that the data can be stored up to a
certain time (expires-at), or in cases where data needs to be real-time, that
the response should not be cached by the client.
By enabling this critical constraint, you will not only greatly reduce the
number of interactions with your API, reducing internal server usage, but
also provide your API users with the tools necessary to provide the fastest
and most efficient apps possible.

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Keep in mind that caching is done on the client side. While you may be able
to cache some data within your architecture to perform overall performance,
the intent is to instruct the client on how it should proceed and whether or
not the client can store the data temporarily.
Uniform Interface
The key to the decoupling client from server is having a uniform interface
that allows independent evolution of the application without having the
application’s services, or models and actions, tightly coupled to the API
layer itself. The uniform interface lets the client talk to the server in a single
language, independent of the architectural backend of either. This interface
should provide an unchanging, standardized means of communicating
between the client and the server, such as using HTTP with URI resources,
CRUD (Create, Read, Update, Delete) and JSON.

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However, to provide a truly uniform interface, Fielding identified four
additional interface constraints: identifying resources, manipulation through
representations, self-describing messages and HATEOAS (hypermedia as
the engine of application state).
We’ll talk about these more in a later chapter.
Layered System
As the name implies, a layered system is a system comprised of layers,
with each layer having a specific functionality and responsibility. If we think
of a Model View Controller framework, each layer has its own
responsibilities, with the models comprising how the data should be formed,
the controller focusing on the incoming actions and the view focusing on
the output. Each layer is separate but also interacts with the other.
In REST, the same principle holds true, with different layers of the
architecture working together to build a hierarchy that helps create a more
scalable and modular application. For example, a layered system allows for
load balancing and routing (preferably through the use of an API
Management Proxy tool which we’ll talk about in Chapter 11). A layered
system also lets you encapsulate legacy systems and move less commonly
accessed functionality to a shared intermediary while also shielding more
modern and commonly used components from them. A layered system also
gives you the freedom to move systems in and out of your architecture as
technologies and services evolve, increasing flexibility and longevity as long
as you keep the different modules as loosely coupled as possible.

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There are substantial security benefits of having a layered system since it
allows you to stop attacks at the proxy layer, or within other layers,
preventing them from getting to your actual server architecture. By utilizing
a layered system with a proxy, or creating a single point of access, you are
able to keep critical and more vulnerable aspects of your architecture
behind a firewall, preventing direct interaction with them by the client.
Keep in mind that security is not based on single “stop all” solution, but
rather on having multiple layers with the understanding that certain security
checks may fail or be bypassed. As such, the more security you are able to
implement into your system, the more likely you are to prevent damaging
attacks.
Code on Demand
Perhaps the least known of the six constraints, and the only optional
constraint, Code on Demand allows for code or applets to be transmitted
via the API for use within the application. In essence, it creates a smart
application that is no longer solely dependent on its own code structure.
However, perhaps because it’s ahead of its time, Code on Demand has
struggled for adoption as Web APIs are consumed across multiple
languages and the transmission of code raises security questions and
concerns. (For example, the directory would have to be writeable, and the
firewall would have to let what may normally be restricted content through.)

13
Just the same, I believe that while it is currently the least adopted constraint
because of its optional status, we will soon see more implementations of
Code on Demand as its benefit becomes more apparent.
Together, these constraints make up the theory of Representational State
Transfer, or REST. As you look back through these you can see how each
successive constraint builds on top of the previous, eventually creating a
rather complex—but powerful and flexible—application program interface.
But most importantly, these constraints make up a design that operates
similarly to how we access pages in our browsers on the World Wide Web.
It creates an API that is not dictated by its architecture, but by the
representations that it returns, and an API that—while architecturally
stateless—relies on the representation to dictate the application’s state.
Please keep in mind that this is nothing more than a quick overview of the
constraints of REST as defined by Dr. Fielding. For more information on the
different constraints, you can read his full dissertation online at
http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm.
Architectural Diagrams provided courtesy of Roy Fielding, from
Architectural Styles and the Design of Network-based Software
Architectures. Doctoral dissertation, University of California, Irvine, 2000.

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Planning Your API
While understanding which type of API you are building is a vital step in
creating the perfect one for your company—and one that users will love—it
is just as important to carefully plan out your API’s capabilities.
Surprisingly, while being one of the most crucial steps in API development,
this step is usually rushed through by companies excited to generate a
product map and start working on code.
In many ways, building an API is like building a house. You can say, “I want
my house to look like this picture,” but without the blueprints, chances are
it isn’t going to turn out exactly the way you had hoped. Yet while we
carefully plan and build houses, APIs have been plagued by the “agile
methodology.”
While I am a fan of agile and I find it to be one of the greatest
advancements in project management, like all good things, it has its time
and place. Unlike many Software as a Service applications built for today’s
web, an API’s interface is a contract, and as such cannot be constantly
changing. With increases in the use of hypertext links and hypermedia,

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perhaps some day in the future that may no longer be true, but right now
many developers are hardcoding resources and actions, and as such
changing the resource name, moving it or changing the data properties can
be detrimental to their application.
It’s also important to understand that developers won’t just be relying on
your API to do cool things—they’ll also be relying on it for their livelihood.
When you put out an API, you are giving developers your company’s word,
and when you break the API, you are not just breaking your company’s
word, you’re undermining their ability to provide for their families. So it’s
important that you create a solid foundation for your API, just as you would
a house. It’s important that you understand what your API should be able to
do, and how it will work. Otherwise, you may lock yourself into a poor
design or worse—create an API that doesn’t meet your developers’ needs.
If that happens, you’ll find yourself taking a lot more time and spending a
lot more money to fix what you should have planned up front instead.
Who is Your API For
As developers, we tend to like to jump to what our API will do before we
think about what our API should do, or even what we want it to do. So
before we get started, we need to take a step back and ask ourselves,
“Who will be using our API?”
Are you building your API for your application’s customers? For their
business partners? For third party developers so that your platform can be
extended upon? Often times the answer tends to be a combination of the
above, but until you understand for whom you are building your API, you
aren’t ready to start planning it.
To Which Actions Do They Need Access?
All too often I hear companies say, “We want to build an API to expose our
data,” or “We want to build an API to get people using our system.” Those

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are great goals, but just because we want to do something doesn’t mean
we can truly accomplish it without a solid plan.
After all, we all want people to use our APIs, but why should they? While it
may seem a little harsh, this is the very next question we need to carefully
ask ourselves: Why would our users (as we identified them above) want to
use our API? What benefit does it offer them?
Another common mistake is answering the question of “why?” with, “our
reputation.” Many companies rely on their name rather than their
capabilities. If you want to grow a strong and vibrant developer community,
your API has to do something more than just bear your name.
What you should be doing is asking your potential users, “Which actions
would you like to be able to accomplish through an API?” By speaking
directly to your potential API users, you can skip all the guesswork and
instead find out exactly what they are looking for and which actions they
want to be able to take within your API, and also isolate your company’s
value to them. Often, the actions your users will want are different from the
ones you anticipated, and by having this information you can build out your
API while testing it for real use cases.
Remember, when building an application for a customer, we sit down either
with them or the business owners to understand what it is they want us to
build. Why aren’t we doing the same thing with APIs? Why aren’t we sitting
down with our customers, the API users, and involving them in the process
from day one? After all, doing so can save us a lot of time, money and
headaches down the road.
Plus, by asking your users what they want, you may find you have better
selling points to get the support from other areas within the business. Most
importantly, you can answer the vital question of why you are building the
API in the first place.

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List out the Actions
Now that you know what your developers want to do, list out the actions.
All too commonly, developers jump into the CRUD mindset, but for this
exercise, you should simply create categories and add actions to them
based on what part of the application or object it will affect.
For example, your developers will probably want access to their users, the
ability to edit a user, reset a password, change permissions, and even add
or delete users.
Assuming your application has a messaging system, they may also want to
create a new draft, message a user, check messages, delete messages, etc.
As you isolate these different categories and actions, you’ll want to chart
them like so:
Now, you may notice that in the above chart we have duplicate entries. For
example, we have “message a user” under both “Users” and “Messages.”
This is actually a good thing, because it shows us how the different actions
work together across different categories, and potentially, different
resources.

19
We now know that there is a viable use case for messages not only within
users, but also within messages, and we can decide under which it makes
the most sense. In the case of “send a message” it would probably make
most sense under the “messages” resource, however because of the
relationship, we might want to include a hypertext link when returning a
user object in the “users” resource.
By doing this exercise, not only can we quickly isolate which actions we
need to plan for, but also how they’ll work together and even how we
should begin designing the flow of our API.
This step may seem incredibly simple, but it is one of the most crucial steps
in the planning process to make sure you are accounting for your
developers’ needs (I would even recommend showing your potential API
users your chart to see if they think of anything else after the fact), while
also understanding how the different resources within your API will need to
work together, preventing you from having to rewrite code or try to move
things around as you are coding your API.
Explain How Your API Will Interact with Existing Services
Unless you are starting from ground zero and taking an API-first approach,
there’s a good chance you have other applications and services that your
API may need to interact with.
You should take time to focus on how your API and application will interact.
Remember, your application can change over time, but you’ll want your
API’s interface to remain consistent for as long as possible.
You’ll also want to take time to isolate which services the API will need to
interact with. Even before building the API you can ensure that these
services are flexible enough to talk to your API while keeping it decoupled
from your technology stack.

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Along with understanding any technical risks involved with these services, if
they are organizationally owned, developers can start focusing on
transitioning them to an API-focused state, ensuring that by the time you
are ready to connect them to your API, they are architecturally friendly. It is
never too early to plan ahead.
How Are You Going to Maintain Your API?
Remember that building an API is a long-term commitment, because you
won’t just be creating it, you will be maintaining it as well.
It’s very possible to build a good API that doesn’t need much work after its
release, but more typically than not, especially if you’re an API-driven
company, you’ll find that not only are there bugs to fix, but developers’
demands will increase more and more as they use your API for their
applications.
One of the advantages to the Spec-Driven Development approach to APIs
is that you start off by building the foundation of your API, and then slowly
and carefully adding to it after that. This is the recommended approach, but
regardless you shouldn’t plan on just launching it and leaving it, but rather
having dedicated resources that can continue to maintain, patch bugs, and
hopefully continue to build upon your API as new features are released
within your application.
How Are You Going to Version Your API
Along with maintaining your API, you should also plan on how you are
going to version your API. Will you include versioning in the URL such as
http://api.mysite.com/v1/resource, or will you return it in the content-type
(application/json+v1), or are you planning on creating your own custom
versioning header or taking a different approach altogether?

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Keep in mind that your API should be built for the long-term, and as such
you should plan on avoiding versioning as much as possible, however,
more likely than not there will come a time when you need to break
backwards incompatibility, and versioning will be the necessary evil that
lets you do so.
We’ll talk about versioning more, but in essence you should try to avoid it,
while still planning for it—just as you would plan an emergency first aid kit.
You really don’t want to have to use it, but if you do – you’ll be glad you
were prepared.
How Are You Going to Document Your API
Along with maintenance, developers will need access to documentation,
regardless if you are building a hypermedia driven API or not.
And while documentation may seem like a quick and easy task, most
companies will tell you it is one of their biggest challenges and burdens
when it comes to maintaining their API.
As you update your API you will want to update your documentation to
reflect this, and your documentation should have a clear description of the
resource, the different methods, code samples, and a way for developers to
try it out.
We’ll look at documentation more in-depth towards the end of this book,
but fortunately the steps we take in the next chapter will help us efficiently
create (and maintain) our documentation. Whether or not you elect not to
follow these steps, you should have a plan for how you are going to
maintain your API’s documentation. This is one area you should not
underestimate since it has proven to be the crux of usability for most public
APIs.

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How will Developers Interact with Your API
Another important aspect to plan for is how developers will interact with
your API. Will your API be open like the Facebook Graph API, or will you
utilize an API Key? If you’re using an API key, do you plan on provisioning
your API to only allow certain endpoints, or set limits for different types of
users? Will developers need an access token (such as OAuth) in order to
access user’s data?
It’s also important to think about security considerations and throttling.
How are you going to protect your developer’s data, and your service
architecture? Are you going to try and do this all yourself, or does it make
more sense to take advantage of a third-party API Manager such as
MuleSoft?
The answers to these questions will most likely depend on the requirements
that you have for your API, the layers of security that you want, and the
technical resources and expertise you have at your company. Generally,
while API Management solutions can be pricey, they tend to be cheaper
than doing it yourself.
We’ll talk more about these considerations and using a proxy or
management solution in another chapter.
How Are You Going to Manage Support
Another consideration, along with documentation is how are you going to
manage support when things go wrong? Are you going to task your
engineers with API support questions? If so, be warned that while this may
work in the short-term, it is typically not scalable.
Are you going to have a dedicated API support staff? If so, which system(s)
will you use to manage tickets so that support requests do not get lost, can
be followed up on, escalated, and your support staff do not lose their minds.

23
Are you going to have support as an add-on, a paid service, a partner perk,
or will it be freely available to everyone? If support is only for certain levels
or a paid subscription, will you have an open community (such as a forum)
for developers to ask questions about your API, or will you use a third-party
like StackOverflow? And how will you make sure their questions get
answered, and bugs/ documentation issues get escalated to the
appropriate source?
Don’t Be Intimidated
In this chapter we’ve asked a lot of questions, hopefully some thought-
provoking ones. The truth is that building and running a successful API is a
lot of work, but do not let these questions scare you off.
The reason we ask these questions now is so that you are thinking about
them, and so that you have the answers and can avoid surprises when the
time comes. These questions are designed to prepare you, not overwhelm
you, so that you can build a truly successful program and avoid costly
mistakes. Because a strong API program doesn’t just drive adoption, it
drives revenue, and it drives the company forward.

“I
can
definitely
say
that
one
of
the
best
investments

we
ever
made
as
a
company
was
in
our
API.
It’s

probably
the
best
marketing
we’ve
ever
done.”
– Ben Chestnut, Owner of MailChimp

25
3
Designing the Spec
Once you understand why you are building your API, and what it needs to
be able to accomplish you can start creating the blueprint or spec for your
API. Again, going back to the building a house scenario, by having a plan
for how your API should look structurally before even writing a line of code
you can isolate design flaws and problems without having to course correct
in the code.
Using a process called Spec-Driven Development, you will be able to build
your API for the long-term, while also catching glitches, inconsistencies and
generally bad design early on. While this process usually adds 2–4 weeks
onto the development cycle, it can save you months and even years of
hassle as you struggle with poor design, inconsistencies, or worse—find
yourself having to build a brand-new API from scratch.
The idea behind a REST API is simple: it should be flexible enough to
endure. That means as you build your API, you want to plan ahead—not
just for this development cycle, not just for the project roadmap, but for
what may exist a year or two down the road.

26
26
This is really where REST excels, because with REST you can take and
return multiple content types (meaning that if something comes along and
replaces JSON, you can adapt) and even be fluid in how it directs the client
with hypermedia. Right off the bat you are being setup for success by
choosing the flexibility of REST. However it’s still important that you go in
with the right mindset—that the API you build will be long-term focused.
Versioning – A Necessary Evil
As previously mentioned, versioning is important to plan for, but all too
often companies look at an API the same way they do desktop software.
They create a plan to build an API—calling it Version 1—and then work to
get something that’s just good enough out the door. But there’s a huge
difference between creating a solid foundation that you can add onto and a
half-baked rush job just to have something out there with your name on it.
After all people will remember your name for better or worse.
The second problem is they look at versions as an accomplishment. I
remember one company that jumped from Version 2 to Version 10 just
because they thought it sounded better and made the product look more
advanced. But with APIs, it’s just the opposite. A good API isn’t on Version
10, it’s still rocking along at Version 1, because it was that well thought out
and designed in the first place.
If you go into your API design with the idea that you are “only” creating
Version 1, you will find that you have done just that—created a version that
will be short lived and really nothing more than a costly experiment from
which you hopefully learned enough to build your “real” API. However, if
you follow the above steps and carefully craft your API with the idea that
you are building a foundation that can be added onto later, and one that will
be flexible enough to last, you have a very good chance of creating an API
that lives 2–3 years—or longer!

27
Think about the time and cost it takes to build an API. Now think about the
time it takes to get developers to adopt an API. By creating a solid API now,
you avoid all of those costs upfront. And in case you’re thinking, “It’s no big
deal, we can version and just get developers to upgrade,” you might want
to think again. Any developer evangelist will tell you one of the hardest
things to do is to get developers to update their code or switch APIs. After
all, if it works, why should they change it? And remember, this is their
livelihood we’re talking about—they can spend time making money and
adding new features to their application, or they can spend time losing
money trying to fix the things you broke—which would you prefer to base
your reputation upon?
Versioning an API is not only costly to you and the developer, it also
requires more time on both ends, as you will find yourself managing two
different APIs, supporting two different APIs, and confusing developers in
the process. In essence, when you do version, you are creating the perfect
storm.
You should NOT version your API just because you’ve:
§ Added new resources
§ Added data in the response
§ Changed technologies (Java to Ruby)
§ Changed your application’s services
Remember, your API should be decoupled from both your technology stack
and your service layer so that as you make changes to your application’s
technology, the way the API interacts with your users is not impacted.
Remember the uniform interface—you are creating separation between
your API and your application so that you are free to develop your
application as needed, the client is able to develop their application as
needed, and both are able to function independently and communicate
through the API.

28
28
However, you SHOULD consider versioning your API when:
§ You have a backwards-incompatible platform change, such as
completely rewriting your application and completely changing the
user interface
§ Your API is no longer extendable—which is exactly what we are
trying to avoid here
§ Your spec is out of date (e.g. SOAP)
Again, I cannot stress enough the importance of going into your API design
with a long-term focus, and that means versioning becomes a last-resort or
doomsday option for when your API can no longer meet your users’—or the
company’s—needs.
Understand You’re Poor at Design
The next thing that’s important to understand is that we, as developers, are
poor at long-term design.
“People
are
fairly
good
at
short-‐term
design
and

usually
awful
at
long-‐term
design.”
– Dr. Roy Fielding, Creator of REST
Think about a project you built three years ago, even two years ago, even
last year. How often do you start working on a project only to find yourself
getting stuck at certain points, boxed in by the very code you wrote? How
often do you look back at your old code and ask yourself, “What was I
thinking?” I don’t know about you, but I do that on an almost daily basis
(and usually with yesterday’s—or this morning’s—code).

29
The simple fact is that we can only see what we can see. While we may
think we are thinking through all the possibilities, there’s a good chance
we’re missing something. I can’t tell you how many times I’ve had the
chance to do peer-programming where I would start writing a function or
method, and the other developer would ask why I didn’t just do it in two
lines of code instead!? Of course their way was the right way, and super
simple, but my mind was set, and I had developer tunnel vision—something
we all get that is dangerous when it comes to long-term design.
By accepting that we, by ourselves, are not good at long-term design, we
actually enable ourselves to build better designs. By understanding that we
are fallible and having other developers look for our mistakes (in a
productive way), we can create a better project and a longer-lasting API.
After all, two heads are better than one!
In the past this has been difficult, especially with APIs. Companies struggle
to afford (or even recognize the benefit of) peer programming, and building
out functional mock-ups of an API has proven extremely costly. Thankfully,
advances in technology have made it possible to get feedback—not just
from our coworkers, but also from our potential API users—without having
to write a single line of code! This means that where before we would have
to ship to find inconsistencies and design flaws, now we can get feedback
and fix them before we even start coding our APIs, saving time and money
not only in development, but also support.
To take advantage of this new technology to the fullest, we can use a
methodology that I am calling Spec-Driven Development, or the
development of our API based on a predefined specification that has been
carefully tested and evaluated by our potential API users.

30
30
Spec-Driven Development
Spec-Driven Development is designed to take advantage of newer
technologies in order to make the development, management and
documentation of our API even more efficient. It does this by first dividing
design and development into two separate processes.
The idea behind Spec-Driven Development is that agility is a good thing,
and so is agile user testing/ experience. However, what we do not want to
see is agile development of the API design when it comes time to write the
code. Instead, Spec-Driven Development encourages separating the design
stage from the development stage, and approaching it iteratively. This
means that as you build out your design using a standardized spec such as
RESTful API Modeling Language (RAML), you can test that spec by
mocking it up and getting user feedback.
MuleSoft’s API Contract Design Cycle demonstrates a well-thought-out
flow for perfecting your spec. It begins with designing your API, then moves
to mocking/simulating the API, soliciting feedback, and finally—depending
on that feedback—either determining that the spec is ready for
development or returning to the design stage where the cycle continues.

31
Once you have finished getting user feedback and perfecting the design in
the spec, then you can use that specification as your blueprint for design.
In essence, you are keeping agile user testing, and agile development, but
splitting them so you’re not doing agile user testing as you do the actual
development (as your code should be written to match the spec’s design,
and thus the previously tested and affirmed user experience).
It’s important to note that with Spec-Driven Development, there is no back
and forth. Once you move into the development phase, you are moving
forward with the assumption that the spec has been perfected, and that
you have eliminated 99 percent of the design flaws/ inconsistencies. Should
you find an issue with the design, rather than correcting it in the
development cycle, you need to stop and go back to the design cycle,
where you fix the spec and then retest it.
The reasoning behind this is pretty simple—we’re usually awful at long-term
design. As such, rather than make changes on the fly or try to fix things
with a short-sighted view, Spec-Driven Development encourages “all hands

32
32
on deck” to ensure that the changes you make (no matter how simple or
insignificant they might seem) do not cause any design inconsistencies or
compounding problems, either in the short-term or down the road.
In order to be successful with Spec-Driven Development, you should follow
these six constraints:
1. Standardized
Spec-Driven Development encourages the use of a standardized format
applicable to the type of application you are building. In the case of
building an API, for example, the following specs would be considered
standard or common among the industry: RAML, Swagger, API
Blueprint, IO Docs. Utilizing a standard spec ensures easy portability
among developers while also ensuring that the spec your application
relies on has been thoroughly tested by the community to ensure that it
will meet both your short-term and long-term needs while maintaining
consistency in its own format.
2. Consistent
In developing your spec, you should utilize pattern-driven design as well
as code reuse when possible to ensure that each aspect of your spec is
consistent. In the event of building an API, this would mean ensuring
your resources are all formatted similarly and your methods all operate
in a similar format—both in regards to the request and available
responses. The purpose of consistency is to avoid confusion in both the
development and use of your application so all aspects of the
application work similarly, providing the end user with the freedom to
move seamlessly from one focus to another.
3. Tested
Spec-Driven Development requires a strong, tested spec in order to
build a reliable application. This means that the spec has to be carefully
crafted and then tested with both internal and external uses to ensure
that it accomplishes its goals and meets the needs of all parties.

33
The spec should be crafted, mocked/prototyped and tested to retrieve
user feedback. Once user feedback is received, the spec should be
modified appropriately, mocked and tested again, creating a continuous
cycle until you have perfected the spec—or at the least eliminated a
large majority of the design issues to ensure spec and application
longevity.
4. Concrete
The specification should be the very foundation of your application or, in
essence, the concrete foundation of the house you are building. The
spec should encompass all aspects of your application, providing a
solid blueprint to which your developers can code. The spec does not
have to encompass future additions, but it should have taken as many
of them into consideration as possible. However, there is nothing that
relates to the spec that is coded outside of existing inside of the spec.
5. Immutable
The spec is the blueprint for development and is unchangeable by code.
This means that at no time is the code to deviate from or override the
spec. The spec is the ultimate authority of the application design, since
it is the aspect that has been most thought out and carefully designed,
and has also been tested by real-world users. It is important to realize
that short-term coding implementations can be detrimental to an
application’s longevity, and as such have no place in Spec-Driven
Development.
6. Persistent
All things evolve, and the application and spec are no different.
However, each evolution must be just as carefully thought out as the
original foundation. The spec can change, but each change must be
justified, carefully evaluated, tested and perfected. In the event of
redevelopment, if the spec is found not to be renderable, it is important

34
34
to go back and correct the spec by re-engaging in user testing and
validation, and then updating the code to match to ensure that it is
consistent with your spec, while also ensuring that the necessary
changes do not reduce the longevity of your application.
The last constraint of Spec-Driven Development, the Persistent constraint,
explains how you can use Spec-Driven Development to continue building
out and adding additions to your API. For every change you make to the
API, you should start with the design stage, testing your new additions for
developers to try out, and then once validated, start adding the code and
pushing the changes to production. As described above in the Immutable
constraint, your API should never differ from the spec or have
resources/methods that are not defined in the spec.
Along with helping ensure that your API is carefully thought out, usable and
extendable for the long-term, use of these common specs offers several
additional benefits, including the ability auto generate documentation and
create interactive labs for developers to explore your API. There are even
Software Development Kit (SDK) generation services such as APIMatic.io
and REST United that let you build multi-language SDKs or code libraries
on the fly from your spec!
This means that not only have you dramatically reduced the number of
hours it will require you to fix bugs and design flaws, handle support or
even rebuild your API, but you are also able to cut down on the number of
hours you would be required to write documentation/create tools for your
API, while also making your API even easier for developers to use and
explore.
Of course, this leads us to choosing the right spec for the job.

35
Choosing a Spec
Choosing the best specification for your company’s needs will make
building, maintaining, documenting and sharing your API easier. Because
Spec-Driven Development encourages the use of a well tested,
standardized spec, it is highly recommended that you choose from RAML,
Swagger or API Blueprint. However, each of these specs brings along with
it unique strengths and weaknesses, so it is important to understand what
your needs are, as well as which specification best meets those needs.
RAML
The baby of the three most popular specs, RAML 0.8, was released in
October 2013. This spec was quickly backed by a strong working group
consisting of members from MuleSoft, PayPal, Intuit, Airware, Akana
(formally SOA Software), Cisco and more. What makes RAML truly unique
is that it was developed to model an API, not just document it. It also
comes with powerful tools including a RAML/ API Designer, an API Console
and the API Notebook—a unique tool that lets developers interact with your
API. RAML is also written in the YAML format, making it easy to read, and
easy to edit—regardless of one’s technical background.
Swagger
The oldest and most mature of the specs, Swagger, just recently released
Version 2.0, a version that changes from JSON to the YAML format for
editing and provides more intuitive tooling such as an API Designer and an
API Console. Swagger brings with it the largest community and has
recently been acquired by SmartBear, while also being backed by Apigee
and 3Scale. However, with the transition from JSON to YAML, you may find
yourself having to maintain two different specs to keep documentation and
scripts up to date. Swagger also lacks strong support for design patterns
and code reusability—important aspects of Spec-Driven Development.

36
36
API Blueprint
Created by Apiary in March of 2013, API Blueprint is designed to help you
build and document your API. However, API Blueprint lacks the tooling and
language support of RAML and Swagger, and utilizes a specialized
markdown format for documenting, making it more difficult to use in the
long run. Just the same, API Blueprint has a strong community and does
excel with its documentation generator.
Overall, RAML offers the most support for Spec-Driven Development and
provides interactive tools that are truly unique. You can also use’s free
mocking service to prototype your API instead of having to install
applications and generate the prototype yourself - as you currently have to
do with Swagger. On the other hand, Swagger offers the most tooling and
largest community.

38
38
Be sure to take a look at the chart on the previous page to see which
specification best matches your business needs. However, I would
personally recommend that unless you have needs that cannot be met by
RAML, that you strongly consider using this spec to define your API. Not
only will you have the ability to reuse code within your spec, but RAML
offers the strongest support for Spec-Driven Development, has strong
support for documentation and interaction, has unique, truly innovative
tools and offers strong support for SDK generation and testing.
In the next chapter we’ll take a look at using RAML, but again, this is only a
recommendation. What is most important is that you choose the spec that
meets your individual needs, as these three specs all have their own
strengths and weaknesses.

39
4
Using RAML
One of the easiest ways to start working with RAML is with the API
Designer, a free open source tool available on the RAML website at
http://raml.org/projects.
To get started even faster, MuleSoft also offers a free, hosted version of its
API Designer. . You can take advantage of this free service by visiting
https://anypoint.mulesoft.com/apiplatform/.
Because RAML is defined in the YAML (Yet Another Markup Language)
format, it is both human- and machine-readable, and with the API Designer
you will receive auto suggestions, tooltips, and available options, while also
having a visual preview to the right of your screen.

40
40
RAML requires that every API have a title, a version and a baseUri. These
three aspects help ensure that your API can be read, versioned and
accessed by any tools you choose to implement.
Describing these in RAML is as easy as:
#%RAML
0.8

title:
My
Book

version:
1

baseUri:
http://server/api/{version}

The nice thing is that the API Designer starts off by providing you the first
three lines, so all you need to add is your baseUri. You’ll also notice that
RAML has a version placeholder, letting you add the version to the URI if
desired.
To add a resource to your RAML file, simply declare the resource by using
a slash “/” and the resource name followed by a colon “:” like so:

41
#%RAML
0.8

title:
My
Book

version:
1

baseUri:

/my-‐resource:

YAML is tab-delimited, so once we have declared /my-resource, we can set
the properties of the resource by tabbing over once.
#%RAML
0.8

title:
My
Book

version:
1

baseUri:

/my-‐resource:

displayName:
My
Resource

description:
this
is
my
resource,
it
does
things

To add a method, such as GET, POST, PUT, PATCH or DELETE, simply
add the name of the method with a colon:
#%RAML
0.8

title:
My
Book

version:
1

baseUri:

/my-‐resource:

displayName:
My
Resource

description:
this
is
my
resource,
it
does
things

GET:

POST:

42
42
You can then add descriptions, query parameters, responses with
examples and schemas, or even additional nested endpoints, letting you
keep all of your resources grouped together in an easy-to-understand
format:
#%RAML
0.8

title:
My
Book

version:
1

baseUri:

/my-‐resource:

displayName:
My
Resource

description:
this
is
my
resource,
it
does
things

get:

description:
this
is
my
GET
method

queryparameters:

name:

responses:

200:
…

post:

description:
this
is
my
post
method

/sub-‐resource:

displayName:
Child
Resource

description:
this
is
my
sub
resource

URI Parameters
Because resources often take dynamic data, such as an ID or even a
search filter, RAML supports URI Parameters/placeholders. To indicate

43
dynamic data (which can either be defined by RAML or the URI), just use
the braces as you did with {version} in the baseUri:
#%RAML
0.8

title:
My
Book

version:
1

baseUri:

/my-‐resource:

/sub-‐resource/{id}:

/{searchFilter}:

Query Parameters
As shown above, you can easily add query parameters or data that is
expected to be sent to the server from the client when making an API call
on that resource’s method. RAML also lets you describe these parameters
and indicate whether or not they should be required:
/my-‐resource:

get:

queryParameters:

name:

displayName:
Your
Name

type:
string

description:
Your
full
name

example:
Michael
Stowe

required:
false

dob:

displayName:
DOB

type:
number

description:
Your
date
of
birth

example:
1985

required:
true

44
44
As you’ve probably noticed, RAML is very straight-forward and uses
descriptive keys. (displayName) for how you want it to be displayed in the
documentation, a description to describe what it does, the type (string,
number, etc), an example for the user to see, and whether or not the
parameter is required (boolean).
Responses
Likewise, RAML tries to make documenting method responses fairly
straight-forward by first utilizing the responses key and then displaying the
type of responses a person might receive as described by their status code.
(We’ll look at the different codes later.) For example, an OK response has
status code 200, so that might look like this:
/my-‐resource:

get:

responses:

200:

Within the 200 response we can add the body key to indicate the body
content they would receive back within the 200 response, followed by the
content-type (remember APIs can return back multiple formats), and then
we can include a schema, an example, or both:
/my-‐resource:

get:

responses:

200:

body:

application/json:

example:
|

{

"name"
:
"Michael
Stowe",

45

"dob"
:
"1985",

"author"
:
true

}

To add additional content-types we would simply add a new line with the
same indentation as the “application/json” and declare the new response
type in a similar fashion (e.g.: application/xml or text/xml).
To add additional responses, we can add the response code with the same
indentation as the 200, using the appropriate status code to indicate what
happened and what they will receive.
As you are doing all of this, be sure to look to the right of your editor to see
your API being formed before your very eyes, letting you try out the
different response types and results.
ResourceTypes
As you can imagine, there may be a lot of repetitive code, as you may have
several methods that share similar descriptions, methods, response types
(such as error codes) and other information. One of the nicest features in
RAML is resourceTypes, or a templating engine that lets you define a
template (or multiple templates) for your resource to use across the entire
RAML spec, helping you eliminate repetitive code and ensuring that all of
your resources (as long as you use a standardized template/ resourceType)
are uniform.
resourceTypes:

-‐
collection:

description:
Collection
of
available

<<resourcePathName>>

get:

description:
Get
a
list
of
<<resourcePathName>>.

responses:

46
46

200:

body:

application/json:

example:
|

<<exampleGetResponse>>

301:

headers:

location:

type:
string

example:
|

<<exampleGetRedirect>>

400:

/my-‐resource:

type:

collection:

exampleGetResponse:
|

{

"name"
:
"Michael
Stowe",

"dob"
:
"1985",

"author"
:
true

}

exampleGetRedirect:
|

http://api.mydomain.com/users/846

/resource-‐two:

type:

collection:

exampleGetResponse:
|

{

"city"
:
"San
Francisco",

"state"
:
"1985",

"postal"
:
"94687"

47
}

exampleGetRedirect:
|

http://api.mydomain.com/locations/78

In the above example we first define the resourceType “collection,” and
then call it into our resource using the type property. We are also taking
advantage of three placeholders <<resourcePathName>> that are
automatically filled with the resource name (“my-resource,” “resource-two”),
and <<exampleGetResponse>> and <<exampleGetRedirect>>, which we
defined in our resources. Now, instead of having to write the entire resource
each and every time, we can utilize this template, saving substantial
amounts of code and time.
Both “my-resource” and “resource-two” will now have a description and a
GET method with 200, 301 and 400 responses. The 200 response returns
back an application/json response with the example response we provided
using the <<exampleGetResponse>> placeholder, and a redirect in the
case of a 301 with <<exampleGetRedirect>>.
Again, we will get all of this without having to write repetitive code by taking
advantage of resourceTypes.
Traits
Like resourceTypes, traits allow you to create templates, but specifically for
method behaviors such as isPageable, isFilterable and isSearchable.
traits:

-‐searchable:

queryParameters:

query:

description:
|

JSON
array

48
48
[{"field1","value1","operator1"},…]

<<description>>

example:
|

<<example>>

/my-‐resource:

get:

is:
[searchable:
{description:
"search
by
location

name",
example:
"["city","San
Fran","like"]"}]

To utilize traits, we first define the trait that we want to use, in this case
“searchable” with the query parameters that we want to use, including the
description (using the <<description>> placeholder) and an example (using
the <<example>> placeholder).
However, unlike with resourceTypes, we pass the values for these
placeholders in the searchable array within the “is” array (which can hold
multiple traits).
Again, like resourceTypes, traits are designed to help you ensure that your
API is uniform and standard in its behaviors, while also reducing the
amount of code you have to write by encouraging and allowing code reuse.
Going Further
Hopefully this has given you a good start with RAML, but there are many
more tools and features to make defining your API even easier (including
easy implementation of common authorization methods such as basic auth
and OAuth1 & 2). Be sure to go through the RAML 100 and 200 Tutorials on
the RAML website (http://raml.org), and take a look at the RAML spec
(http://raml.org/spec.html) to learn even more pro tips and tricks.
Now, once you have your API defined, we can quickly prototype it using the
same MuleSoft tools, getting feedback from our potential users and
identifying potential design flaws and bugs!

49
5
Prototyping &
Agile Design
As you design your spec, one of the most important things you can do is
involve your users, getting crucial feedback to ensure it meets their needs,
is consistent and is easily consumable.
The best way to do this is to prototype your API and have your potential
users interact with it as if it was the actual API you are building.
Unfortunately, until recently this hasn’t been possible due to constraints in
time and budget resources. This has caused companies to utilize a “test it
as you release it” method, where they build the API to what they think their
users want, and after doing internal QA, release it in the hope that they
didn’t miss anything. This Wild West style of building APIs has led to
numerous bugs and inconsistencies, and greatly shortened API lifetimes.
Thankfully, RAML was designed to make this process extremely simple,
allowing us to prototype our API with the click of a button, creating a mock

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API that relies on example responses that can be accessed from anywhere
in the world by our users.
Likewise, Swagger and API Blueprint offer some building and mocking tools,
however, right now there isn’t anything quite as simple or easy to use as
MuleSoft’s free mocking service.
Mocking Your API
MuleSoft’s API designer not only provides an intuitive way to visually design
your API, as well as interact and review resources and methods for
completeness/documentation purposes, but it also provides an easy toggle
to quickly build a hosted, mocked version of your API that relies on the
“example” responses.
To turn on the Mocking service, one only needs to click the “Mocking
Service” toggle switch into the “On” setting:
When set to “On” MuleSoft’s free API Designer will comment out your
current baseUri and replace it with a new, generated one that may be used
to make calls to your mock API.

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This new baseUri is public, meaning that your potential users can access it
anywhere in the world, just as if your API was truly live. They will be able to
make GET, POST, PUT, PATCH, DELETE and OPTIONS calls just as they
would on your live API, but nothing will be updated, and they will receive
back example data instead.
MuleSoft’s mocking service currently supports RAML and Swagger—
although when importing Swagger it is converted to RAML.
Again, what makes prototyping so important is that your users can actually
try out your API before you even write a line of code, helping you catch any
inconsistencies within the API, such as inconsistencies in resource naming,
method interaction, filter interactions or even in responses:
After all, as you build out your API, you want all of your data to be
consistent, and you want to ensure that all of your user interactions are
uniform, letting developers quickly utilize your API without having to isolate
and debug special use cases where they need to do something unique or
different just for that one particular case.
Alternative Mocking Methods
While the MuleSoft API Designer is a completely free, SaaS-hosted tool
with no lock-in (there’s no subscription or anything you have to buy), there
are other ways to mock your RAML API.

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Mockable.io and Apiary both provide tools that let you build out your API
with a step-by-step (resource-by-resource) creation tool that creates a
mock that your users can interact with. Mockable.io is also working on
supporting spec importing (hoping to support RAML, Swagger, API
Blueprint, Mashery IO Docs and WADL) but at the time of this writing, none
of these imports were functional. Apiary also lets you import your API
Blueprint spec.
Swagger also provides multiple clients for building out mocks of your API,
although these clients must be downloaded and then hosted by you.
RAML specs can also be mocked out and hosted manually by using
MuleSoft’s Anypoint Studio with APIKit. This may be especially useful if you
are also planning on building out your API with such a tool and need to
integrate with multiple services, databases and third-party APIs.
But again, this requires not only downloading software and then either
installing a runtime on your own server or hosting it on CloudHub, it also
requires a manual process to generate the mock in the first place.

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Last, but certainly not least, there are multiple language-based frameworks
that allow the importing of specs, which we won’t cover here.
Keep in mind that when choosing a service that is manually driven (not a
SaaS such as MuleSoft, Apiary or Mockable.io) you are responsible for
keeping the mock up to date with the spec—an exercise that proves to be
both tedious and time-consuming.
Getting Developers Involved
One of the hardest parts of the mocking/prototyping process is getting
developers involved. Or as Gareth Jones, the Principle API Architect at
Microsoft OneNote, points out:
“Developers
do
not
want
to
code
against
an
API
that
is

just
going
to
be
thrown
away
or
might
change
the
next

day.”
– Gareth Jones, Principle API Architect, Microsoft OneNote
Instead, to get developers involved with the testing and evaluation of your
prototype, you have to make the process as simple as possible. This means
providing your potential users with tools that let them interact with your API
without having to write code.
For some, this may be as easy as pointing them to tools like Google
Chrome’s Advanced REST API Client (http://bit.ly/chromeRestAPIClient)
that lets developers make REST calls right from their browser.

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However, this tool still requires an in-depth introduction of your API to
developers, something that can be easily provided with MuleSoft’s API
Portal and API Console that generates interactive documentation from the
RAML spec but still requires developers to spend the time reading through
these docs and manually transition from one call to another.
Ideally, you will want to have comprehensive documentation on your mock
API for developers to access (again, something that can be easily
generated from the RAML spec), but you do not want this to be the primary
way your developers interact with and explore your API. Instead, the
documentation should act as a supplementary tool, so that as developers
want to do more complex calls or better understand your API, they can
quickly access the information. But to get to this point, you need a quick
on-ramp tool that will help teach developers what your API does and how
they can use it—in five minutes or less.

55
One of the most useful tools for prototyping, as well as “teaching” purposes,
is MuleSoft’s RAML-based API Notebook (http://www.apinotebook.com).
The API Notebook lets you create API experiences through markup that
developers can then edit and try out in real time without having to write any
code. But to do more advanced calls, manipulate data and even have it
interact with other data/APIs, the user just needs to know some basic
JavaScript.
The API Notebook also lets users explore multiple resources by guiding
them through all of the available options. It also lets them clone or create
their own Notebooks that they can then share with you, providing you with
valuable examples of real-use cases, bugs and inconsistencies that you
can try out for yourself.
This also keeps developers from having to write advanced code to try out
your API for their custom-use cases, and from having to share proprietary
code either during the prototype or production phases!

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Currently, the API Notebook is only available for RAML, and so far I have
been unable to find an equivalent tool for Swagger or API Blueprint.
Getting Feedback
Once you provide your potential API users with a prototype and the tools to
try it out, the next step is to provide a simple way for them to give you
feedback. Ideally, during this stage you’ll have a dedicated resource such
as an API engineer or a Project Manager that can interact with your testers
to not only get their feedback, but also have conversations to fully
understand what it is that they are trying to do, or what it is that they feel
isn’t as usable as it should be. Keep in mind you’ll also want to encourage
your testers to be as open, honest and blunt as possible, as they may try to
be supportive by ignoring issues or sugarcoating the design flaws that
bother them at first—a kind but costly mistake that will ultimately harm both
you and your potential users.
This step provides two valuable resources to the company. First, it provides
a clearer understanding of what it is you need to fix (sometimes the
problem isn’t what a person says, but rather what the person is trying to do),
while also telling your users that you listen to them, creating ownership of
your API.
Many companies talk about creating a strong developer community, but the
simplest way is to involve developers from day one. By listening to their
feedback (even if you disagree), you will earn their respect and loyalty—and
they will be more than happy to share your API with others, since they will
be just as proud of it as you are.
It’s also important to understand that people think and respond differently.
For this reason you’ll want to create test cases that help your testers
understand what it is you are asking of them. However, you should not
make them so restrictive or “by the book” that testers cannot veer off
course and try out “weird” things (as real users of your API will do). This can

57
be as simple as providing a few API Notebooks that walk developers
through different tasks and then turning them loose on those notebooks to
create their own scenarios. Or it can be as complex as creating a written
checklist (as is typically used in user experience testing).
If you take the more formal route, it’s important to recognize that you will
have both concrete sequentials (“I need it in writing, step by step”) and
abstract randoms (“I want to do this. Oh, and that.” “Hey look—a squirrel!”),
and you’ll want to empower them to utilize their unique personalities and
learning/working styles to provide you with a wide scope of feedback.
Your concrete sequential developers will already do things step by step, but
your abstract randoms are more likely not to go by the book—and that’s
okay. Instead of pushing them back onto the scripted testing process,
encourage them to try other things (by saying things like, “That’s a really
interesting use case; I wonder what would happen if…”) as again, in real life,
this is exactly what developers will do, and this will unlock issues that you
never dreamed of.
The purpose of the prototyping process isn’t to validate that your API is
ready for production, but to uncover flaws so that you can make your API
ready for production. Ideally, in this stage you want to find 99 percent of the
design flaws so that your API stands as a solid foundation for future
development while also remaining developer-friendly. For that reason it’s
important not to just test what you’ve already tested in-house, but to let
developers test every aspect of your API. The more transparent your API is,
and the more feedback you get from your potential API users, the more
likely you are to succeed in this process.
Remember, there’s nothing wrong with finding problems. At this point, that
is the point. Finding issues now lets you circle back to the design phase
and fix them before hitting production. Take all of the developers’ feedback

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to heart—even if you disagree—and watch out for weird things or common
themes.
You’ll know your API is ready for the real world when you send out the
prototype and, after being reviewed by a large group of potential API users
(a minimum of 10; 20–50 is ideal), you get back only positive feedback.

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6
Authorization &
Authentication
Another important aspect of APIs for SaaS providers is authentication, or
enabling users to access their accounts via the API. For example, when you
visit a site and it says, “Log in with Facebook,” Facebook is actually
providing an API endpoint to that site to enable them to verify your identity.
Early on, APIs did this through the use of basic authorization, or asking the
user for their username and password, which was then forwarded to the
API by the software consuming it. This, however, creates a huge security
risk for multiple reasons. The first is that it gives the developers of the
software utilizing your API access to your users’ private information and
accounts. Even if the developers themselves are trustworthy, if their
software is breached or hacked, usernames and passwords would become
exposed, letting the hacker maliciously use and access your users’
information and accounts.

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To help deal with this issue, Open Authentication—or OAuth—a token-
based authorization format was introduced. Unlike basic authorization,
OAuth prevents the API client from accessing the users’ information.
Instead it relays the user to a page on your server where they can enter
their credentials, and then returns the API client an access token for that
user.
The huge benefit here is that the token may be deleted at any time in the
event of misuse, a security breach, or even if the user decides they no
longer want that service to have access to their account. Access tokens
can also be used to restrict permissions, letting the user decide what the
application should be able to do with their information/account.
Once again, Facebook is a great example. When you log in to Facebook, a
popup comes up telling you that the application wants to access your
account and asking you to log in with your Facebook credentials. Once this
is done it tells you exactly which permissions the application is requesting,
and then lets you decide how it should respond.
This example shows you the Facebook OAuth screen for a user who is
already logged in (otherwise it would be asking me to log in), and an
application requesting access to the user’s public profile, Friends list and
email address:

61
Notice that this is a page on Facebook’s server, not on Digg. This means
that all the information transmitted will be sent to Facebook, and Facebook
will return an identifying token back to Digg. In the event I was prompted to
enter a username/password, that information would also be sent to
Facebook to generate the appropriate token, keeping my information secure.
Now you may not need as complex as a login as Facebook or Twitter, but
the principles are the same. You want to make sure that your API keeps
your users’ data (usernames and passwords) safe and secure, which means
creating a layer of separation between their information and the client. You
should never request login credentials through public APIs, as doing so
makes the user’s information vulnerable.

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Generating Tokens
It’s also extremely important to ensure that each token is unique, based
both on the user and the application that it is associated with. Even when
role-based permissions are not required for the application, you still do not
want a generic access token for that user, since you want to give the user
the ability to have control over which applications have access to their
account. This also provides an accountability layer that allows you to use
the access tokens to monitor what an application is doing and watch out
for malicious behaviors in the event that they are hacked.
It’s also smart to add an expiration date to the token, although for most
applications this expiration date should be a number of days, not minutes.
In the case of sensitive data (credit cards, online banking, etc) it makes
more sense to have a very short time window during which the token can
be used,, but for other applications, doing so only inconveniences the user
by requiring them to login again and again. Most access tokens last
between 30 and 90 days, but you should decide the timeframe that works
for you.
By having the tokens automatically expire, you are adding another layer of
security in the event that the user forgets to manually remove the token and
are also helping to limit the number of applications that have access to your
users’ data. In the event that the user wants that application to be able to
access their account, they would simply reauthorize the app by logging in
through the OAuth panel again.
Types of OAuth
Unfortunately, implementing OAuth may not be as simple as one would
hope, as there are two very distinct versions—OAuth 1 and OAuth 2—with
OAuth 2 having several different possible implementations. Many
companies also choose to deviate from standards and create their own
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63
In general, OAuth 2 is considered to be less secure than OAuth 1, as it
relies on SSL instead of signed certificates. But what it may lack in security,
it makes up for in flexibility and usability. Because OAuth 2 does not require
custom-signed certificates for communication, but instead relies on SSL, it
is easier for third-party developers to implement. OAuth 2 also reduces the
number of tokens needed from two down to one—a single token that is
designed to be short-lived and easily removed either through timed
expiration or by the user at any point. Lastly, OAuth 2 offers a unique layer
of flexibility by letting the user choose which actions the calling application
has access to, which limits what third-party companies can do with their
data and account.
When choosing a type of OAuth, it’s important to understand the key
differences between the two, and how choosing one or the other might
impact usability and security. Regardless of whether you choose OAuth 1
or OAuth 2, you’re still choosing a much better route than giving calling
applications access to your user’s credentials.
In the rest of this chapter we’ll be focusing on OAuth2, as this offers the
most flexibility and may be the best choice for most APIs. But this is a
question that is best answered by security experts who can truly
understand what you are trying to accomplish with your API. It’s also
important to understand that both of these formats are still heavily relied on
today, with Twitter utilizing OAuth 1.0A and Facebook taking advantage of
a slightly customized version of OAuth 2.

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Twitter expands on OAuth 1 by restricting access to protected resources
(“This application will be able to..., but will not be able to….). This in itself is
not a feature of OAuth 1, but shows that even when using OAuth Version
1.0, an application can expand upon it to limit what a token can access. See
http://oauth.net/core/1.0/#anchor42 for more information on.
OAuth2
In a two-legged OAuth 2 process, the application consuming your API first
prompts the user to log in using your service. This is usually done through
the use of a “log in with” button, as shown on the Klout website:
Within this button or link is crucial information to complete the hand shake,
including what the application is requesting (a token), the URI to which your

65
application should respond (such as http://theirsite.com/oauth.php), the
scope or permissions being requested and a client ID or unique identifier
that allows their application to associate your response with that user.
Now, when the user clicks the button (or link), they are then redirected to
your website (with all the above information being transmitted), where they
are able to log in and determine which permissions they want the
application to have access to (as shown in the OAuth screenshots for
Twitter and Facebook above).
Your application then generates an access token based on both the user
and the application requesting access. In other words, the access token is
tightly coupled to both the user and the application, and is unique for this
combination. However, the access token can be independent of access
permissions or scope, as you may choose to let the user dictate (or change)
these permissions from within your application. By having the scope remain
changeable or decoupled from the hash of the token, users are able to have
any changes they make regarding the scope from within your application
applied immediately without needing to delete or regenerate a new token.
The access token created should also have a set expiration (again, usually
days, but this should depend on your API’s needs). This is an additional

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security measure that helps protect a user’s information by requiring them
to occasionally reauthorize the application requesting access to act on their
behalf. (This is often as simple as clicking “reauthorize” or “login with….”)
Once the access token has been generated, your application then responds
back to the URI provided by the application to provide the unique identifier
or client ID and access token that the application may utilize to perform
actions or request information on their behalf.

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Because this information is not being handled through signed certificates, it
is important that the information being transmitted is handled over SSL.
However, to be truly secure, BOTH parties must implement SSL. This is
something to be aware of as many API users may not realize this and
instead create insecure callback URLs such as
“http://theirdomain.com/oauth.php” instead of “https://theirdomain.com/oauth.php.”
For this reason, you may want to build in validation to ensure that you are
passing data back to a secured URL in order to prevent interception of the
access token by malicious third parties.
As an added security measure, you can also restrict the access token to the
domain of the calling application.
Once the application receives the access token and client ID or identifier, it
can then store this information in its system, and the handshake is
complete until the access token either expires or is deleted by the user. At
that time, should the user choose to reauthorize the application, the
handshake starts back at the beginning.
In a three-legged OAuth process, the flow is the same, with the exception
of having one more party involved (such as an OAuth service provider) who
would then act as the middle leg and provide your application with the
information.

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OAuth and Security
When implementing OAuth it’s important to understand that it is the only
thing preventing free access to your users’ accounts by the application—
and any malicious users who try to hijack or abuse it.
This means that you need to take a security-first approach when building
out your OAuth interface, and that before building anything on your own it is
important to first understand your own security needs, and secondly
understand the different security aspects (and vulnerabilities) of OAuth.
Brute Force and Man-in-the-Middle Attacks
Attackers may attempt to use brute force attacks against your OAuth
solution or utilize a man-in-the middle attack (pretending to be your server
and sneaking into the calling application’s system that way).
Improper Storage of Tokens
It’s also important to remember that your users’ information is only as
secure as their access tokens. I’ve already mentioned being sure to make
all calls over SSL, but you should also work with your API users to ensure
they are properly and securely storing access tokens.
Many years ago while working on a shopping cart system for a client, I
came across a file freely available in the root called “config.xml.” This
config file contained their database username and password, where the
user was not restricted by IP or host. In other words, anyone who found
that file had access to everything in their database! When I talked to the
client about it, they shrugged it off, saying, “Well who would ever look for
that?”
Unfortunately, that is not the only frightening security tale I have. I’ve also
seen access tokens being passed over JavaScript—freely visible in HTML
code—and on non-secure pages.

69
The point is that just because some things are common sense to you, you
should not assume the same of your API users—especially when it involves
your users’ data. Be sure to educate them and guide them along the way to
help ensure these vital access tokens remain secure and out of the public
eye.
Session Fixation/Hijacking
If you apply the principles of REST to your OAuth solution and keep it
stateless, you should be able to avoid many of the security risks associated
with session hijacking and fixation. However, this becomes a real threat
under two circumstances—first when the issue is on the API user’s side,
and the second when the OAuth screen is being accessed via a public or
shared computer.
Unfortunately there is not much you as the provider can do in the first place
other than watch for multiple access token requests for the same user. (If
you start seeing several requests per second for the same user or client
ID/call identifier, chances are something isn’t right —it could be a loop in
their system or an indication that they have a session/client ID issue).
In the second case, many applications such as Facebook automatically
jump to the permissions screen instead of forcing the user to log in—if they
are already logged into their platform. While this is incredibly convenient for
the user, it may lead to unintended or unauthorized access of their
accounts by third parties if they forget to log out. This is something you
should consider when determining how to handle OAuth sessions on your
side.
Security Isn’t Easy
As you can see, security in regards to OAuth isn’t just about what you build
or the code you write. It extends into how information is stored and

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communicated, since the ability to access user information and accounts is
priceless to hackers with malicious intent. This makes your API—especially
as your application grows—a prime source for such attacks.
One of the biggest mistakes developers make when dealing with security is
trying to secure systems without truly understanding the different security
measures necessary. Perhaps the best talk I have heard on encryption, by
Anthony Ferrara, ended with this piece of advice: “Don’t. Hire a
professional.”
The same goes with implementing OAuth. Unless you are comfortable with
signed certificates, RSA tokens, HMAC, consumer/token secrets, SHA1,
etc.—and I mean REALLY comfortable—you may want to either look at
bringing in someone with experience to help you build this out instead, or
take advantage of a common OAuth library or an OAuth service provider
such as OAuth.io or StormPath.
Adding OAuth to RAML
The good news is that once you have an OAuth service, adding it to your
API’s definition in RAML, and making it accessible through the different
tools available, is extremely easy.
For OAuth 1, you would simply need to state that it is securedBy oauth_1_0
and provide a requestTokenUri, an authorizationUri and the
tokenCredentialsUri as shown below in the Twitter RAML example:
securitySchemes:

-‐
oauth_1_0:

type:
OAuth
1.0

settings:

requestTokenUri:

https://api.twitter.com/oauth/request_token

authorizationUri:

https://api.twitter.com/oauth/authorize

71

tokenCredentialsUri:

https://api.twitter.com/oauth/access_token

securedBy:
[
oauth_1_0
]

For OAuth 2, you would likewise state that it is securedBy oauth_2_0 and
provide the accessTokenUri and authorizationUri. Because OAuth 2 only
uses one token, we are able to combine the requestTokenUri and
tokenCredentialsUri URIs into the same request (accessTokenUri). However,
because OAuth 2 utilizes scope, we will need to add that in using the
scope’s property. We’ll also need to add the information on how to send
the access token via the Authorization header:
securitySchemes:

-‐
oauth_2_0:

type:
OAuth
2.0

describedBy:

headers:

Authorization:

description:
|

Used
to
send
valid
access
token

type:
string

settings:

authorizationUri:

https://api.instagram.com/oauth/authorize

accessTokenUri:

https://api.instagram.com/oauth/access_token

authorizationGrants:
[
code,
token
]

scopes:

-‐
basic

-‐
comments

-‐
relationships

-‐
likes

securedBy:
[
oauth_2_0
]

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You can learn more about using OAuth within RAML in the RAML spec
under “Security” at http://raml.org/spec.html#security. But thankfully the
process of implementing existing OAuth services into your RAML-based
applications is far more simple than actually creating them, and it makes it
easy for your developers to access real information when debugging or
exploring your API.

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7
Designing Your
Resources
Resources are the primary way the client interacts with your API, and as
such it’s important to carefully adhere to best practices when designing
them, not only for usability purposes, but to also ensure that your API is
long-lived.
In REST, resources represent either object types within your application or
primary gateways to areas of your application. For example, the /users
resource would be used to interact with and modify user data. In a CRM
application you may have a /users resource that would represent the users
of the application, and a /clients resource that would represent all of the
clients in the application. You might also have a /vendors resource to
manage suppliers, /employees to manage company employees, /tickets to
manage customer or sales tickets, etc. Each resource ties back into a
specific section of the CRM, providing a general gateway to reach and
interact with that specific resource.

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But what makes REST truly unique is that the resources are designed to be
decoupled from their actions, meaning that you can perform multiple
actions on a single resource. This means that you would be able to create,
edit and delete users all within the /users resource.
Decoupled Architecture
If we look at the constraints of REST, we have to remember that REST is
designed to be a layered system that provides a uniform interface. It must
also allow the server to evolve separately from the client and vice versa.
For this reason, resources are designed to be decoupled from your
architecture and tied not to specific methods or classes, but rather to
generalized application objects. This is a big change from SOAP, where the
calls are tied to the class methods, and RPC, where naming conventions
tend to be tightly coupled to the action you’re taking (getUsers).
By decoupling your resources from your architecture, you are ensuring that
you can change backend services or technologies without impacting how
the client interacts with your API while also providing flexibility in how the
client interacts with your API through the use of explicit methods or
representations.
Use Nouns
One of the best ways to ensure that your resources are decoupled is to
think of them as webpage URIs. For example, if sharing information about
your company, you would probably send the user to an “about” section on
your website. This might look something like “yourdomain.com/about” or
“yourdomain.com/company.”
In the same way, you can build out resources using that same navigational
principle. As mentioned above in the CRM example, users could be

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directed to the /users resource, clients to the /clients resource, vendors to
/vendors, etc.
Another way to be sure that you are enforcing this navigational style and
avoiding tight coupling of your resources to methods or actions is to utilize
nouns for the name of your resource. If you find yourself using verbs
instead, such as “/getUsers” or “/createVendor,” there’s a good chance
you’re creating a tightly coupled resource that is designed to only perform
one or two actions.
Resources should also take advantage of the plural form. For example,
/users represents the full scope of the user object, allowing interaction with
both a collection (multiple records) and an item (a single user). This means
that the only time you would want to take advantage of a singular noun is if
the only possible action that can be taken is specific to a single item or
entity. For example, if you were creating a shopping cart API, you may elect
to utilize the singular form “/cart” for the resource rather than “/carts.” But
again, in general, the plural form will offer you the most flexibility and
extendibility, as even when a feature is not built into your application, or
there are current restrictions (for example, only letting companies have one
location) that may change in the future. And the last thing you want is to
have both a plural and singular form of the resource. For example, imagine
users having to decide whether to use /location or /locations.
In other words, only use the singular format when there’s no possibility of
the resource having multiples—a scenario that is extremely rare. After all,
even in the /cart example, you may decide someday to give users multiple
shopping carts (saved carts, business carts, personal carts, etc.). So as you
build out your resources, you should be thinking not just about planning for
now, but planning for what could happen down the road as well.

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Content-types
Resources should also be able to support multiple content-types, or
representations of that resource. One of the most common mistakes
developers are making today is building their API for only one content-type.
For example, they will building out their REST API and only return JSON in
the response. This by itself is not necessarily a bad thing, except that they
are failing to inform their clients that other media types are not allowed (see
status codes and errors), and even more importantly, they have not built out
their architecture to be flexible enough to allow for multiple content-types.
This is very shortsighted, as we forget that only a few years ago XML was
king. And now, just a short while later, it is repeatedly mocked by
“progressive developers,” and the world is demanding JSON (and for good
reason).
With the emergence of JSON, many enterprises were caught off guard,
stuck serving XML via SOAP APIs with no new way to meet their customers’
needs. It is only now that we are seeing many enterprises in a position to
provide RESTful APIs that serve JSON to their customers.
The last thing we want to do is put ourselves in this position again. And with
new specs emerging every day, it is just a matter of time. For example,
YAML (Yet Another Markup Language) is already gaining popularity, and
while it may not be the primary choice for most developers today, that
doesn’t mean some of your most influential clients won’t ask you for it.
By preparing for these types of scenarios, you also put yourself in a
position to meet all of your clients’ needs and provide an extremely flexible
and usable API. By letting developers decide which type of content-type
they are utilizing, you let them quickly and easily implement your API in their
current architecture with formats they are comfortable with. Surprisingly,
along with functionality and flexibility, this is something that many
developers are looking for.

77
Using the Content-type Header
Today, when you browse the Web your client (browser) sends a content-
type header to the server with each data-sending request, telling the server
which type of data it is receiving from the client.
This same principle can be applied to our HTTP based REST API. For
example, you can use the content-type header to let clients tell your API
what format of data they are sending you, such as: XML (text/xml,
application/xml), JSON (application/JSON), YAML (application/yaml) or any
other format that you support.
Once the server receives this content-type, it not only knows what data it
has received but, if it is a recognized format, how to process it as well. This
same principle can be applied to your API, letting you know which type of
data your client is sending and how to consume it. It also tells you which
data format they are working with.
To go a step further, you can also take a look at the Accept header to see
which type of data they are expecting in return. Hypothetically, when you
build out your architecture your client should be able to send you XML by
declaring it in the content-type and expect JSON in return by declaring a
desired JSON response in the Accept header.
This creates the most flexibility for your API and lets it act as a mediator
when used in conjunction with other APIs. However, it also provides a lot of
opportunity for confusion. Because your API is designed to have a uniform
interface, I would recommend not taking advantage of this wonderful
header, but rather relying on the content-type to determine which data
format they are working with, and then passing back that same format.

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Building Your Architecture
In order to best accommodate different content-types, you will want to
build out two specific areas of your API’s architecture—an incoming
content handler and an outbound view renderer.
Unless you have to create special schemas for the data, it’s recommended
to keep the data architecture the same regardless of which format you are
using. This lets you easily break down the incoming data using an incoming
content-type handler, formatting it quickly into a usable object that can
easily be interpreted by your API service layer and passed on to the
necessary services to generate the data necessary for a response.
The problem with having specialized architectures or schemas is that you
will need a custom class to handle the deserialization of your incoming data
and assign it to the appropriate class/property architecture so that it can be
processed by your underlying service architecture. While this can quickly
be done, unless it is carefully planned out, you may find yourself having to
build this interpretation layer for each resource and add new components
to it each time a new component is added.

79
By having a standardized architecture, or having your data organized in the
same manner across multiple formats (XML, JSON, YAML, etc.), you will be
able to reduce the amount of work needed to provide this layer of flexibility.
This is not always possible, however, and tools like MuleSoft’s Anypoint
Studio with Mule ESB may make the task easier through the use of their
DataMapper.
In the case of output, you’ll want to take advantage of a view renderer. This
lets you build your architecture in such a way that the response format is
separate from the data processing and generation. Again, if you have
specialized schemas, you will have to write special classes/objects to
handle them, but if you have a standardized response architecture, you
should be able to simply pass the response data to the view layer based on
the content-type used by the client.
When building your view renderer, you may want to create a separate view
layer for handling hypertext links, which will let you generate the
appropriately formatted content response and then pull in the hypermedia
link representations formatted in the requested content-type (HAL, JSON
API, CPHL, Siren, etc.).

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80
Many of today’s popular MVC frameworks already take advantage of view
renderers and partials (incomplete view aspects such as those that could
be used to generate the hypertext links), making this process extremely
simple.
But in the event that you have to build it out, remember by creating your
content-handler and content-response views in a layered format, you will
be able to quickly add new response types with little to no work on your
part. For some frameworks with a standardized data architecture, this is as
easy as adding a new view renderer for the content-type you want to
support, and then adding that content-type to the list of accepted types. Of
course, you still have to build the content-handler to deserialize the data,
but with the layered system, supporting a new content-type may only take
a matter of hours or days (not including testing) instead of taking months,
years or a major rewrite. This will allow you to quickly adapt and support
your clients’ needs as technology evolves.
XML
Defined by the W3C, XML or the Extensible Markup Language was
designed to present a format that was both machine- and human-readable.
Some of the more common formats of XML include RSS (commonly used
for feeds), Atom, XHTML and, of course, SOAP.

81
XML also encourages the use of strict schemas and was the choice format
for many enterprises, causing the move to JSON to be more challenging.
However, while descriptive, XML takes up more bandwidth than JSON, and
while commonly used, does not have the same broad language support.
While there are many libraries for interpreting XML, many of these are used
as add-ons rather than core libraries.
<books>

<book>

<title>This
is
the
Title</title>

<author>Imag
E.
Nary</author>

<description>

<![CDATA[Once
upon
a
time
there
was
a
great
book]]

</description>

<price>12.99</price>

</book>

<book>

<title>Another
Book</title>

<author>Imag
E.
Nary</author>

<description>

<![CDATA[This
is
the
sequel
to
my
other
book]]

</description>

<price>15.99</price>

</book>

</books>

JSON
JSON, or the JavaScript Object Notation, was designed as an alternative to
XML, consisting of key/value pairs in a human-readable format. Originally
created by Douglas Crockford for use within JavaScript, JSON is now a
language agnostic, being described in two separate RFCs, and has quickly

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become one of the most commonly used formats due to its broad language
support and ability to serialize objects.
JSON is represented by the application/json content-type and typically
takes advantage of the .json extension.
{[

{

"title"
:
"This
is
the
Title",

"author"
:
"Imag
E.
Nary
",

"description"
:
"Once
upon
a
time
there
was
a
great

book
",

"price"
:
"12.99",

},

{

"title"
:
"Another
Book",

"author"
:
"Imag
E.
Nary
",

"description"
:
"This
is
the
sequel
to
my
other

book",

"price"
:
"15.99",

},

]}

You can also define strict JSON through the use of JSON Schemas,
although these are not as commonly used.
YAML
YAML, or Yet Another Markup Language/YAML Ain’t Markup Language
(depending on if you go by the original meaning or the latter acronym), was
originally created by Clark Evans, Ingy döt Net and Oren Ben-Kiki with the
goal of being a simpler, more human-readable format.

83
To accomplish this goal, YAML utilizes whitespace to identify properties,
eliminating the need for opening/closing brackets in most instances.
However, support for YAML has been slow, with many languages lacking
core libraries for its serialization and deserialization. This may be due in part
to the complex rules YAML incorporates, providing users with useful
shortcuts in building their files out, but making the actual deserialization
process much more difficult.
While YAML hasn’t been widely adopted as a response format, it has
become the format of choice for API definition and modeling languages,
including both RAML and Swagger.
Books:

-‐

title:
This
is
the
Title

author:
Imag
E.
Nary

description:
|

Once
upon
a
time
there
was
a
great
book

price:
12.99

-‐

title:
Another
Book

author:
Imag
E.
Nary

description:
|

This
is
the
sequel
to
my
other
book

price:
15.99
Versioning
I cannot stress enough that when it comes to building an API, your goal
should be to create one that is so amazing that you can avoid versioning
altogether. However, as hard as we try, there is a good chance (at least with
today’s technology) that at some point in time we will find ourselves having
to version our API for one reason or another.

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84
There have been several suggested methods for versioning, but the first
thing to remember is that, fundamentally, versioning is a bad thing. It’s
important to understand that the lower the version number, the better. In
the desktop software world, we push for that next number, creating Version
1, 2 and—in some cases—skipping numbers altogether to make the
product sound more advanced than it is! But in the API world, the sign of a
success is not having to version.
This means you should consider avoiding minor versioning altogether, since
it serves no real purpose in an API. Any feature changes should be made
immediately and seamlessly available to developers for their consumption.
The only strong argument for minor versioning is in tightly coupled SDKs, or
saying this SDK supports the features of Version 1.1, whereas access to
recently added features would require an upgrade to Version 1.2.
You could also make the argument that minor versioning lets developers
quickly know there’s been a change to your API—an argument that makes
sense on the surface. Of course the counter argument is that you may have
developers who misunderstand minor versioning and instead rush to try
and upgrade their system to the new API without needing any of the new
features (or while they’re already taking advantage of them without realizing
it). This may result in unnecessary support calls and emails, as well as
confusion (“Can I do this in 1.1 or only in 1.2? And how do I access version
1.1 instead of 1.2?”).
The other counterpoint to this argument is that if you build a strong
developer community, developers will talk about new features (although not
everyone will be involved in the community), and if you utilize registration to
gain an API key (spoiler alert— you should) you can keep in touch with all of
your developers via email. (They may not be read, but then again, minor
versioning in the code might not be seen either.)
So with this in mind, let’s take a look at the three different mainstream
schools of thought regarding how to version your API.

85
In the URI
This method includes the version number in the base URI used for API calls,
making developers explicitly call the API version that they want. For
example, to access Version 1 of the API, one would use
api.domain.com/v1/resource to access the API, whereas for Version 2 they
would call api.domain.com/v2/resource. This means that when reading
documentation and implementing your API, developers will be forced to
look at the version number, since they may not notice it when briefly
looking over code samples unless it is predominately called out. This makes
this method preferable for APIs that are catering to newer developers.
One argument against the URI method is that it doesn’t allow the API to be
hypermedia driven, or that the content-type method makes this easier. This
is partially because REST is designed to be hypermedia driven and not
tightly coupled to URIs, which URI versioning does. Also, most hypermedia
specs rely on relative path text links that utilize the base URI, meaning that
unless there is an explicit change made by the developer, the client will
always call the same version of the API, staying within the current realm
and not being able to move back and forth between versions automagically.
However, even with the content-type, we currently have no good way to
know what the client supports, So when calling Version 2 from a client that
only supports certain Version 2 segments, we’re still likely to get back
Version 2 links in the hypertext response, causing the client application to
fail.
In other words, the problem cannot be avoided regardless of whether
you’re using the URI or the content-type to denote the version. Although as
applications become more advanced with machine learning and code-on-
demand, we may see this change, but I feel that this can be
accommodated regardless of which method you are using, just perhaps not
as cleanly as with the content-type versioning method.

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86
One advantage of URI versioning is that it tells developers which version
they are using, and is easier for newer developers to implement. It also
helps prevent confusion if developers forget to append the version on top
of the content-type version type (which if using this method should throw
an error to prevent ambiguity).
Of course, it’s also very easy for the base URI to become hidden
somewhere in an include, meaning that developers may not explicitly know
which version of the API they are using without having to dig into their own
code. Just the same, the other methods run this same risk depending on
how the client’s application is architected.
In the Content-type Header
This method is arguably cleaner and far less coupled than the URI method.
With this method, developers would append the version to the content-type,
for example:
Content-‐type:
application/json+v1

This lets developers quickly modify the version for the calls needed and
reinforces the use of representations to communicate with the server. It
also allows for a more dynamic and hypermedia-led API, as one could
implement the Accept header to return back a specific version. (For
example, if I make a call on a V2 feature, I may ask to only return V1 links in
the response, as other applications of my API may not be compatible).
Although doing this could create an architectural nightmare (What happens
if you do a create? Do you only return V2 responses for that item, and then
V1 for resources that have a shared relationship? What about compatibility
issues?).
This also raises questions regarding a uniform interface, as you are
transitioning the user between two incompatible versions of your API to
accomplish different things. On the other hand, this may help developers

87
transition from one version to another, as they can do it over time instead of
all at once. Just the same, I can’t say it is recommended, as I believe that
depending on business needs and implementation, it may cause far more
harm than good .
Another issue with the content-type is that developers have to know that
they need to call this out. This means that you have to not only have clear
documentation, but also validation regarding whether or not this
information is provided.
You must also have a central routing mechanism between your two APIs,
which presents a possible domain challenge. Since a key reason you are
versioning is that your current version no longer meets your needs, you are
probably not just rebuilding one section of the API, but rather its very
foundation. This may make taking advantage of the content-type method of
versioning far more complex than having multiple, but explicit, URIs.
Perhaps the biggest benefit of the content-type method is if you have two
different versions of your application (some customers are on V1, some on
V2) and you want to provide an API that can accommodate both. In that
case you’re not really versioning your API, but rather letting customers tell
you which version of your application they’re on so you can provide them
with the appropriate data structures and links. This is an area where the
content-type method absolutely excels.
Outside of this use case, content-type method falls prey to many of the
same problems as the URI method, in addition to creating more work and
opening you up to “out of the box” use cases (such as people trying to take
advantage of the Accept header in conjunction with the content-type
header to go between versions).

88
88
In a Custom Header
The custom header is very similar to the content-type header, with the
exception that those using this method do not believe that the version
belongs in the content-type header, and instead makes sense in a custom
header, such as one called “Version.”
Version:
1

This helps prevent the whole “Accept” conundrum, but it also runs into the
same issues of the content-type header as well as forces developers to
veer into the documentation to understand what your custom header is,
eliminating any chance of standardization (unless everyone decides on a
standard custom header, such as “version”).
This also opens up confusion, as developers may ask how to send a
version number through a header and get multiple answers ranging from
other API’s custom headers to using the content-type header.
For that reason, I cannot recommend using the custom header. And while I
personally agree that the content-type header may not be the best place
either, I think using a pre-existing, standard header is better than creating
an offshoot—at least until a new header is established and standardized for
this purpose.
Caching
One of the primary constraints of REST, caching is critical to scaling your
API. As you build out your API’s application and services layer, you will of
course want to take advantage of caching mechanisms within your own
code. But while this will help reduce the data load and memory usage,
letting developers cache their calls by providing them with the necessary
information eliminates unnecessary calls. This of course lets your API do
more and helps protect you against memory leaks (hopefully letting you find

89
them before they crash your server), while also allowing you to scale
without increasing cost.
However, before developers can cache their calls, they need to know that
the call is cacheable and how often they should renew their cache when the
data for that call expires. Surprisingly, this is one of the most forgotten
aspects of REST style APIs.
You can share this information with developers through the headers you
send, including the Cache-Control and Expires header. In the Cache-
Control header you can declare whether or not the cache should be public
or private (used with CDNs) or whether or not the data should not be
cached (no-cache) or stored (no-store). You can also add a max-age (in
seconds) for which that the data should be stored and a number that is
designed to override the Expires header (but this is reliant on client
implementation).
Because you can use either the Expires or the max-age, your cache
headers might look like:
Cache-‐Control:
public,
max-‐age=3600

With this example we are declaring that the cache is public (meaning it can
be cached by both a CDN or the client), and that the cache should expire in
1 hour (or 3600 seconds). Using the Expires header instead, your headers
would look like this:
Cache-Control: public
Expires: Mon, 09 February 2015 17:00:00 GMT
In this example, we are still sending back data that is publicly cacheable
but expires explicitly on February 9, 2015 at 5 p.m. GMT. In the event we
did not want the developer caching the response (for when they need real

90
90
time data) we could instead send back the cache-control header with the
no-cache (do not cache the data) and no-store (do not store the data)
declarations.
Cache-‐Control:
no-‐cache,
no-‐store

However, telling developers that your API is cacheable in the headers alone
is not enough. You’ll also want to inform them in the documentation, letting
them know which resources are cacheable and encouraging them to do so.
If you provide an SDK or a code wrapper, you will also want to implement
caching into your code to help developers who are looking for a plug-and-
play solution.

91
8
Designing Your
Methods
Similar to having a class with methods for performing different actions on
an object, REST utilizes methods within the resources. For a web-based
REST API that will be accessed over the HTTP or HTTPS, protocol we can
take advantage of the predefined, standardized HTTP methods. These
methods represent specific actions that can then be tied to the CRUD
acronym.
Utilizing CRUD
CRUD stands for Create, Read, Update and Delete and is an acronym
commonly used when referring to database actions. Because databases
are data-driven, like the Web, we can apply these same principles to our
API and how our clients will interact with the methods.
This means that we will be utilizing specific methods when we want to
create new objects within the resource, specific methods for when we want

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92
to update objects, a specific method for reading data and a specific
resource for deleting objects.
However, before we can apply CRUD to our API, we must first understand
the difference between interacting with items verses collections, as well as
how each method affects each one. This is because multiple methods can
be used for both creating and updating, but each method should only be
used in specific cases.
Items versus Collections
Before we continue, we need to define the difference between an item and
a collection. An item is a single result set, or a single data object, such as a
specific user. A collection, on the other hand, is a dataset comprised of
multiple objects or multiple users. This differentiation is important because
while some methods are appropriate for collections, other methods are
appropriate for dealing with an item.
For example, when dealing with a collection you have to be very careful
when allowing updates or deletes, as an update on a collection will modify
every record within it, and likewise a delete will erase every single record.
This means that if a client accidentally made a delete call on a collection,
they would effectively (and accidentally) delete every single user from the
system.
For this reason, if you plan to let your users do mass edits/ deletes, it’s
always a good idea to require an additional token in the body to ensure that
they are doing exactly what they are intending. Remember, REST is
stateless, so you should not force them to make multiple calls, as you have
no way of carrying over state on the server side.
For single, pre-existing records, it makes perfect sense to let a user edit or
even delete the record. However it doesn’t make much sense to let them

93
create a new record from within a specific record. Instead, creation should
be reserved for use on the collection.
While this can be a little confusing at first, with proper documentation and
the use of the OPTIONS method, your API users will be able to quickly
identify which methods are available to them. As they work with your API,
this will eventually become second nature as long as you remain consistent
in their usage.
HTTP Methods
You’re probably already quite familiar with HTTP methods, or HTTP action
verbs. In fact, every time you browse the Web with your browser, you are
taking advantage of the different methods—GET when you are requesting a
website and POST when you are submitting a form.
Each HTTP method is designed to tell the server what type of action you
want to take, ranging from requesting data, to creating data, to modifying
data, to deleting data, to finding out which method options are available for
that given collection or item.
Of the six methods we’re going to look at, five can be mapped back to
CRUD. POST is traditionally used to create a new object within a collection,
GET is used to request data in a read format, PUT and PATCH are used
primarily for editing existing data and DELETE is used to delete an object.
However, there is some crossover among the different methods. For
example, while POST is predominately used to create objects, PUT can
also be used to create an object within a resource—if it doesn’t already
exist. Likewise, PUT and PATCH can both be used to edit existing data, but
with very different results. For that reason it’s important that you
understand what each method is intended to do, and which ones you
should use for what purpose. This is also something you’ll want to explain

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in your documentation, as many developers today struggle with
understanding how to use PUT to create, as well as the difference between
PUT and PATCH.
GET
The GET HTTP Method is designed explicitly for getting data back from a
resource. This is the most commonly used HTTP Method when making
calls to a webpage, as you are getting back the result set from the server
without manipulating it in any way.
In general, a GET response returns a status code 200 (or ok) unless an error
occurs, and relies on a querystring (domain.com/?page=1) to pass data to
the server.
The GET method should be used any time the client wants to retrieve
information from the server without manipulating that data first.
POST
One of the most versatile HTTP Methods, POST was designed to create or
manipulate data and is used commonly in Web forms. Unlike GET, POST
relies on body or form data being transmitted and not on the query string.
As such, you should not rely on the query string to transmit POST data, but
rather send your data through form or body data, such as in JSON.

95
While extremely versatile, and used across the Web to perform many
different functions due to its common acceptance across multiple servers,
because we need an explicit way to define what type of action should be
taken within a resource, it is best to only use the POST method for the
creation of an item within a collection or a result set (as with a multi-filtered
search).
When creating an object (the function for which POST should
predominately be used), you will want to return a 201 status code, or
Created, as well as the URI of the created object for the client to reference.
PUT
Less well known is the PUT Method, which is designed to update a
resource (although it can create the resource if it doesn’t exist).
Traditionally, PUT is used to explicitly edit an item, overwriting the object
with the incoming object. When using the PUT method, most developers
are not expecting an object to be created if it doesn’t exist, so taking
advantage of this clause within this method should be done with extreme
care to ensure that developers know exactly how your API uses it. It’s also
important that your usage of PUT remains consistent across all resources.
(If it creates an object on one resource, it should do the same on all the
others.)

96
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If you elect to utilize PUT to create an item that doesn’t exist (for example,
calling “/users/1” would create a user with the ID of 1), it is important to
return the Created status code, or 201. This tells your consumers that a
new object was created that may (or may not) have been intended.
It’s also important to understand that you cannot use PUT to create within
the resource itself. For example, trying a PUT on /users without explicitly
stating the user ID would be a violation of the standardized specification for
this spec.
For this reason I would highly recommend not creating an object with PUT,
but rather returning an error informing the client that the object does not
exist and letting them opt to create it using a POST if that was indeed their
intention. In this case, your request would simply return status code 200
(okay) if the data was successfully modified, or 304 if the data was the
same in the call as it was on the server.
It’s also important to explain to your users that PUT doesn’t just overwrite
the object data that they submit, but all of the object data. For example, if I
have a user with the following structure:
{"firstName"
:
"Mike",

"lastName"
:
"Stowe",

"city"
:
"San
Francisco",

"state"
:
"CA"}

And I submit the following request using a PUT:
{"city"
:
"Oakland"}

97
The object on the server would be updated as such, reflecting a complete
override:
{"firstName"
:
"",

"lastName"
:
"",

"city"
:
"Oakland",

"state"
:
""}

Of course, this is traditionally not what the user wants to do, but is the
effect of PUT when used in this case. What the client should do when
needing to patch a portion of the object is to make that same request using
PATCH.
PUT should never be used to do a “partial” update.
PATCH
Another lesser-known method, PATCH has created some confusion, as it
operates differently as an HTTP method than when used in bash/shell
commands.
In HTTP, PATCH is designed to update only the object properties that have
been provided in the call while leaving the other object properties intact.
Using the same example we did for PUT, with PATCH we would see the
following request:
{"city"
:
"Oakland"}

Which would return the following data result set from the server:

98
98

{"firstName"
:
"Mike",

"lastName"
:
"Stowe",

"city"
:
"Oakland",

"state"
:
"CA"}

Like PUT, a PATCH request would return either a 200 for a successful
update or a 304 if the data submitted matched the data already on record—
meaning nothing had changed.
DELETE
The DELETE Method is fairly straight forward, but also one of the most
dangerous methods out there. Like the PUT and PATCH methods,
accidental use of the DELETE method can wreck havoc across the server.
For this reason, like PUT and PATCH, use of DELETE on a collection (or the
main gateway of the resource: /users) should be disallowed or greatly
limited.
When making a DELETE request, the client is instructing the server to
permanently remove that item or collection.
When using a DELETE, you will most likely want to return one of three
status codes. In the event that the item (or collection) has been deleted and
you are returning a content or body response declaring such, you would

99
utilize status code 200. In the event that the item has been deleted and
there is nothing to be returned back in the body, you would use status code
204. A third status code, 202, may be used if the server has accepted the
request and queued the item for deletion but it has not yet been erased.
OPTIONS
Unlike the other HTTP Methods we’ve talked about, OPTIONS is not
mappable to CRUD, as it is not designed to interact with the data. Instead,
OPTIONS is designed to communicate to the client which of the methods
or HTTP verbs are available to them on a given item or collection.
Because you may choose not to make every method available on each call
they may make (for example not allowing DELETE on a collection, but
allowing it on an item), the OPTIONS method provides an easy way for the
client to query the server to obtain a quick list of the methods it is allowed
to use for that collection or item.
When responding to the OPTIONS method, you should return back either a
200 (if providing additional information in the body) or a 204 (if not providing
any data outside of the header fields) unless, ironically, you choose not to
implement the OPTIONS method, which would result in a 405 (Method Not
Allowed) error. However, given that the purpose of the OPTIONS method is

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to declare which methods are available for use, I would highly recommend
implementing it.
Just the same, this is perhaps one of the most underused methods of the
six. You can learn more about this—and the other HTTP methods—by
visiting http://bit.ly/HTTPMethods.

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9
Handling
Responses
Since APIs are designed to be consumed, it is important to make sure that
the client, or consumer, is able to quickly implement your API and
understand what is happening. Unfortunately, many APIs make
implementation extremely difficult, defeating their very purpose. As you
build out your API you want to ensure that you not only provide
informational documentation to help your developers integrate/ debug
connections, but also return back relevant data whenever a user makes a
call—especially a call that fails.
While having a well formatted, coherent body response is extremely
important (you want something that can easily be deserialized, iterated and
understood), you’ll also want to provide developers with quick references
as to what happened with the call, including the use of status codes. And in
the case of a failure, you will want to provide descriptive error messages
that tell the client not just what went wrong, but how to fix it.

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HTTP Status Codes
When implementing REST over HTTP, we are able to take advantage of the
HTTP Status Codes, a structure that most developers are familiar with, (For
example, most developers can tell you that 200 is “okay,” 404 is “page/
resource not found” and 500 is a server error.
Using the current HTTP status codes prevents us from having to create a
new system that developers must learn, and creates a standard of
responses across multiple APIs, letting developers easily integrate your API
with others while using the same checks.
It is important, however, that you stick to standardized or accepted status
codes, as you’ll find plenty of status codes in use across APIs that do not
really exist. This creates the opportunity for confusion, for example, as
Twitter uses its famous 420 status code (Enhance Your Calm) for too many
requests, while Java’s Spring Framework used to return 420 to refer to a
method failure (Now Deprecated).
In this use case Twitter, while opting for a humorous “Easter Egg” response,
could have instead opted for status code 429 (Too Many Requests), and it
may have been wiser for Spring Framework to return a 500 to represent a
generic server error.
As you can see, someone utilizing the Spring Framework at this time while
making a call to Twitter might be confused by what 420 really meant, and
whether the method was not allowed (405) or there was a server error (500)
instead of realizing they simply made too many calls. Imagine how much
time and energy that confusion could cause them in debugging and trying
to fix an application that is already working perfectly.
It’s also important to use status codes because the behavior of the server
may be different from the expected behavior of the client. For example, if a
client does a PUT on an item and the item doesn’t exist, per the RFC the
item/object can then be created on the server—but not all APIs adhere to

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this idea. As such, by returning a 201 (Created) instead of a 200 (OK), the
client knows that the item did not previously exist and can choose to delete
it (if it was accidentally created) or update their system with the data to
keep everything in sync. Likewise, a 304 response would inform them that
nothing was modified (maybe the data was identical), and a 400 would
inform them that it was a bad request that could not be handled by the
server (in the event where you elect not to create the item).
Some of the more common HTTP status codes you may run into, and
should consider using, include the following:
Status Code Definition
1XX
Informational
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Switching Protocols (if moving the client from HTTP to
another, less common, protocol)
2XX
Successful
200
Ok – The request has succeeded.
201
Created (e.g. a new resource or object within a
collection)
202
Accepted – Your request has been accepted but has not
yet been completed (e.g. delayed creates).
204
No Content – The request was successful, but there’s
nothing being returned in the body.
3XX
Redirection
301
Resource Moved Permanently
304
Not Modified (Nothing was modified by the request, e.g.
PUT/PATCH)

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307
Resource Moved Temporarily
4XX
Client Error
400
Bad Request – The request could not be understood or
performed by the server due to malformed syntax and
requires modifications before it can be performed.
401
Unauthorized – Authorization credentials are required, or
the user does not have access to the resource/method
they are attempting.
403
Forbidden – The request is understood but is being
refused.
404
Resource not found – The URI is not recognized by the
server.
405
Method not allowed – The attempted method is not
allowed on the collection/item being called upon.
408
Timed out – The client did not provide information within
the allotted timeframe.
409
Conflict – Unable to perform the action, usually on a
PUT due to incompatible data sets (think of a GIT
conflict that needs to be manually fixed before merging)
410
Gone – A resource or item that previously existed was
permanently deleted and is no longer available for
access. In the event it is unknown whether or not the
deletion is permanent, or if the item pre-existed, a 404
should be used instead.
413
Request entity too long – The content the server is being
asked to process is too large and must be broken down
or resubmitted in a shorter/more compact format.
414
Request URI too long – The URI provided is too long
and is not acceptable by the server. This is a fairly
common error when trying to lengthen calls on Google’s
MAP API

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Of course, there are more HTTP status codes you can take advantage of if
desired, one being the popular 418 (I’m a Teapot). However, this 1998 April
Fool’s joke—while now an official HTTP Status Code—should only be used
if the responding server is, in fact, a teapot. And while not quite as common
as the other status codes, as more and more teapots become connected to
the Internet, perhaps this is a code we’ll be seeing a lot more of!
Handling Errors
Unfortunately, no matter how hard you try and how carefully you document
your API, errors will always be a reality. Developers will skip over
documentation, misunderstand it or simply discover that calls which were
previously valid no longer work.
415
Unsupported Media Type – The requested media type
(content-type) is not supported. For example, if the user
tries to submit a request using XML but you only
support JSON.
429
Too many requests – The client has sent too many
requests to the server in the given timeframe.
5XX
Server Error
500
The server ran into an unexpected error and was not
able to perform the requested action (the generic
“something went wrong on our end” error)
501
Not Implemented – Used when the client sends a
request that the server is incapable of fulfilling due to
lack of functionality
503 Service Unavailable – The server is temporarily unable to
respond to the request (for example, due to
maintenance) and the client should try again shortly.

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Because APIs are designed to be consumed, it is vital that you provide your
clients with the tools, resources and information to help consume them.
This means investing in your error messaging, an aspect of APIs that is
often overlooked. In many cases companies have opted for generic error
messages, as they have failed to consider the long-term ramifications.
Generic error messages tell developers that “something went wrong,” but
fails to tell them exactly what went wrong and how to fix it. This means that
developers must spend hours debugging their code—and your API—in
hopes of finding the answer. Eventually they’ll either just give up (often
finding a competitor’s API instead) or contact support, requiring them to go
through everything to track down oftentimes veiled and abstruse issues.
In the end, it costs you far more to have generic error messages than it
does to provide descriptive error messages that alleviate developer
frustration (“Oh, I know exactly what happened”); reduce development,
debug, and integration time; and reduce support requirements. And by
having descriptive error messages, when support is needed, they will have
a good idea of what the issue is and where to look, saving you resources in
that department as well (and keeping your API support team happy).
Bad Error Messages Good Error Messages
Restricted Access Invalid API Key/Access Token. If you need an
API key, you can register for one at
http://developers.mydomain.com/register
Permission Denied Your API Key does not have access to this
resource/ or the method attempted on this
resource. Please contact our support team if
you need access to this resource/ method.

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An additional advantage of providing descriptive error messages is that
generic messages tend to be ambiguous and confusing. For example, what
is the difference between “restricted access” and “permission denied?”
Well, in the above cases, quite a bit. But someone unfamiliar with your API
or naming conventions may not realize that.
A descriptive error should include an identifier for support (something that
is short, such as a code that they can ask for and be able to look up quickly
within their system), a description of what went wrong (as specific as
possible), and a link to documentation where the developer can read more
on the error and how to fix it.
Descriptive Error Formats
Thankfully, despite their fairly rare usage, descriptive error messaging is
nothing new, and there are several different formats out there that already
incorporate the above information, providing an easy way to implement
descriptive errors in a standardized and recognized format. Three of the
most popular ones include JSON API, Google Errors, and error.vnd.
JSON API
JSON API was created to serve as a way of returning back JSON-based
response metadata, including hypertext links (which we’ll discuss in
Chapter 12), as well as handling error bodies.
Request Failed The request was missing required data such
as first name, last name email, etc. Additional
detail may be found in our documentation by
using the link provided below.

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Rather than returning back just a single error, JSON API, as well as the
other error specs we’ll look at, lets you return back multiple errors in a
single response, each containing a unique identifier, an error code to the
correlating HTTP status, a brief title, a more in-depth message describing
the error, and a link to retrieve more information.
{

"error":
{

"errors":
[{

"id":
"error1_firstName",

"code":
"XB500",

"status":
"400",

"title":
"User
first
name
cannot
be
empty",

"detail":
"The
first
name
field
is
required
to

have
a
value",

"href":
"http://docs.domain.ext/users/post"

},

{

"id":
"error2_lastName",

"code":
"XB501",

"status":
"400",

"title":
"User
last
name
cannot
be
empty",

"detail":
"The
last
name
field
is
required
to

have
a
value",

"href":

}]

}

}

It is important to note that with JSON API errors, no other body data should
be returned, meaning that the error should be the primary response and not
embedded or returned with other resource body content or JSON.
JSON API Errors allow for the following properties to be used:

109
Property Usage
error.errors
An array containing all of the errors that
occurred (For example, if the form failed
because of missing data, you could list
out which fields are missing here with
an error message for each of them.)
error.errors[].id
A unique identifier for the specific
instance of the error (Please note that
this is not the status code or an
application-specific identifier.)
error.errors[].href
A URL that the developer can use to
learn more about the issue (for example,
a link to documentation on the issue)
error.errors[].status
The HTTP status code related to this
specific error, if applicable
error.errors[].code
An application-specific code that
identifies the error for logging or
support purposes
error.errors[].title
A short, human-readable message that
briefly describes the error
error.errors[].detail
A more in-depth, human-readable
description of the error and how to
resolve it
Additional information regarding JSON API errors may be found at
http://jsonapi.org/format/#errors

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110
Google Errors
Google Errors is the error handling format Google created for its own APIs,
and is made up of two parts—a generalized error code and message, and
an array of what caused the generalized error message—letting you break
down exactly what went wrong and what the developer needs to fix.
Another advantage to Google Errors is that it is property-, not format-based,
meaning that it can be used across multiple formats including XML and
JSON.
Within the errors array, the Google error format lets you return back the
domain (area of the API) where the error occurred, the primary reason, a
human-readable message, the location (based on setting the locationType
property) and a URL for additional information.
For example, you can describe errors in JSON as shown below:
{

"error":
{

"code":
400,

"message":
"The
user
was
missing
required
fields",

"errors":
[{

"domain":
"global",

"reason":
"MissingParameter",

"message":
"User
first
name
cannot
be
empty",

"locationType":
"parameter",

"location":
"firstName",

"extendedHelp":


},

{

"domain":
"global",

"reason":
"MissingParameter",

"message":
"User
last
name
cannot
be
empty",

"locationType":
"parameter",

111
"location":
"lastName",

"extendedHelp":


}]

}

}

Or as described in an XML format:
<?xml
version="1.0"
encoding="UTF-‐8"
?>

<errors>

<code>400</code>

<message>The
user
was
missing
required
fields</message>

<error>

<domain>global</domain>

<reason>MissingParameter</reason>

<message>User
first
name
cannot
be
empty</message>

<location
type="parameter">firstName</location>

<extendedHelp>http://docs.domain.ext/users/post</ext
endedHelp>

</error>

<error>

<domain>global</domain>

<reason>MissingParameter</reason>

<message>User
last
name
cannot
be
empty</message>

<location
type="parameter">lastName</location>

<extendedHelp>http://docs.domain.ext/users/post</ext
endedHelp>

</error>

</errors>

112
112
Google Errors allows for the following properties to be used:
Property Usage
error.code
An integer representing the HTTP status error
code
error.message
A brief description of what the overall,
generalized error is. You will be able to explain
exactly what caused the overall error in the
errors array later.
error.errors
An array containing all of the errors that
occurred. For example, if the form failed
because of missing data, you could list out
which fields are missing here with an error
message for each of them.
error.errors[].domain
The name of the service, class or other identifier
for where the error occurred to better provide an
understanding of what the issue was
error.errors[].reason
A unique identifier for the error, or the specific
type of error. For example, “InvalidParameter.”
error.errors[].message
A description of the error in a human-readable
format.
error.errors[].location
The location of the error with interpretation
determined by the location type (In the case of a
parameter error, it might be the form field where
the error occurred, such as “firstName.”)
error.errors[].locationType
Determines how the client should interpret the
location property (For example, if the location is
specific to a form field/ query parameter, you
would use “parameter” to describe the
locationType.)
error.errors[].extendedHelp
A URL that the developer can use to learn more
about the issue (For example, a link to
documentation on the issue.)
error.errors[].sendReport
A URI to be used to report the issue if the
developer believes it was caused in error and/or
to share data regarding the error.

113
Additional information regarding Google Errors may be found at
http://bit.ly/1wUGinJ

114
114
vnd.error
Created by Ben Longden in 2012, vnd.error is a concise JSON error
description format that allows for embedding and nesting errors within a
response. It was specifically designed to be fully compatible with HAL-
based hypertext linking format (we’ll discuss HAL in the next chapter) and is
expressed by using the content-type: application/vnd.error+json.
Unlike JSON API errors, a vnd.error can be nested within a larger response.
However, in the case of an error, this is most likely not recommended, as
you would not want the client to continue the request before fixing any
client-based issues.
vnd,error is also far more concise in its response, returning back a message,
an error code identifier and a URL the developer may use to learn more
about the issue.
{

"total":
2,

"_embedded":
{

"errors":
[
{

"message":
“User
first
name
cannot
be
empty",

"logref":
“XB500”,

"_links":
{

"help":
{
"href":
}

}

},

{

"message":
“User
last
name
cannot
be
empty",

"logref":
“XB501”,

"_links":
{

"help":
{
"href":
}

}

}]

}

}

vnd.error allows for the following properties to be used:

115
Property Usage
errors[]
An array containing all of the errors
that occurred (For example, if the form
failed because of missing data, you
could list out which fields are missing
with an error message for each of
them.)
errors[].message
A description of the error in a human-
readable format.
errors[].logref
A unique identifier for the error or the
specific type of error (For example
“InvalidParameter” to be used for
logging purposes)
errors[].path
The resource for which the error is
relevant. This becomes useful when
using the “about” property in the links
section when an error can apply to
multiple resources.
errors[].links[].help
A URL that the developer can use to
learn more about the issue (For
example, a link to documentation on
the issue)
errors[].links[].describes
The correlating server side error that
the response error is describing, if
applicable (See RFC6892.)
errors[].links[].about
A link to the resource to which the
error applies (See RFC6903.)
Additional information regarding vnd.error may be found at
https://github.com/blongden/vnd.error

116
116
Usability is Key
Remember, one of the keys to your API’s success is usability. It’s easy to
return a “Permission Denied” error to a resource, but it is far more helpful if
you return “Invalid Access Token” with additional resources telling the
developer exactly what the problem is.
Making your API easy to consume with HTTP Status Codes and Descriptive
Error Messaging may be the difference between an award-winning
application on your API or an award-winning application on your
competitor’s.

117
10
Adding
Hypermedia
In his dissertation, Dr. Fielding described four sub-constraints of the
Uniform Interface, one being Hypermedia as the Engine of Application State.
Unfortunately, while a common occurrence in the computer industry,
hypermedia has presented a challenge in both building a hypermedia-
driven API and consuming it.
Before we can truly work to incorporate hypermedia into our API, we must
first understand what it is. The term, coined back in 1965 by Ted Nelson, is
simply an expansion on hypertext—something you may recognize as part
of today’s World Wide Web.
In fact, we use hypertext every single day, as it is a key component in the
Hypertext Markup Language (HTML). This means that chances are—
possibly without even realizing it—you have been not only using, but also
writing hypertext and hypermedia on a frequent basis.

118
118
To put it succinctly, hypertext is simply regular text with links to other
documents. By adding these links, we create a structured document that
users can access and browse, guiding them through the possible actions
and relevant resources.
For example, if you visit the Yahoo! home page, you’ll see several different
links for different sections, including the news. If you click on “News,” it
takes you to a whole new page, which loads up hundreds of more links
directing you to different news articles. In the most basic sense, this is
hypertext linking at work. The links guide you to where you want to go and
what your possible actions from within that area are.
In the same way, the easiest way to incorporate these links (or hypertext/
hypermedia) is to incorporate links within your API response. Or, as Keith
Casey explains it, “provide your users with a ‘choose their own adventure’
book.” It’s a way for them to say, “I want to take this action; now what are
the available actions based on that?”
Take for example, if in our API we had created a new user, we might get
back a response that looks something like this:
{

"users":
[

{

"id":
1,

"firstName":
"Mike",

"lastName":
"Stowe",

"_links":
{

"update":
{

"description":
"edit
the
user",

"href":
"/api/users/1",

"methods":
["put","patch"]

},

119

"delete":
{

"description":
"delete
the
user",

"href":
"/api/users/1",

"methods":
["delete"]

},

"message":
{

"description":
"message
the
user",

"href":
"/api/message/users/1",

"methods":
["post"]

}

}

}

]

}
You’ll notice that within the user response we have a section called _links,
in this case taking advantage of CPHL, or the Cross Platform Hypertext
format (experimental). Each one of these links tells the client which
resources are available to them based on the user they just created. For
example, they can now edit the user, delete the user, message the user,
view the user’s timeline and post to the user’s timeline. Different hypertext
specs may look and handle this slightly differently, but ultimately they all tell
us which resources are available through links.
Along with directing us to the possible actions of an item, hypertext links
can also make navigating collections easier, especially in regards to
pagination.
{

"_links":
{

"beginning":
{

"description":
"first
page",

"href":
"/api/users?page=1",

"methods":
["get"]

120
120

},

"prev":
{

"description":
"previous
page",

"href":

"methods":
["get"]

},

"next":
{

"description":
"next
page",

"href":

"methods":
["get"]

},

"last":
{

"description":
"last
page",

"href":

"methods":
["get"]

}

}

}

Now, adding hypermedia/hypertext links to an API can be scary for several
reasons. First, all one has to do is search for something called HATEOAS,
or Hypermedia as the Engine of Application of State, to find complex
explanations and diagrams. On the flip side, you’ll also find arguments
about the lack of a “hypermedia client,” among others.
I think its important to point out right off the bat that hypermedia is neither a
cure-all nor a perfect solution. Part of the reason is that while it’s been
around since 1965, it’s still relatively new for RESTful APIs. As such, we’re
still working on trying to figure out the exact formula, and new ideas are
being tested every day.
However, that doesn’t mean that you shouldn’t understand how
hypermedia works in a RESTful API, or implement it in your own. While
there are plenty of challenges (which we’ll take a look at shortly), there are

121
far more benefits, and by using hypermedia within your API, you will
increase its longevity and long-term usability.
Hypermedia as the Engine of Application State
Perhaps the most daunting of terms is Hypermedia as the Engine of
Application State (HATEOAS), over which the debate is endless. In fact,
today there are still arguments over how to pronounce it. Is it “HATE-E-
OSS”? “Hat-E-OSS”? “Hat-E-AS”? The list goes on.
To make the concept more difficult, Fielding doesn’t explain what
HATEOAS is in his dissertation, simply saying that it’s a key component to
having a uniform interface. While this has started some debates (as there
always are in computational theories), the easiest way to describe
HATEOAS is this—for every object you have, you have possible actions that
can be applied to it. However, these actions may not be applicable to all
objects in a collection, as all objects are not necessarily the same.
In other words, if you have a collection of users, you may have users who
haven’t confirmed their email addresses yet, standard users, admins,
superusers, etc. Because each user is unique, the actions you can take on
each user might also be unique. For example, you might have a link to
confirm an unconfirmed email user’s email address, but not have that
action for other users. You may have admin links that you will provide to
your admin and superuser, but not to your standard users.
The real question is, how do you know which actions are available for which
user when those actions are dynamic? How do you know which actions a
user has access to if he or she is a superuser?
Now, you can have the client try to hardcode these rules into their system,
check to determine the user’s role, and then assume those actions are
available, or you can communicate this to the client through the API in real-

122
122
time. As complex as it sounds, this is what it means to use hypermedia as
the engine of application state.
Wait—I know what you’re thinking! REST is supposed to be stateless! And
it is; with REST, all state is handled by the client. That means the server
does not keep track of the calls the client is making, but rather receives one
call and returns the data based on the call it received. With HATEOAS, you
are not changing this. Instead, you are providing them with a representation
of the object’s or application’s state (in other words, which actions they can
take with the item or collection they have).

123
This prevents the client from having to guess and hardcode your
application’s rules into their own application, and instead lets them rely on
your rules and the data in real-time. Rather than having clients making calls
for actions they cannot do, by implementing hypermedia, you are telling
them the state of that item and what it is they can do.
As the state of that item changes, so does the response they receive back
from the server—again, represented by the hypertext links that are included
within the response.
Implementing Hypermedia
Of course, one of the main arguments against hypermedia is it requires
taking substantial amounts of time to think of every possible action a
person could take within an item or collection—something that is absolutely
true. In order to provide the most helpful and complete set of links, you will
need to know how your API interacts with itself—something you should
already have a good grasp on.
And if by chance you started building out your API as described in Chapter
2, you already have the linking relationships defined:

124
124
As simple as this chart may seem, it helps us identify the different
resources that might need to interact with each other and helps us
eliminate the need to go back and locate the different resources and/or
create complex diagrams or charts to define them. Instead, we can simply
look at the “Users” section, which we will assume became our “/users”
resource, and we now know which actions to which the client may need
access, or which links we should provide (based on our application’s rules,
of course).
Using the above chart, we can quickly determine that there are multiple
actions within the /users resource to which they may need access (and it’s
quite possible some of these are their own resources), but they would also
need access to the URI for sending a message, which we determined in
Chapter 2 belonged in the /messages resource.
Essentially, we’ve done the hard work already. Now we just need to add
these links into our response using a flexible architecture and, preferably,
one of the specs below.
Common Hypermedia Specs
Thankfully, to make Hypermedia implementation even easier, there are
linking specifications already in existence. Each specification is slightly
different, with some offering broader language support, some focusing on
actions over resources, and some incorporating documentation and form
data.
Unfortunately, none of these are perfect, and each specification has its own
strengths and weaknesses. While HAL and JSON API are the clear choice
for most hypermedia implementations today, others (such as PayPal) have
elected to create their own specification to meet their needs. For this
reason, it’s important to look at how you want developers interacting with
your API and which spec best meets their—and your—needs.

125
Collection+JSON http://bit.ly/17P84eR
Collection+JSON is a JSON-based read/write hypermedia-type designed
by Mike Amundsen back in 2011 to support the management and querying
of simple collections. It’s based on the Atom Publication and Syndication
specs, defining both in a single spec and supporting simple queries through
the use of templates. While originally widely used among APIs,
Collection+JSON has struggled to maintain its popularity against JSON API
and HAL.
{
"collection"
:

{

"version"
:
"1.0",

"href"
:
"http://example.org/friends/",

"links"
:
[

{"rel"
:
"feed",
"href"
:

"http://example.org/friends/rss"},

{"rel"
:
"queries",
"href"
:

"http://example.org/friends/?queries"},

{"rel"
:
"template",
"href"
:

"http://example.org/friends/?template"}

],

"items"
:
[

{

"href"
:
"http://example.org/friends/jdoe",

"data"
:
[

{"name"
:
"full-‐name",
"value"
:
"J.
Doe",

"prompt"
:
"Full
Name"},

{"name"
:
"email",
"value"
:
"jdoe@example.org",

"prompt"
:
"Email"}

],

"links"
:
[

126
126

{"rel"
:
"blog",
"href"
:

"http://examples.org/blogs/jdoe",
"prompt"
:
"Blog"},

{"rel"
:
"avatar",
"href"
:

"http://examples.org/images/jdoe",
"prompt"
:
"Avatar",

"render"
:
"image"}

]

}

]

}

}

Strong choice for collections, templated
queries, early wide adoption, recognized as a
standard
JSON only, lack of identifier for documentation,
more complex/ difficult to implement
JSON API http://jsonapi.org/
JSON API is a newer spec created in 2013 by Steve Klabnik and Yahuda
Klaz. It was designed to ensure separation between clients and servers (an
important aspect of REST) while also minimizing the number of requests
without compromising readability, flexibility or discovery. JSON API has
quickly become a favorite, receiving wide adoption, and is arguably one of
the leading specs for JSON-based RESTful APIs. JSON API currently bears
a warning that it is a work in progress, and while widely adopted, it is not
necessarily stable.
{

"links":
{

"posts.author":
{

"href":
"http://example.com/people/{posts.author}",

Weaknesses:
Strengths:

127

"type":
"people"

},

"posts.comments":
{

"href":

"http://example.com/comments/{posts.comments}",

"type":
"comments"

}

},

"posts":
[{

"id":
"1",

"title":
"Rails
is
Omakase",

"links":
{

"author":
"9",

"comments":
[
"5",
"12",
"17",
"20"
]

}

}]

}

Simple versatile format, easy to
read/implement, flat link grouping, URL
templating, wide adoption, strong community,
recognized as a hypermedia standard
JSON-only, lack of identifier for documentation, still
a work in progress
HAL http://bit.ly/1ELp7LP
HAL is an older spec, created in 2011 by Mike Kelly to be easily consumed
across multiple formats including XML and JSON. One of the key strengths
of HAL is that it is nestable, meaning that _links can be incorporated within
each item of a collection. HAL also incorporates CURIES, a feature that
makes it unique in that it allows for inclusion of documentation links in the
Weaknesses:
Strengths:

128
128
response, though they are tightly coupled to the link name. HAL is one of
the most supported and widely used hypermedia specs out there today,
and is supported by a strong and vocal community.
{

"_links":
{

"self":
{
"href":
"/orders"
},

"curies":
[{
"name":
"ea",
"href":

"http://example.com/docs/rels/{rel}",
"templated":
true

}],

"next":
{
"href":
"/orders?page=2"
},

"ea:find":
{

"href":
"/orders{?id}",

"templated":
true

},

"ea:admin":
[{

"href":
"/admins/2",

"title":
"Fred"

},
{

"href":
"/admins/5",

"title":
"Kate"

}]

},

"currentlyProcessing":
14,

"shippedToday":
20,

"_embedded":
{

"ea:order":
[{

"_links":
{

"self":
{
"href":
"/orders/123"
},

"ea:basket":
{
"href":
"/baskets/98712"
},

"ea:customer":
{
"href":
"/customers/7809"

}

},

"total":
30.00,

129

"currency":
"USD",

"status":
"shipped"

},
{

"_links":
{

"self":
{
"href":
"/orders/124"
},

"ea:basket":
{
"href":
"/baskets/97213"
},

"ea:customer":
{
"href":

"/customers/12369"
}

},

"total":
20.00,

"currency":
"USD",

"status":
"processing"

}]

}

}

Dynamic, nestable, easy to read/implement,
multi-format, URL templating, inclusion of
documentation, wide adoption, strong
community, recognized as a standard
hypermedia spec, RFC proposed
JSON/XML formats architecturally different,
CURIEs are tightly coupled
JSON-LD http://json-ld.org/
JSON-LD is a lightweight spec focused on machine-to-machine readable
data. Beyond just RESTful APIs, JSON-LD was also designed to be utilized
within non-structured or NoSQL databases such as MongoDB or CouchDB.
Developed by the W3C JSON-LD Community group, and formally
recommended by W3C as a JSON data linking spec in early 2014, the spec
has struggled to keep pace with JSON API and HAL. However, it has built a
Weaknesses:
Strengths:

130
130
strong community around it with a fairly active mailing list, weekly meetings
and an active IRC channel.
{

"@context":
"http://json-‐ld.org/contexts/person.jsonld",

"@id":
"http://dbpedia.org/resource/John_Lennon",

"name":
"John
Lennon",

"born":
"1940-‐10-‐09",

"spouse":
"http://dbpedia.org/resource/Cynthia_Lennon"

}

Strong format for data linking, can be used
across multiple data formats (Web API and
databases), strong community, large working
group, recognized by W3C as a standard
JSON-only, more complex to integrate/interpret, no
identifier for documentation
CPHL http://bit.ly/18iXlcC
The spec I created in 2014, CPHL, or the Cross Platform Hypertext
Language, is an experimental specification based on HAL that incorporates
methods, documentation, code on demand, and API definition
specifications (such as RAML or Swagger). Unlike the other specifications,
CPHL is designed to be action-first, not resource-first, by focusing on what
the link does verses what resource it is. CPHL is also designed to provide
an architecturally similar structure across different formats such as XML,
YAML and JSON, making it easier to incorporate in an application after
deserialization. Just the same, CPHL is listed as a brainstorming document,
is not stable, and does not have a strong community or broad adoption.
Weaknesses:
Strengths:

131
{

"_definition":
{

"raml":
"http://api.domain.com/docs/api/raml",

"swagger":

"http://api.domain.com/docs/api/swagger"

},

"_links":
{

"update":
{

"title":
"Edit
User",

"description":
"edit
the
user",

"href":
"/api/resource",

"methods":
[

"put",

"patch"

],

"formats":
{

"json":
{

"mimeType":
"application/json",

"schema":

"http://api.domain.com/docs/api/editSchema.json"

},

"xml":
{

"mimeType":
"text/xml",

"schema":

"http://api.domain.com/docs/api/editSchema.xml"

}

},

"docHref":
"http://api.domain.com/docs/edit",

"code":
{

"php":
{

"href":

"http://code.domain.com/phplib/edit.tgz",

"md5":

"0cc175b9c0f1b6a831c399e269772661",

132
132

"recordSpecific":
false

},

"java":
{

"href":

"http://code.domain.com/javalib/edit.tgz",

"md5":

"0cc175b9c0f1b6a831c399e269772661",

"recordSpecific":
false

},

"ruby":
{

"href":

"http://code.domain.com/rubylib/edit.tgz",

"md5":

"0cc175b9c0f1b6a831c399e269772661",

"recordSpecific":
false

}

}

}

}

}

Designed for cross-platform consistency; allows
loosely-coupled documentation; incorporates
API definitions, methods and code on demand;
and allows for multiple formats while also
providing a strict naming structure for common
actions
Poor adoption/not heavily tested, can become
bloated, work in progress, listed as a brainstorming
document
Weaknesses:
Strengths:

133
Siren http://bit.ly/1F72aAK
Created in 2012 by Kevin Swiber, Siren is a more descriptive spec made up
of classes, entities, actions and links. It was designed specifically for Web
API clients in order to communicate entity information, actions for executing
state transitions and client navigation/discoverability within the API. Siren
was also designed to allow for sub-entities or nesting, as well as multiple
formats including XML—although no example or documentation regarding
XML usage has been provided. Despite being well intentioned and versatile,
Siren has struggled to gain the same level of attention as JSON API and
HAL. Siren is still listed as a work in progress.
{

"class":
[
"order"
],

"properties":
{

"orderNumber":
42,

"itemCount":
3,

"status":
"pending"

},

"entities":
[

{

"class":
[
"items",
"collection"
],

"rel":
[
"http://x.io/rels/order-‐items"
],

"href":
"http://api.x.io/orders/42/items"

},

{

"class":
[
"info",
"customer"
],

"rel":
[
"http://x.io/rels/customer"
],

"properties":
{

"customerId":
"pj123",

"name":
"Peter
Joseph"

},

"links":
[

{
"rel":
[
"self"
],
"href":

"http://api.x.io/customers/pj123"
}

134
134

]

}

],

"actions":
[

{

"name":
"add-‐item",

"title":
"Add
Item",

"method":
"POST",

"href":
"http://api.x.io/orders/42/items",

"type":
"application/x-‐www-‐form-‐urlencoded",

"fields":
[

{
"name":
"orderNumber",
"type":
"hidden",

"value":
"42"
},

{
"name":
"productCode",
"type":
"text"
},

{
"name":
"quantity",
"type":
"number"
}

]

}

],

"links":
[

{
"rel":
[
"self"
],
"href":

"http://api.x.io/orders/42"
},

{
"rel":
[
"previous"
],
"href":

},

{
"rel":
[
"next"
],
"href":

}

]

}

135
Provides a more verbose spec, query templating
and form fields; incorporates actions, multi-
format
Poor adoption, lacks documentation, work in
progress
Other Specs
Because there is no standardized way of doing hypermedia, and because
we still face challenges today in implementing hypermedia into our APIs
and getting it to work exactly how we would like, new specs are being
created every day. A quick look at the specs we just covered will show that
of the six, half were created in the last two years.
Other newborn specs include Mike Amundsen’s UBER, Jorn Wildt’s Mason
and Niels Krijger’s Yahapi [Yet Another Hypermedia(ish) API Specification].
Each tries to challenge the way we look at hypermedia APIs and each, like
CPHL, tries to find a better way of doing things.
However, while it’s important to innovate when building your API, it’s also
important to adhere to the tools and standards that developers know, and
the specifications that have been carefully tried and tested. While I would
personally urge you to try specs such as CPHL, Siren, UBER, Yahapi and
others in your own personal projects, I have to recommend utilizing HAL or
JSON API for your production API unless you have a strong reason to
choose a different specification.
Unfortunately, no hypermedia specification is perfect, as we will talk about
in the next section.
Weaknesses:
Strengths:

136
136
Hypermedia Challenges
While hypermedia dominates the Internet through formats such as HTML,
there are distinct advantages that are lacking in the API space, making the
implementation and usage of hypermedia more challenging.
For example, unlike HTML, we have already listed several different
mainstream specs that you can use to describe your links—with more being
created every single day. That means that as of right now, there is no
universal standard for how to format your links. In fact, many companies
are even creating their own specs, forcing developers to learn and apply
different formats to every API they utilize.
The other challenge is intelligence, as HTML—when interpreted by the
browser—presents the links to humans for them to determine the next
action they want to take. This allows for rather creative linking formats, as
links can be placed wherever, and with whatever text the designer wants.
API hypertext links, however, must be interpreted by machines, making it
far more difficult to add creative linking throughout the response.
These two challenges alone greatly hinder the advancement and usage of
hypermedia in APIs today. But again, even with these limitations, when
used as the engine of application state, hypermedia presents a level of
state that would not otherwise be available while also providing a layer of
flexibility to your API. This benefit should not be discarded or overlooked
because of these other challenges.
Client Resource URI Dependence
Another challenge with purely hypermedia-driven APIs as described by
Fielding is that there is a single point of entry to determine which resources
are available. While in certain cases this works extremely well (for example,
with StormPath’s API), but for more complex APIs with multiple pathways,
this requires the client to make multiple calls to the API just to get to a
starting point.

137
Many developers choose to hardcode URIs, either because they don’t fully
understand the benefits of a Hypermedia API, or they don’t know that the
API is not truly fluid (or that the resource they are calling shouldn’t change).
This is also one of the key arguments against the use of HATEOAS—that it
may break resource URIs.
The benefit to this, however, is that it forces our API to act just like a
browser, and when a URI is changed, we simply get the updated link and
click on it. For example, going back to Yahoo!—when you visit, there are
key URIs that you utilize, such as the base domain to access the site, and
then all available links are presented to you. This means that you are able to
get the latest information without having to remember the links, and that if a
link changes, it does not negatively affect you unless you saved the link in
your browser (i.e. hardcoded it).
This means that as your API continues to evolve, and as resources are
added, changed, and removed, that in a purely hypermedia-driven API,
your developers and clients would not be adversely affected (assuming the
client is smart enough to handle the different paths/actions/responses).
Unfortunately, because hypermedia in the sense of APIs is so new,
pioneering into the world of true hypermedia APIs presents substantial
challenges and could negatively impact usage. As such, it is important to
understand that regardless of whether or not you build a truly hypermedia
driven API, unless you force your users to start at the base URI, they will
most likely hardcode the resources that they want to use in their application.
This means that regardless of the format in which you build your API, you
should be wary about changing resource paths and understand that doing
so—even in a hypermedia driven API—may have adverse and unintended
effects.
The good news is that, again, this doesn’t detract from the state benefit
that HATEOAS brings with it, and as technology and usage of hypermedia

138
138
driven APIs increases, so will developers’ understanding of its benefits and
implementation. Just like REST was confusing for many developers at first,
we should expect to see a learning curve as more and more developers
consume hypermedia-driven APIs. This means that should you choose to
build a hypermedia-driven API, as REST dictates, you should be prepared
to educate your users through documentation, tutorials and examples.
One of the best ways to do this is through the use of the API Notebook,
which lets you walk your users through exactly how the API works in any
given scenario—something we will cover in Chapter 12.

139
11
Managing Your API
with a Proxy
Once you have designed and built your API, one of the most crucial
aspects is protecting your system’s architecture and your users’ data, and
scaling your API in order to provide downtime and meet your clients
demands.
The easiest and safest way to do this is by implementing an API manager,
such as MuleSoft’s API Gateway. The API manager can then handle API
access (authentication/provisioning), throttling and rate limiting, setting up
and handling SLA tiers and—of course—security.
A hosted API manager also provides an additional layer of separation, as it
can stop DDoS attacks, malicious calls and over-the-limit users from ever
reaching your system’s architecture, while also scaling with demand—
meaning the bad requests are killed at a superficial layer, while valid
requests are passed through to your server.

140
140
Of course, you can build your own API manager and host it on a cloud
service such as AWS, but you have to take into consideration both the
magnitude and the importance of what you’re building. Because an API
manager is designed to provide both scalability and security, you’ll need to
make sure you have system architects who excel in both, as one mistake
can cost hundreds of thousands—if not millions—of dollars.
And like any large-scale system architecture, trying to design your own API
manager will most likely prove costly—usually several times the cost of
using a third-party API manager when all is said and done.
For this reason, I highly recommend choosing an established API
management company such as MuleSoft to protect and scale your API, as
well as provide you with the expertise to help your API continue to grow
and live a long, healthy life.
API Access
Controlling access is crucial to the scale and security of your API. By
requiring users to create accounts and obtain API keys or keys for their
application, you retain control. The API key acts as an identifier, letting you
know who is accessing your API, for what, how many times, and even what
they are doing with that access.
By having an API key, you can monitor these behaviors to isolate malicious
users or potential partners who may need a special API tier. If you choose
to monetize your API, monitoring API access will also allow you to identify
clients who may want or need to upgrade to a higher level.
Typically, an API key is generated when a user signs up through your
developer portal and then adds an application that they will be integrating
with your API.

141
However, this isn’t always the case, as some companies create singular API
keys that can be used across multiple applications. However, by setting it
at the application level, you can see exactly the types of applications for
which the client is utilizing your API, as well as which applications are
making the most calls.

142
142
OAuth2 and More
A good API manager goes beyond just the basic management of API keys,
and will also help you implement security for restricting access to user
information, such as OAuth2, LDAP, and/ or PingFederate. This lets you
take advantage of systems you are already utilizing, or the flexibility of
using a third party service to handle OAuth if you choose not to build it
yourself (remember chapter 6):
Throttling
Throttling and rate limiting allow you to prevent abuse of your API, and
ensure that your API withstands large numbers of calls (including DoS and
accidental loops). With Throttling, you can set a delay on calls after an SLA
tier has been exceeded, slowing down the number of calls an API client is
making.

143
Rate limiting lets you set a hard number for how many calls the client may
make in a specific time frame. Essentially, if a client is making too many
calls, you can slow down the responses or cut the client off to prevent the
system from being overrun or disrupting your other users.

144
144
This is especially helpful in negating malicious attacks, as well as the
dreaded accidental infinite loop that pounds your API with calls. While this
practice may seem harsh at first, it is widely adopted to ensure the best
quality of service for everyone.
SLA Tiers
SLA tiers, or Service Level Agreements let you set up different rules for
different groups of users. For example, you may have your own mobile
apps, premium partners, paid API users, and standard/free users. You may
want to limit the access of each of these groups to ensure the highest
quality engagement for your users, while also helping prevent loops by
inexperienced developers testing out your API. For example, you can
ensure premium partners and your mobile apps have priority access to the
API with the ability to make thousands of calls per second, while the
standard API user may only need four calls per second. This ensures that
the applications needing the most access can quickly obtain it without any
downtime, while your standard users can also rely on your API without
having to worry about someone abusing the system, whether accidentally
or on purpose.

145
An API manager that incorporates SLA tiers should let you create the
different levels and specify the number of requests per second users in this
tier are allotted. You should also be able to determine whether or not users
of the tier require your approval. For example, you may offer automatic
approval to premium partners, but require special agreements and manual
intervention in order for basic or free users to take advantage of the higher
throughput.
Once you have set up your SLA tiers, you should be able to assign
applications to that tier.

146
146
Analytics
Another valuable tool your API manager should provide you is analytics,
letting you quickly see which of your APIs are the most popular and where
your calls are coming from. These analytics can help you identify which
types of devices (OS) the clients are using, the top applications using your
API and the geographical location of those accessing your API.
Along with identifying your API’s usage trends, as well as being able to
monitor API uptime/spikes/response times (especially in regards to your
own server architecture), analytics also help you prove the business use

147
case for your API, as you are able to show both its popularity and how it is
being used.
These metrics are especially key when making business decisions,
reporting to business owners/stakeholders, and designing a Developer
Relations/Evangelism program (as often times API Evangelist’s
performances are based on the number of API keys created and the
number of calls to an API).
Security
Security is an essential element of any application, especially in regards to
APIs, where you have hundreds, to thousands, to hundreds of thousands of
applications making calls on a daily basis.
Every day, new threats and vulnerabilities are created, and every day,
companies find themselves racing against the clock to patch them.
Thankfully, while an API manager doesn’t eliminate all threats, it can help
protect you against some of the most common ones. And when used as a
proxy, it can prevent malicious attacks from hitting your architecture.
It’s important to understand that when it comes to security, you can pay a
little up front now, or a lot later. After all, according to Stormpath, in 2013
the average cost of a personal information breach was $5.5 million. When
Sony’s PlayStation network was hacked, exposing 77 million records, the
estimated cost to Sony was $171 million for insurance, customer support,
rebuilding user management and security systems.
This is in part why you should seriously consider using a pre-established,
tested API manager instead of trying to build your own, because not only
do you have the development costs, but also the risks that go along with it.
When it comes to security, if you don’t have the expertise in building these
types of systems, it’s always best to let those with expertise do it for you.

148
148
Cross-Origin Resource Sharing
Cross-Origin Resource Sharing, or CORS, allows resources (such as
JavaScript) to be used by a client outside of the host domain. By default,
most browsers have strict policies to prevent cross-domain HTTP requests
via JavaScript due to the security risks they can pose.
CORS lets you enable these cross-domain calls, while also letting you
specify host limitations (for example, only calls from x-host will be allowed;
all other hosts will be denied) using the Access-Control-Allow-Origin header.
The idea is that this will help reduce the security risks by only letting certain
hosts take advantage of this functionality.
With that said, simply restricting hosts does not ensure that your
application will be safe from JavaScript attacks, as clients can easily
manipulate the JavaScript code themselves using freely available browser-
based tools (such as Firebug or Inspector).
Being client-side, CORS can also present other security risks. For example,
if a developer chooses to control your API via client-based JavaScript
instead of using a server-side language, they may expose their API key,
access tokens and other secret information.
As such, it’s important to understand how users will be utilizing CORS to
access your API to ensure that secret or detrimental information is not
leaked.
Keep in mind that every API manager operates differently. Some API
managers may let you set up a list of allowed hosts, while others may issue
a blanket statement allowing every host to make cross-origin requests.

149
As such, the golden rule for CORS, unless you have a specific-use case, is
to leave it disabled until you have a valid, well-thought-out reason for
enabling it.
IP Whitelisting/Blacklisting
While IP whitelisting/blacklisting is not an acceptable method for securing
an API by itself (as IP addresses can be spoofed and hackers often times
use multiple addresses), it does help provide an added layer of security to
prevent or allow known IPs based on previous interactions. For example,
you can choose to whitelist your or your partner’s dedicated IPs, or
blacklist IPs that have a history of making malicious calls against your
server or API. You can also block IPs from users that have abused your API
to prevent their server from communicating with it (something they could
easily do if you have a free tier by simply creating a new account with a
different email).

150
150
As we look at IP whitelisting/blacklisting, it’s important to remember that
security functions in layers. For example, if you think of a prison, it’s
important to have a multitude of security clearances so that if by chance
someone bypasses the first layer, they will be stopped at the second or
third. In the same way, we need to layer our security processes for our API,
both on the API management/proxy side, and within our architecture.
IP whitelisting/blacklisting fits very nicely into this layered system and can
help kill some calls before running any additional checks on them. But
again, it’s important to remember that IP addresses may change and can
be spoofed, meaning you may not always be allowing “good” users, and
you may not always be stopping malicious traffic. However, if you are
selective in this process, the chances that you will disrupt well-intentioned
users from interacting with your API are pretty slim.
XML Threat Protection
With the growth and focus of SOA in enterprise architectures, hackers
worked to find a new way to exploit vulnerabilities, often times by injecting
malicious code into the data being passed through the system. In the case

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of XML services, users with malicious intent could build the XML data in
such a way as to exhaust server memory, hijack resources, brute force
passwords, perform data tampering, inject SQL, or even embed malicious
files or code.
In order to exhaust server memory, these attackers might create large and
recursive payloads, something that you can help prevent by limiting the
length of the XML object in your API manager, as well as how deep into the
levels the XML may go.
Along with memory attacks, malicious hackers may also try to push through
malicious XPath/XSLT or SQL injections in attempt to get the API layer to
pass along more details than desired to services or the database.
Malicious attacks may also include embedding system commands in the
XML Object by using CDATA or including malicious files within the XML
payload.
Of course, there are several more XML-based attacks that can be utilized to
wreak havoc on an API and the underlying architecture, which is why
having XML threat protection in place is key to ensuring the safety of your
API, your application and your users. Again, since security is built in layers,
while the API manager can help prevent some of these threats, monitor for
malicious code and limit the size of the XML payload or the depth it can
travel, you will still want to be meticulous in building your services
architecture to ensure that you are eliminating threats like SQL and code
injection on the off chance they are missed by your API gateway.
JSON Threat Protection
Similar to XML, JSON falls victim to several of the same malicious attacks.
Attackers can easily bloat the JSON and add recursive levels to tie up

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memory, as well as inject malicious code or SQL that they anticipate your
application will run.
As with XML threat protection, you will want to limit the size and depth of
the JSON payload, as well as constantly be on the watch for security risks
that might make it through the API gateway including SQL injections and
malicious code the user wants you to evaluate.
MuleSoft’s API Manager lets you set up custom policies to help prevent
XML and JSON Threat Protections:

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12
Documenting &
Sharing Your API
Since the goal of any API is developer implementation, it’s vital not to forget
one of your most important assets—one that you should be referencing
both in error messages and possibly even in your hypermedia responses:
documentation.
Documentation is one of the most important factors in determining an API’s
success, as strong, easy-to-understand documentation makes API
implementation a breeze, while confusing, out-of-sync, incomplete or
convoluted documentation makes for an unwelcome adventure—one that
usually leads to frustrated developers utilizing competitor’s solutions.
Unfortunately, documentation can be one of the greatest challenges, as up
until now we have been required to write the documentation as we go, try
to pull it from code comments or put it together after the API has been
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The challenge is that not only should your documentation be consistent in
its appearance, but also consistent with the functionality of your API and in
sync with the latest changes. Your documentation should also be easily
understood and written for developers (typically by an experienced
documentation team).
Until recently, solutions for documentation have included expensive third-
party systems, the use of the existing CMS (Content Management System),
or even dedicated CMS’s based on open source software such as
Drupal/WordPress.
The challenge is that while expensive API documentation-specific solutions
may provide consistency regarding the look and feel of your API (something
harder to maintain with a CMS), they still rely on the manual effort of the
developer (if derived from the code) or a documentation team to keep them
in sync.
However, with the expansion of open specs such as RAML—and the
communities surrounding them—documentation has become incredibly
easier. Instead of trying to parse code comments and have inline
descriptions written (usually) by developers, the documentation team is still
able to provide descriptive documentation in the spec, and all code
parameters/examples are already included, making the transition to
documentation a snap.
And with the explosion of API documentation software-as-a-service (SaaS)
companies that utilize and expand on these specs, creating an effective API
portal and documentation has never been easier or less expensive.
However, before we jump into the different documentation tools, it’s
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Writing Good Documentation
Good documentation should act as both a reference and an educator,
letting developers quickly obtain the information they are looking for at a
glance, while also reading through the documentation to glean an
understanding of how to integrate the resource/method they are looking at.
As such, good documentation should be clear and concise, but also visual,
providing the following:
• A clear explanation of what the method/resource does
• Call outs that share important information with developers, including
warnings and errors
• A sample call with the correlating media type body
• A list of parameters used on this resource/method, as well as their
types, special formatting, rules and whether or not they are required
• A sample response, including media type body
• Code examples for multiple languages including all necessary code
(e.g. Curl with PHP, as well as examples for Java, .Net, Ruby, etc.)
• SDK examples (if SDKs are provided) showing how to access the
resource/method utilizing the SDK for their language
• Interactive experiences to try/test API calls (API Console, API
Notebook)
• Frequently asked questions/scenarios with code examples
• Links to additional resources (other examples, blogs, etc.)
• A comments section where users can share/discuss code
• Other support resources (forums, contact forms, etc.)
One such service that does an excellent job regarding documentation as a
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Of course, the ReadMe.io example isn’t perfect, but their interface does let
you add additional items, and overall it gives you an example of a visual
interface that developers can access to quickly obtain information about
the type of call they are trying to make while also having access to code
examples and being alerted to potential issues.
Another good example of API documentation is Constant Contact’s setup,
which provides chronological steps for developers to aid them through the
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Constant Contact also sections their documentation, providing links to the
different sections at the top so that developers can quickly scroll to the
information they are looking for, as well as giving a strong overview and
description of the resource as a whole.

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They also provide a nice, clean interface for describing the query
parameters:
The available response codes:

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And the overall structure of the API response:
As well as an example response to inform their developers of what they
should expect to receive back, letting developers prepare their script to
handle the data and compare what they are getting back to what they
should be getting back:

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This format is extremely user-friendly and helps developers get up and
running with the Constant Contact API quickly. It also has the added benefit
of a “Test API” button located at the top of each section, letting developers
read about the resource/method and then quickly try it out in the interactive
console.
Last but not least, Constant Contact integrates comments into their API
documentation through the use of Disqus, a simple and free comment
management tool that only requires a snippet of code to be placed on your
site.
However, you’ll notice that the Constant Contact documentation, as well
written and thorough as it is (kudos to Richard Marcucella), lacks code
examples—something that was a constant pain point for getting developers
started with the resource while I was working as a Developer Evangelist at
Constant Contact.
For many developers, code examples provide the key to integrating with
your API, allowing them to quickly copy and paste it in without having to
spend the time to “learn your API.” While not necessarily ideal, this is a
reality of how a large portion of programming works today, and something
developers will be looking for on your site.
Twilio, a communications company built as an API provider, does a pretty
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eight different languages based on their SDK. This means that if you’ve
installed the Twilio SDK, integrating with their service takes only minutes, as
you can literally paste in the code example, only having to replace the
placeholders with your own values.
Twilio has also mastered the “five-minute demo” in regards to utilizing their
API. As you build out your documentation, this five-minute window should
be your goal. From reading your documentation, developers should be able
to create a working example (even if it’s super basic) in five minutes or less.
If it takes longer than five minutes, you may need to evaluate whether or not
your documentation is as clear and concise as it should be.
Twilio’s documentation with code examples:

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Documentation Tools
One of the key benefits to Spec-Driven Development is that instead of
having to rely on costly and complex vendors—or trying to build an ad-hoc
API documentation solution out of a CMS like WordPress or Drupal—specs
like RAML, Swagger and API Blueprint have strong open source
communities surrounding them that offer prebuilt documentation tools you
can use.
While each offers its own unique toolset, and each spec has its own
strengths and weaknesses (refer to Chapter 3), since we’ve been using
RAML so far, we’ll focus on the tools available from the RAML community.
The RAML community has already put together parsers for several different
languages including Java, Ruby, PHP and Node, as well as full scripts to
manage API documentation while providing interactive environments such
as the API Console and API Notebook. These tools help you provide
documentation as shown in the ReadMe.io, Constant Contact and Twilio
examples above with little to no work on your part (other than the
installation and, of course, defining your RAML).
The following tools are available at http://raml.org/projects and can be
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RAML to HTML
RAML to HTML is a Node.js script that generates documentation into a
singular HTML page from the RAML spec. This provides a quick overview
of your code, in a similar format as Swagger’s console, letting developers
view and try out your API.
The RAML to HTML script also has third-party plugins for both Gulp and
Grunt.
RAML to Markdown
In the event that you are using a CMS that takes advantage of markdown,
you can use this Node.js script to quickly transform your RAML spec in
markdown that can be copied and pasted in your markdown CMS of choice.

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RAML to Wiki
This Node.js script is designed to let you easily port your RAML to
Atlassian’s Confluence or Jira style markdown, letting you integrate
documentation into your external or internal wikis, as well as bug tracking
systems.
RAML 2 HTML for PHP
Based on RAML to HTML, RAML to HTML for PHP renders your RAML in
real time, creating a full-fledged set of documentation that developers can
navigate, with unique pages for each resource and method. HTML to RAML
for PHP is also based on a templated system, letting you customize the
look and feel of your development portal and even add/remove different
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While these tools may not encompass all of the items that you are looking
for in documentation (e.g. code examples, call outs, comments, etc.) they
do provide a good starting point. And in the case of RAML 2 HTML for PHP,
they provide a toolset that lets you easily integrate these features into your
documentation.
As mentioned, there are also a growing number of third-party SaaS API
documentation services such as ReadMe.io that let you pull in your spec to
build out your API documentation. The trick is you’ll want to be sure that
whatever solution you choose can pull in any updates to your spec without
overwriting the additional data you’ve provided (e.g. callouts) and that
everything you are documenting is in fact documented in your spec (to
prevent this from happening).
Along with providing textual documentation, RAML also provides two
powerful, interactive tools to help developers interact with, and better
understand, your API. The first is the API Console.
API Console
The API Console is available both as an open source project (on the RAML
website) and as a freely hosted service from MuleSoft
(anypoint.mulesoft.com/apiplatform). The API Console provides a unique
way of documenting, demonstrating and letting developers interact with
your API.
In the latest version, documentation plays an even heavier role, letting
developers quickly view the different endpoints available and then—when
drilling down into a resource—providing complete and detailed
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Once the user selects the method of the resource they are wanting to learn
more about, they are presented with a reference panel for that method,
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As with the Constant Contact API documentation, there is a “Try It” button
that pulls up an interactive console that lets the developer enter the
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The API Console provides you with a quick way to share your
documentation with developers, while also providing a powerful tool for
testing out API calls and debugging calls when something goes wrong in
their code.
But what the API Console doesn’t do is provide a way for you to set up
example use-case scenarios for your API, or let developers provide
feedback/their use cases without sharing proprietary code. This next tool,
the API Notebook provides an intuitive way to highlight your API’s abilities,
let developers explore your API and garner feedback.
API Notebook
Another freely available JavaScript based tool, the API Notebook lets you
create scenarios/examples of API usage for your developers while walking
them through what each step does.
For example, in the screenshot below, we will first create an API client by
grabbing the Twitter RAML URL, and then we will authenticate into Twitter
using OAuth (as described in the RAML file):

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To run this notebook, we would simply click “Play” on the step(s) we want
to run (for example, if we click “Play” for the API.authenticate step, it would
run all previous steps), or we can click the “Play All” button (not shown) to
run all the steps.
Once we have created an API client, we now have the freedom to explore
that client by simply adding a “.” to the API alias (in this case twitterRestApi).
This means that after reviewing your example, your developers can branch
off and create their own, exploring your API and the resources available to
them without having to read any documentation.
As each call is made, the response from the server is made available to the
developer, letting them view the headers as well as the body response.
Along with acting as a useful tool by letting developers try out scenarios
you have created, developers can also copy or create their own notebooks,
sending information back to you. In the case of support, this lets a
developer show you what they are trying to do with your API and what
errors they are getting without revealing proprietary code. This also means
your support team can see first-hand exactly what the issue is in a way that
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The API Notebook also lets you integrate any other API defined with RAML
into your calls, letting developers try your API in conjunction with other APIs
they may be using, as well as letting them manipulate and modify the
output in the notebook using JavaScript.
For example, going back to the sample Twitter API Notebook, we could
determine who our last follower was by adding in the following JavaScript
code:
The API Notebook provides a whole new way for developers to interact with,
test and learn about your API, while also making the job of support that
much easier. The ability to provide explorative scenarios for your
developers, while also letting developers create their own scenarios that
can be shared with you or others using markdown, make the API Notebook
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The API Notebook is currently only available for RAML, and again, is freely
available on the RAML website (raml.org/projects). The API Notebook is
also available as part of MuleSoft’s API Portal (a tool for building your own
API Portals – anypoint.mulesoft.com/apiplatform) or as a universally
accessible tool at apinotebook.com.
Support Communities
Along with providing documentation, you’ll want to make sure that your
developers have a support community where they can interact with other
developers to get the answers to their questions. This may be a dedicated
support channel (either through email or a support ticket system), or an
unmanned support community, such as a forum/redirection to support sites
such as StackOverflow.
When evaluating how to set up your support community, it’s important to
understand what your resources and objectives are, as well as the
community that you’ll be serving. For an internal API, this may be
something as simple as having the API development team handle support
tickets/emails.
For a public API, you may want to consider setting up a forum as a primary
source for answering questions and directing people to the forum if they are
unable to find the answer in your documentation or an FAQ. Even if you
choose to offer free, dedicated support to your developers, by creating and
hosting a forum you enable the community to share ideas, become more
engaged and reduce the number of support tickets. However, you’ll need to
carefully manage and moderate your forum, being careful to eliminate
spam/trolling without affecting the free speech of your developers.
Forums also present a way for you to hear the complaints of your
developers, as well as get broad feedback on different ideas and
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that lets users vote up forum questions and replies in a similar manner to
StackOverflow, while also creating a separate board where developers can
up vote feature requests and ideas (helping you gauge the demand of these
items).
Some companies have elected to do away with support altogether, instead
pushing developers to third-party services such as StackOverflow. This is
perfectly acceptable, however, you must carefully weigh the pros and cons
before doing it, as it may adversely affect your community, your ability to
solicit feedback, developer usage and, ultimately, your company if
questions on these third-party sites go unanswered. Regardless of whether
they are your primary source of support, you should still have your team
monitor these services when hosting public/open APIs to ensure that
developers are able to get the answers they need, while also highlighting
pain-points in terms of integrating your API.
Keep in mind, if you are looking at building out a Developer Relations
program around your API, the first step is to create a community of support
that brings developers in contact with your company. By having this
community already in place, your Developer Relations team will be able to
converse with these developers, and you will already have advocates who
are excited to talk about and share your API with their friends, co-workers
and peers.
All Developer Relations programs start with a focus on community.
SDKs and Client Libraries
Software Development Kits, or SDKs, present an interesting paradox in
terms of developer onramp and support. On one hand they can greatly
reduce the onramp, as developers can simply drop in the SDK and begin
working with your API, using just the code or aspects that they need. On
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changes are made to it in order to access newer aspects of the API, and
also requires you to maintain and support them.
The biggest advantage of an SDK is that it doesn’t require the developer to
learn your API and understand the rules of REST. Instead, all they see is
code—not necessarily resources or even methods. They do not need to
understand the difference between POST, PUT and PATCH—all they need
to do is add some code, and it works.
But again, on the flip side, complex SDKs require developers to learn a new
set of code, even if they already understand how to interact with your API.
Perhaps more than full-fledged SDKs I am a fan of code wrappers, or
intuitive interfaces that take away the call logic (e.g. CURL for PHP). These
tend to be more lightweight, with less dependencies, and are far more
flexible and forgiving. However, they require that the developer have at
least a partial understanding of your API.
Regardless of your decision, if there are code libraries/ SDKs out there on
your public API, your company will need to support them. Perhaps not in
regards to contributing code, but from a “how to use it” standpoint, as
developers will be asking about this.
If you choose to provide your own SDKs, keep in mind that you will want to
provide ones for the most popular languages your clients are using, and
this could mean—for example, in the case of Twilio—supporting eight
different languages and having eight different SDKs. As you build out the
SDKs, you want developers who are proficient in each language producing
them, which creates a contracting/outsourcing nightmare for most
companies.
Because SDKs are so popular, but also a pain for companies to build and
maintain, we’ve seen a couple businesses pop up with the solution to this

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problem as their sole focus. Companies like APIMATIC.io and REST United
create SDKs built on demand to your specification to meet your client’s
needs. For example, APIMATIC.io currently generates SDKs for Android,
Windows, iOS, Java, PHP, Python, Ruby and Angular JS, all with the press
of a button. They also provide a widget that allows developers to download
the SDK directly from your site instead of having to visit the APIMATIC.io
directory.
REST United (restunited.com) goes a step further, having both
documentation with code samples and SDKs for Android, C#, ActionScript,
Java, Objective-C, PHP, Python, Ruby and Scala.
Of course, one challenge when using generated SDKs is quality, and both
of these companies are still relatively new and still testing the power of their
solutions. But this may be a good starting point, as they have the ability to
generate a bulk amount of the code and then clean it up, making your SDK
easier both to build and maintain.
The simple reality of an SDK or client library is this—it can be an effective
and very helpful tool for your users, but in the process of solving one
problem—if you’re not careful—you may end up creating more. Simply
having an SDK isn’t enough; you also need to form a plan for how the SDK
will be maintained (whether by you or the community) and how it will be
supported (since again, regardless of your official support policy, people
will ask your support teams how to integrate your API using both the SDK
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In an early presentation I ended with the following quote: “SDKs save lives.”
The truth is, I think code samples save lives. And regardless of your SDK
decision, code samples will be integral to the success of developers when
trying to integrate your API.

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13
A Final Thought
One of the challenges of writing a “generalized” API book is just that. It
becomes generalized—a jack-of-all-trades but master of none. There is so
much more that could have been covered in every chapter if we had the
time and space to cover it. Of course that would also require you finding
the time and space to read it. Each chapter should be looked at as a
starting point, not a complete reference, as it only touches the tip of the
iceberg. But hopefully this book has provided you with a strong base on
which to build the design of your API.
But as you proceed with your API, I want to share just one more lesson life
has taught me. Oftentimes, especially as developers, we want to push the
envelope and create new things. But an API that will be relied upon by your
company and—potentially—third parties, is neither the right time nor place
to try new things and travel through uncharted waters. Instead, it is the time
to apply the best practices that we know work—the practices we know will
ensure a long and healthy API life.
That doesn’t mean that one shouldn’t innovate. In fact, I believe quite the
opposite. I can’t encourage innovation enough and strongly believe that this

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is one of the greatest benefits the open source community gives us—the
chance to innovate and the chance to validate our creations. Continue to
innovate and try new things, but test your thoughts and processes in
personal projects and get community validation before trying them in a
production-level environment. Remember, pushing an API to production is
like squeezing toothpaste out of the tube—it’s really easy to do, but
extremely difficult to take back.
Growing up, my father would constantly remind me of the KISS principle:
“Keep it simple, stupid.” It became a running joke in my family, but this
principle applies to APIs perhaps more than anything else, since when we
are creating an API, we are constructing the foundation on which we will
need to continue building.
By keeping it simple, we help make our API easier to use, easier to maintain
and easier to extend. Avoid the urge to go wild—to create “amazing” and
“shiny” interactions. Because as enticing as they may be, in the end, the
sole purpose of your API is to let developers interact with your core
application’s services and data, and to make doing so as easy, efficient and
simple as possible.
Again, hopefully this book helps guide you towards best practices,
highlights problems to watch out for, and provides proven methods to help
you build your API in the strongest and most enduring way possible. By
using Spec-Driven Development, you not only create a contract, but also
test the contract before investing time, and then enforce it—not just on the
client’s end, but also your own.
If I had to highlight the key takeaways for building a successful API, I would
say remember these rules of thumb:
By using tools like RAML, you allow yourself to save time and energy,
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expand upon the tools you offer your developers while reducing your own
workload.
By incorporating best practices you create an API that is easily
recognizable and consistent, helping developers take advantage of your
offerings with a reduced learning curve while also making it easier on your
support and maintenance teams.
By utilizing an API Manager you provide yourself the reassurance that your
API has built-in user management, provisioning, scalability and security.
And by providing complete, easy-to-understand documentation, you are
ensuring that when your developers do have questions, they can quickly
obtain the information without having to spend hours searching or contact
support for answers.
With all of these steps, and by remembering the KISS principle, you can be
on your way to building the API of your dreams. And if you choose to follow
the REST constraints, you will have one that provides both you—and your
developers—with peace of mind and undisturbed REST.

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Appendix: More API Resources
API Courses (live training)
MuleSoft – http://training.mulesoft.com
API Directories
ProgrammableWeb – http://www.programmableweb.com
PublicAPIs – http://www.publicapis.com/
APIs.io – http://www.apis.io
API Tutorials
REST API Tutorial – http://www.restapitutorial.com/
API Definition Specs
RAML – http://raml.org
Swagger – http://www.swagger.io
API Blueprint – http://apiblueprint.org
API Hypertext Link Specs
HAL – http://stateless.co/hal_specification.html

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JSON API – http://jsonapi.org/
JSON-LD – http://json-ld.org/
Collection+JSON – http://amundsen.com/media-types/collection/
CPHL – https://github.com/mikestowe/CPHL/blob/master/README.md
Siren – https://github.com/kevinswiber/siren/blob/master/README.md
Uber – http://rawgit.com/uber-hypermedia/specification/master/uber-
hypermedia.html
Yahapi - https://github.com/Yahapi/yahapi/blob/master/pages/home.md
API Descriptive Error Formats
JSON API – http://jsonapi.org/format/#errors
Google Errors – http://bit.ly/1wUGinJ
vnd.error – https://github.com/blongden/vnd.error
API Mocking Services
MuleSoft – https://anypoint.mulesoft.com/apiplatform/
Apiary – http://apiary.io/
Mockaeble – http://mockable.io

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API Frameworks
Apigility – https://www.apigility.org/
Grape – http://intridea.github.io/grape/
Rails API – https://github.com/rails-api/rails-api
Restify – http://mcavage.me/node-restify/
Jersey – https://jersey.java.net/
API Testing
API Science – https://www.apiscience.com/
SmartBear – http://smartbear.com/
SDK Generators
APIMatic – http://www.apimatic.io
REST United – http://www.restunited.com
More Great Reads
A Practical Approach to API Design (2014)
D. Keith Casey Jr. and James Higginbotham
https://leanpub.com/restful-api-design

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Restful API Design (2013)
Leonard Richardson and Mike Amundsen
Published by O’Reilly
API News/ Blogs
MuleSoft – http://blogs.mulesoft.org
MikeStowe.com – http://www.mikestowe.com
ProgrammableWeb – http://www.programmableweb.com
NordicAPIs – http://nordicapis.com/blog/
APIUX – http://apiux.com/
API Evangelist – http://apievangelist.com/blog/
API Handyman – http://apihandyman.io/
API2Cart – https://www.api2cart.com/blog/
API Workshop (VLOG) - http://bit.ly/APIWorkshop
Mike Amundsen – http://amundsen.com/blog/
Roy Fielding, Untangled – http://roy.gbiv.com/untangled/
Just for Fun
{"apis":"the joy"} - http://apijoy.tumblr.com
For a list of up-to-date resources, visit http://bit.ly/apiResources

187
Appendix: Is Your API Ready for Developers?
You’ve taken the time to talk to potential developers and make
sure your API meets their needs (see Chapter 2)
You’ve defined your API in a spec like RAML (raml.org) and had
developers test out your API to make sure it functions as expected
(see Chapters 3, 4, & 5)
You’ve reviewed your API to make sure it’s consistent with best
practices including resource naming, method design, content-type
handling, etc. (see Chapters 7, 8, & 9)
You have a way to manage API users, keys, throttling, provisioning
and scaling your API (see Chapter 11)
You’ve created an API Portal with extensive documentation or
interactive documentation to help your developers use your API.
Remember, by defining your API in RAML you can automagically
generate your documentation and provide more interactive tools.
(see Chapter 12)
You’ve set up API Notebooks (apinotebook.com) to show your
developers sample use cases and flows using your API (see
Chapter 12)
You’ve provided code libraries or links to services such as
APImatic.io to help your developers get started without having to
write custom methods to access your API (see Chapter 12)
You’ve set up a community of support to help your clients use
your API and get their questions answered by technical experts
(see Chapter 12)

About the Author
Michael Stowe is a professional,
Zend Certified Engineer with over 10
years experience building
applications for law enforcement, the
medical field, nonprofits, and
numerous industrial companies.
Over the last two years he has been
focused on API design and ways to
improve industry standards,
speaking at API World & API Strategy
and Design, while being a leading
voice for RAML and Spec Driven
Development.
@mikegstowe
http://www.mikestowe.com
About MuleSoft
MuleSoft provides the most widely used integration platform for connecting
any application, data source or API, whether in the cloud or on-premises.
With Anypoint Platform™, MuleSoft delivers a complete integration
experience built on proven open source technology, eliminating the pain
and cost of point-to-point integration. Anypoint Platform includes
CloudHub™ iPaaS, Mule ESB™, and a unified solution for API
management, design and publishing.
@mulesoft
http://www.mulesoft.com

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