Domain driven design

Domain Driven
Design
presenter: Yuriy Taras

03/29/13 1

Agenda
• What and Why?
• Requirements
• Building blocks
• Higher level patterns

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What is DDD?
Domain-driven design (DDD) is an approach to
develop software for complex needs by connecting
the implementation to an evolving model

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Why?
DDD helps to isolate business logic from other parts
application source code

What to do?
Use inheritance from base class to provide ad-hoc
polymorphism and use monadic transformation to
extract iterator interface, so strategy pattern can
be applied to persistence level from MVC
prospective.

Non-developers hear the same
when you talk to them
You hear the same when non-
developers talk to you

Misunderstanding is a
bug

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Who knows the domain?
Domain experts do!

Talk to them

How to talk to them?
Ubiquitous Language!

All traffic is made up of planes. Each plane takes off
from a departure place, and lands at a destination
place.
The pilots receive a route they must follow. And they
should stay on that route as close as possible.
Before leaving the airport, the pilots receive a
detailed flight plan which includes all sorts of
information about the flight: the route, cruise altitude,
the cruise speed, the type of airplane, even
information about the crew members.

Anaximander world map

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Mercator projection

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Domain model should not be strict or realistic. Instead
domain model should focus on what’s important in
current context and omit what’s not.

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14

Entities
Entity is an object, which has an identity which
remains the same throughout the states of the
software. Each Entity can be strictly identified by it’s
identity
Example:
•US citizen – Social Security number
•Bank account – account number
•Meeting – surrogate key

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Entity implementation
• Entities are considered equal if their identity are
equal
• Entities should stay focused on domain level, don’t
put infrastructure code or code which belongs to
other entities into them
• Entities can provide getters/readers
• Don’t provide setters for your entities, provide
business mutators instead:
o post.status = PUBLISHED # BAD
o post.publish() # GOOD

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CQS
Use Command-Query Separation (not CQRS) – each
method is either a command or query:
!post.publish()
!user.disable()
!order.add_line_item(product, 2)
?order.get_price
?post.get_publish_date

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Value object
Value Object is an object which is used to describe
certain aspects of a domain but doesn’t have an
identity.
Example:
•Address
•Money amount
•2D/3D Point

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Value object
implementation
• Value objects should be immutable – it makes them
shareable
• Value objects can contain other VO’s and even
Entities

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Complex Domain Model
What if domain
is too big and
complex? Too
many relations?

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Aggregate
A collection of objects that are bound together by a
root Entity, otherwise known as an Aggregate Root.
Example:
•Order contains line items
•Car contains wheels and engine
•Blog post contains comments*
•Cinema contains seats

* it depends

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Aggregate
implementation
• All Aggregate Entities should be accessible only
from Aggregate Root
• Reference to internal Entities may be passed to
external object, but those can use it only temporary
and can’t store it
• Internal Entity identity doesn’t have to be unique
across the application, uniqueness across the
Aggregate is enough

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Example
Customer

1

*

Order

Line item
1 1 Product

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Service
Services contain behavior which can’t be considered
as a part of specific entity.
Example:
•Transaction Service transfers money from one
account to another
•Payment Service processes orders
•Route Service provides routes based on route
specification
Services should be stateless.
Don’t mix infrastructure and domain services!

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Layered architecture

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You all know that
• Modules are groups of elements which are
functionally or logically belong together
• Factories are used to create Entities and
Aggregates*

* Factories are not necessary separate objects, it can
be GoF Factory Method or Builder or whatever

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Repository
Repositories encapsulate all the logic needed to
obtain object references.
Note that even if repository implementation can
belong to infrastructure layer, it’s API is a pure domain
model
Example:
•customer_repository.add_customer(customer)
•customer_repository.find_customer(‘12345’)
If we have separate repository for Aggregate Entities,
only Aggregate Root (or it’s repository) should use
them

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Higher level patterns

Footer Text 03/29/13 29

Cargo example
• Customer can ship a cargo from source to
destination
• Route can be changed on it’s way


Cargo example
• Service should provide either cheapest or fastest
route based on customer’s preferences


Bounded contexts

Transition Edge
Cargo
*

1 *

1 Node

Route

1

*

Leg

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Bounded Context
It’s advised to maintain Translation Map, which shows
bound contexts and relationships between them
Bounded Contexts names should be part of the
Ubiquitous Language
Bonded Contexts can be used for team organization
Contexts can relate to each other using Shared
Kernel, Customer-Supplier or Conformist patterns

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Anticorruption layer
Isolate bad code from good code. Point.

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Distillation
Look at your features/use cases/concepts and
separate them into 3 parts:
•Core Domain
•Supporting subdomain
•Generic Subdomain


Generic subdomains
Examples:
•Authentication and authorization
•Mailing*
•Reporting*
You can:
•Buy it
•Outsource it
•Copy it from existing projects
•Implement it
* it depends

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Core and supporting
domain
Supporting is what we have to have.
Core domain is what differentiate us.
Supporting can be crap.
Core can’t.

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When should I use DDD?
• When I have complex business logic. It’s not suited
for your mom’s CRUD app
• When I have access to domain experts – otherwise I
will build other perfect but useless app
• When I have skilled team


Questions?


Links
http://habrahabr.ru/post/61524/ Russian, a lot of links
http://www.infoq.com/minibooks/domain-driven-
design-quickly English, free book
http://dddcommunity.org/ English, community

03/29/13 40

Domain driven design

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