NodeJS: the good parts? A skeptic’s view (jax jax2013)

@crichardson
NodeJS: the good parts?
A skeptic’s view
Chris Richardson
Author of POJOs in Action
Founder of the original CloudFoundry.com
@crichardson
chris.richardson@springsource.com
http://plainoldobjects.com

@crichardson
Presentation goal
How a curmudgeonly
server-side Java developer
discovered an appreciation
for NodeJS and JavaScript

@crichardson
About Chris
http://www.theregister.co.uk/2009/08/19/springsource_cloud_foundry/

@crichardson
vmc push About-Chris
Developer Advocate
Signup at http://cloudfoundry.com

@crichardson
Agenda
Overview of NodeJS
Building a front-end server with NodeJS
Taming tangled asynchronous code with promises

@crichardson
What’s NodeJS?
Designed for DIRTy apps

@crichardson
Small but growing rapidly
Busy!

@crichardson
NodeJS Hello World
app.js
$ node app.js
$ curl http://localhost:1337
http://nodejs.org/
Load a module
request handler

@crichardson
NodeJS
JavaScript
Reactor
pattern
Modules

@crichardson
Dynamic, weakly-typed
Dynamic:
Types are associated with values - not variables
Deﬁne new program elements at runtime
Weakly typed:
Leave out arguments to methods
Access non-existent object properties
Weird implicit conversions: 99 == “99”!
truthy and falsy values
Comprehensive tests are essential

@crichardson
JavaScript is object-oriented
> var fred = {name: “Fred”, gender: “Male”};
undefined
> fred.name
“Fred”
> console.log("reading age=" + fred.age);
reading age=undefined
undefined
> fred.age = 99;
99
> fred
{ name: 'Fred',
gender: 'Male',
age: 99 }
> delete fred.age
true
> fred
{ name: 'Fred', gender: 'Male' }
Unordered key-value pairs
Keys = properties
Add property
Delete property

@crichardson
JavaScript “classes”
function Person(name) {
this.name = name;
}
Person.prototype.sayHello = function () { console.log("Hello " + this.name); };
var chris = new Person("Chris");
chris.sayHello();
Looks like a
constructor?!?
What’s that?!?!
This Java-like syntax is a mess
because JavaScript isn’t class
based
Looks familiar

@crichardson
overrides
JavaScript is a prototypal
language
__proto__
a 99
__proto__
a 100
b 200
inherited
Prototype
X
Y

@crichardson
Prototypal code
$ node
> var person = {};
undefined
> person.sayHello = function () { console.log("Hello " + this.name); };
[Function]
> var chris = Object.create(person, {name: {value: "Chris"}});
undefined
> var sarah = Object.create(person, {name: {value: "Sarah"}});
undefined
> chris.sayHello();
Hello Chris
undefined
> sarah.sayHello();
Hello Sarah
undefined
> chris.sayHello = function () { console.log("Hello mate: " + this.name); };
[Function]
> chris.sayHello();
Hello mate: Chris
undefined
Not defined
here
prototype properties

@crichardson
JavaScript is Functional
function makeGenerator(nextFunction) {
var value = 0;
return function() {
var current = value;
value = nextFunction(value);
return current;
};
}
var inc = makeGenerator(function (x) {return x + 1; });
> inc()
0
> inc()
1
Pass function as an
argument
Return a function
closure

@crichardson
Partial function application
> var join = require("path").join;
undefined
> join("/x", "y")
'/x/y'
> var withinx = join.bind(undefined, "/x");
undefined
> withinx("y");
'/x/y'
>
partially apply join

@crichardson
Created in a hurry with the
goal of looking like Java
The ‘Java...’ name creates expectations that it can’t satisfy
Fake classes: Hides prototypes BUT still seems weird
global namespace
scope of vars is confusing
Missing return statement = confusion
‘function’ is really verbose
‘this’ is dynamically scoped

@crichardson
JavaScript is the only way to
get things done in the
browser

@crichardson
Stockholm syndrome
“Stockholm syndrome ... is a psychological
phenomenon in which hostages ... have
positive feelings toward their captors,
sometimes to the point of defending them...”
http://en.wikipedia.org/wiki/Stockholm_syndrome

@crichardson
Martin Fowler once said:
"...I'm one of those who despairs that a
language with such deep flaws plays such an
important role in computation. Still the
consequence of this is that we must take
javascript seriously as a first-class language
and concentrate on how to limit the damage
its flaws cause. ...."
http://martinfowler.com/bliki/gotoAarhus2012.html

@crichardson
Use just the good parts

@crichardson
Use a better language that
compiles to JavaScript
TypeScript
Typed parameters and ﬁelds
Classes and interfaces (dynamic structural typing)
Dart
Class-based OO
Optional static typing
Bidirectional binding with DOM elements

@crichardson
CoffeeScript Hello World
http = require('http')
class HttpRequestHandler
constructor: (@message) ->
handle: (req, res) =>
res.writeHead(200, {'Content-Type': 'text/plain'})
res.end(@message + 'n')
handler = new HttpRequestHandler "Hi There from CoffeeScript"
server = http.createServer(handler.handle)
server.listen(1338, '127.0.0.1')
console.log('Server running at http://127.0.0.1:1338/')
Classes :-)
Bound method

@crichardson
About the Reactor pattern
Deﬁned by Doug Schmidt in 1995
Pattern for writing scalable servers
Alternative to thread-per-connection model
Single threaded event loop dispatches events on
handles (e.g. sockets, ﬁle descriptors) to event handlers

@crichardson
Reactor pattern structure
Event Handler
handle_event(type)
get_handle()
Initiation Dispatcher
handle_events()
register_handler(h)
select(handlers)
for each h in handlers
h.handle_event(type)
end loop
handle
Synchronous Event
Demultiplexer
select()
owns
notiﬁes
uses
handlers

@crichardson
Benefits:
Separation of concerns - event handlers separated
from low-level mechanism
More efficient - no thread context switching
Simplified concurrency - single threaded

@crichardson
Drawbacks:
Non-pre-emptive - handlers can’t block/take a long
time
Difﬁcult to understand and debug - inverted ﬂow of
control

@crichardson
NodeJS event loop implements
the Reactor pattern

@crichardson
Application code
Event-driven architecture
NodeJS event loop
Basic networking/ﬁle-system/etc.
HTTP DB driver ...
Event
listener
Callback
function
One
time
events
Recurring
events

@crichardson
Getting notiﬁed: Callback
example
var fs = require("fs");
function statFile(path) {
fs.stat(path, function (err, stat) {
if (err) {
console.log("Stat failed: " + path, err);
throw err;
}
console.log("stat result=" + path, stat);
});
};
By convention: ﬁrst
param is error object
By convention: Last
arg is a callback
Callbacks are
good for one
time
notifications

@crichardson
Getting notiﬁed: event
listeners
EventEmitter class - inherit or use
Listener registration methods:
on(eventName, listener)
once(eventName, listener)
Emitting events
emit(eventName, args...)
‘error’ event = special case: no listener print stack trace and
exit!
Good for
recurring
events

@crichardson
Event listener example
var fs = require("fs");
var readStream = fs.createReadStream("events.js", {encoding: "utf-8"});
// ReadStream << ReadableStream << EventEmitter
readStream.on('open', function (fd) {
console.log("opened with fd=", fd);
});
// Node v0.10 has readable instead: this is deprecated
readStream.on('data', function (data) {
console.log("data=", data);
});
Register listener
Register listener

@crichardson
Callback hell
function times2(x, callback) {
setTimeout(function () {
callback(x * 2)}, 500);
}
function plus3(x, callback) {
setTimeout(function (){
callback(x + 3)}, 500);
}
function displayResult(z) {
console.log("The result is=", z);
}
function plus3AndThenTimes2(x, callback)
{
plus3(x, function (y) {
times2(y, callback)
})
}
plus3AndThenTimes2(10, displayResults);
function sum(a, b, callback) {
callback(a + b);
}, 500);
}
function plus3PlusTimes2(x, callback) {
var p3, t2;
function perhapsDone() {
if (p3 & t2)
sum(p3, t2, callback);
};
plus3(x, function (y) {
p3 = y;
perhapsDone();
});
times2(x, function (y) {
t2 = y;
perhapsDone();
});
}
plus3PlusTimes2(10, displayResult);
times2(plus3(x)) times2(x) + plus3(x)

@crichardson
Long running computations
Long running computation blocks event loop for
other requests
Need to run outside of main event loop
Options:
Community: web workers threads
Built-in: NodeJS child processes

@crichardson
Using child processes
var child = require('child_process').fork('child.js');
function sayHelloToChild() {
child.send({hello: "child"});
}
setTimeout(sayHelloToChild, 1000);
child.on('message', function(m) {
console.log('parent received:', m);
});
function kill() {
child.kill();
}
setTimeout(kill, 2000);
process.on('message', function (m) {
console.log("child received message=", m);
process.send({ihateyou: "you ruined my life"})
});
parent.js
child.js
Create child process
Send message to child

Core built-in modules
Basic networking
HTTP(S)
Filesystem
Events
Timers
...

@crichardson
Thousands of community
developed modules
https://npmjs.org/
web frameworks, SQL/NoSQL
database, drivers, messaging, utilities...

@crichardson
What’s a module?
One or more JavaScript ﬁles
Optional native code:
Compiled during installation
JavaScript != systems programming language
Package.json - metadata including dependencies
exports.sayHello = function () {
console.log(“Hello”);
}
foo.js

@crichardson
Easy to install
$ npm install package-name

@crichardson
Easy to use
var http = require(“http”)
var server = http.createServer...
Core module OR
Path to ﬁle OR
module in node_modules
Module’s exports

@crichardson
Developing with NodeJS
modules
Core modules
Community modules
Your modules
Application code

@crichardson
Lots of modules BUT...
Variable quality
Multiple incomplete MySQL drivers, e.g. without
connection pooling!
Often abandoned
...

@crichardson
To summarize
NodeJS
JavaScript
Reactor patternModules
Flawed and
misunderstood
Scalable yet
costly and
often
unnecessary
Rich but
variable quality

@crichardson
Alternative technologies
Atmosphere - portable event delivery
Netty - asynchronous I/O
Vert.x
SpringSource’s Reactor project
...

@crichardson
So why care about
NodeJS?
Easy to write scalable network services
Easy to push events to the browser
Easy to get (small) stuff done
It has a role to play in modern
application architecture

@crichardson
Evolving from a monolithic
architecture....
WAR
Shipping
Service
Accounting
Service
Inventory
Service
StoreFrontUI

@crichardson
... to a micro-service architecture
Store front web application
shipping web application
inventory web application
Accounting
Service
StoreFrontUI
accounting web application
Shipping
Service
Inventory
Service

@crichardson
Browser
WAR
StoreFrontUI
Model
View Controller
Presentation layer evolution....
HTML / HTTP
+
JavaScript

@crichardson
Browser Web application
RESTful
EndpointsModel
View Controller
...Presentation layer evolution
JSON-REST
HTML 5 -
JavaScript
No elaborate, server-side web framework
required
Event
publisher
Events
Static
content

@crichardson
NodeJS as an API gateway
Browser
Service 1
Service 2
Message
Bus
HTML 5/
Java
Script
Socket.io
client
Events
RESTful WS
Server
application
Socket.io
server
Node JS
Service 3
RESTful
WS

@crichardson
Serving static content

@crichardson
Using low-level APIs
var http = require('http'), path = require('path'), mime = require('mime'), fs = require("fs");
var server = http.createServer(function (req, res) {
var filePath;
if (req.url == '/') {
filePath = 'public/index.html';
} else {
filePath = 'public' + req.url;
}
fs.exists(filePath, function (exists) {
if (exists) {
res.writeHead(200, {"content-type": mime.lookup(path.basename(filePath)) });
fs.createReadStream(filePath).pipe(res);
} else {
res.writeHead(404, {'Content-Type': 'text/plain'});
res.write('Error 404: resource not found.');
res.end();
}
});
}
);
server.listen(1337, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1337/');
Uses file
extension
Cool!

@crichardson
Using the express web
framework
var express = require('express')
, http = require('http')
, app = express()
, server = http.createServer(app)
;
app.conﬁgure(function(){
app.use(express.static(__dirname + '/public'));
});
server.listen(8081);

@crichardson
RESTful web services

@crichardson
Implementing RESTful WS
with Express
var express = require('express'),
http = require('http'),
path = require('path');
var app = express();
var server = http.createServer(app);
app.get('/portfolio/:userId', function (req, res) {
var portfolio = retrievePortfolio(req.params.userId);
res.json(portfolio);
});
Easy URL
routing and
destructuring

@crichardson
Proxying to backend server
express = require('express')
request = require('request')
app = express.createServer()
proxyToBackend = (baseUrl) ->
(req, res) ->
callback = (error, response, body) -> console.log("error=", error)
originRequest = request(baseUrl + req.url, callback)
req.pipe(originRequest)
originRequest.pipe(res)
app.get('/restaurant/*', proxyToBackend('http://available-restaurant....com'))
app.post('/orders', proxyToBackend('http://restaurant-management...com'))
app.get('/orders', proxyToBackend(''http://restaurant-management...com'))
Returns a request handler
that proxies to baseUrl

@crichardson
Delivering events to the
browser

@crichardson
NodeJS clock example
Increments every
second

@crichardson
Socket.io - Server side
var express = require('express')
, app = express()
, server = http.createServer(app)
, io = require('socket.io').listen(server)
;
app.conﬁgure(function(){
app.use(express.static(__dirname + '/public'));
});
io.sockets.on('connection', function (socket) {
var counter = 0;
function tick() {
counter = counter + 1;
socket.emit('tick', counter);
};
setInterval(tick, 1000);
});
handle new
connection
Send tick event to
browser every 1 sec
initializes socket.io

@crichardson
Socket.io - client side using the
knockout.js MVVM framework
var socket = io.connect(location.hostname);
function ClockModel() {
self.ticker = ko.observable(1);
socket.on('tick', function (data) {
self.ticker(data);
});
};
ko.applyBindings(new ClockModel());
<html>
<body>
The event is <span data-bind="text: ticker"></span>
<script src="/socket.io/socket.io.js"></script>
<script src="/knockout-2.0.0.js"></script>
<script src="/clock.js"></script>
</body>
</html>
clock.js
Connect to
socket.io
Subscribe
to tick event
Bind to model
Update
model

@crichardson
Distributed NodeJS clock
example
Browser NodeJS NodeJS
RabbitMQ
ProducerConsumer
socket.iosocket.io

@crichardson
AMQP Publisher
var amqp = require('amqp'),
amqpConnection = amqp.createConnection(...),
tickTockExchange;
function tick() {
var message = { tick: Date.now() };
tickTockExchange.publish("tickTock", message, {
mandatory: true,
contentType: "text/plain"
});
};
amqpConnection.on('ready',
function () {
tickTockExchange =
amqpConnection.exchange("tickTock",
options = {passive: false, type: 'fanout'});
tickTockExchange.on('open', function () { setInterval(tick, 1000) });
});
Connect to
AMQP
Ensure
exchange
exists
Publish
message
Initialize timer

@crichardson
AMQP socket.iovar express = require('express')
, amqp = require(‘amqp’)
....;
...
var amqpCon = amqp.createConnection(...);
io.sockets.on('connection', function (socket) {
function amqpMessageHandler(message, headers, deliveryInfo) {
var m = JSON.parse(message.data.toString());
socket.emit(‘tick’, m);
};
amqpCon.queue(“”, {},
function(queue) {
queue.bind(“tickTock”, “”);
queue.subscribe(amqpMessageHandler);
});
});
Connect to
AMQP queue
Subscribe to
AMQP queue
Republish
as socket.io
event
https://github.com/cer/nodejs-clock

@crichardson
Async code = callback hell
Scenarios:
Sequential: A B C
Fork and join: A and B C
Code quickly becomes very messy

@crichardson
Simplifying code with
Promises (a.k.a. Futures)
Functions return a promise - no callback parameter
A promise represents an eventual outcome
Use a library of functions for transforming and
composing promises
Promises/A+ speciﬁcation - http://promises-aplus.github.io/promises-spec
when.js (part of cujo.js by SpringSource) is a popular
implementation

@crichardson
Taming callback hell 1
function times2(x) {
var deferred = when.defer();
deferred.resolve(x * 2)}, 500);
return deferred.promise;
}
times2(plus3(x)) Create a deferred
Return a promise
Eventually supply a value
function plus3AndThenTimes2(x) {
return plus3(x).then(times2);
}
plus3AndThenTimes2(10).
then(displayResult);
Transform value in
promise
function plus3(x) {
deferred.resolve(x + 3) }, 500);
}
Simpler, almost
synchronous-style code

@crichardson
Taming callback hell 2
function sum(a, b) {
deferred.resolve(a + b);
}, 500);
}
function plus3PlusTimes2(x) {
var p3 = plus3(x),
t2 = times2(x);
return when.join(p3, t2).spread(sum);
}
plus3PlusTimes2(10).then(displayResult);
times2(x) + plus3(x)
Combine results
of two promises
Call with array
elements as
arguments

@crichardson
Calling non-promise code
fs.stat(path, function (err, statInfo) {
if (err)
deferred.reject(err);
else
deferred.resolve(statInfo);
}
var promise = deferred.promise;
var nodefn = require("when/node/function");
var promise = nodefn.call(fs.stat, path)
Hides
boilerplate
code

@crichardson
Filesystem scanner example
Read contents of directory
Use stat to determine if directory or ﬁle
Recurse on directories
Merge the results

@crichardson
Read contents of directory
function findFilesInDir(dir) {
var directoryContents =
nodefn.call(self.fs.readdir, dir);
...
}
Returns promise containing
an array of ﬁle names

@crichardson
Create absolute paths
var directoryContents = ...
var toAbsolute = join.bind(undefined, dir)
var absolutePaths = when.map(directoryContents, toAbsolute);
...
}
Partially apply
join()

@crichardson
Use stat to determine if
directory or ﬁle
var directoryContents = ...
var absolutePaths = ...
var statTasks = when.map(absolutePaths, makeStatTask);
var statResults = parallel(statTasks);
...
}
function makeStatTask(path) {
return function () {
function makeStatInfo(stats) {
return {path: path, isdir: stats.isDirectory(),
ctime: stats.ctime};
}
return nodefn.call(self.fs.stat, path).then(makeStatInfo);
};
}
Execute stats in
parallel

@crichardson
Recurse on directories
...
var statResults = ...;
var listOfListsOfFiles =
when.map(statResults, processStatInfo);
...
}
function processStatInfo(statInfo) {
if (statInfo.isdir) {
return findFilesInDir(statInfo.path);
} else {
return [statInfo.path];
}
}
Map each stat
result to a list of
ﬁles

@crichardson
Flatten array of arrays of ﬁle
paths
...
var listOfListsOfFiles = ...;
return when.reduce(listOfListsOfFiles,
concatenateLists, []);
}
function concatenateLists(currentResult, value, index, total)
{
return currentResult.concat(value);
}

@crichardson
Summary
JavaScript is a very ﬂawed language
The asynchronous model is often unnecessary; very
constraining; and adds complexity
BUT despite those problems
Today, NodeJS is remarkably useful for building network-
focussed components

@crichardson
Questions?
@crichardson chris.richardson@springsource.com
http://plainoldobjects.com - code and slides
www.cloudfoundry.com

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