Implementing Messaging Patterns in JavaScript using the OpenAjax Hub

Implementing Messaging
Patterns in JavaScript using
the OpenAjax Hub

Kevin Hakanson
Twin Cities Code Camp 11
8-9 October 2011

Are You In The Right Place?
● This talk:
○ Is your web application a tightly coupled, DOM event handler
mess? Use techniques from the Enterprise Integration Patterns book to
build better components. Concepts including message, publishsubscribe channel, request-reply and message filter will be demonstrated
in JavaScript (along with corresponding tests) using the OpenAjax Hub.

● This speaker:
○ Kevin Hakanson is an application architect for Thomson Reuters where
he is focused on highly, scalable web applications. His background
includes both .NET and Java, but he is most nostalgic about Lotus
Notes. He has been developing professionally since 1994 and holds a
Master’s degree in Software Engineering. When not staring at a
computer screen, he is probably staring at another screen, either
watching TV or playing video games with his family.

Kevin Hakanson
@hakanson
#tccc11

kevin.hakanson@gmail.com

github.com/hakanson/tccc11

stackoverflow.com/users/22514/kevin-hakanson

What to Expect
● "Based on a True Story"
○ derived from production code from large scale, web app
● Enterprise Integration Patterns
○ selected patterns from the book
● JavaScript Libraries
○ OpenAjax, jQuery, QUnit
● Problems + Patterns + Tests
● Living on the edge
○ Google docs presentation, Cloud9 IDE, github
● Questions (OK during presentation)

Problem
● large scale web application
● multiple teams with separate code bases
● iterative development, but long duration project
● continuous integration, build/deploy on check in
● primary touch point is in browser
● automated regression tests
● make all this work without app constantly breaking

Architect Talk
● Treat UI components (widgets) like services in the browser
which operate independent of each other
○ however, some have no UI and are invisible services
● Think in terms of application level messages between
components instead of direct wiring/capturing another
components raw DOM events
○ internal component interaction can be DOM or custom
event based
● Hierarchical channels with naming standard and wildcard
subscriptions. Long lived and should be namespaced.
● Forwards and backwards version compatibility

insert "Ivory Tower Architect" joke here :)

Demo
$( ".slide[title='Demo']" ).focus( function() {
window.location.href = "demo.html";
});

Enterprise Integration Patterns
Enterprise Integration Patterns:
Designing, Building, and Deploying Messaging Solutions
By Gregor Hohpe, Bobby Woolf
Published Oct 10, 2003 by Addison-Wesley Professional. Part
of the Addison-Wesley Signature Series (Fowler) series.
ISBN-10: 0-321-20068-3
ISBN-13: 978-0-321-20068-6

http://www.enterpriseintegrationpatterns.com/
Pattern icon, pattern name, problem and solution
statements, and sketch available under the
Creative Commons Attribution license.

List of Patterns
Messaging Systems:
● Message Channel, Message, Pipes and Filters, Message Router, Message Translator, Message
Endpoint
Messaging Channels:
● Point-to-Point Channel, Publish-Subscribe Channel, Datatype Channel, Invalid Message Channel,
Dead Letter Channel, Guaranteed Delivery, Channel Adapter, Messaging Bridge, Message Bus
Message Construction:
● Command Message, Document Message, Event Message, Request-Reply, Return Address,
Correlation Identifier, Message Sequence, Message Expiration, Format Indicator
Message Routing:
● Content-Based Router, Message Filter, Dynamic Router, Recipient List, Splitter, Aggregator,
Resequencer, Composed Msg. Processor, Scatter-Gather, Routing Slip, Process Manager, Message
Broker
Message Transformation:
● Envelope Wrapper, Content Enricher, Content Filter, Claim Check, Normalizer, Canonical Data Model
Messaging Endpoints:
● Messaging Gateway, Messaging Mapper, Transactional Client, Polling Consumer, Event-Driven
Consumer, Competing Consumers, Message Dispatcher, Selective Consumer, Durable Subscriber,
Idempotent Receiver, Service Activator
System Management:
● Control Bus, Detour, Wire Tap, Message History, Message Store, Smart Proxy, Test Message,
Channel Purger

JavaScript Publish-Subscribe Options
● jQuery custom events (api.jquery.com/category/events/)
○ flat name with optional, restricting namespace
○ reflects DOM structure, including event bubbling
○ API: bind, unbind, trigger
● AmplifyJS Pub/Sub (amplifyjs.com/api/pubsub/)
○ cleaner interface that jQuery custom events
○ subscriptions can request priority and/or prevent
other callbacks from being invoked
○ API: subscribe, unsubscribe, publish
● OpenAjax Hub 1.0 (www.openajax.org)
○ simple and lightweight publish/subscribe engine
○ hierarchical event names with wildcard subscriptions
○ API: subscribe, unsubscribe, publish

QUnit
"QUnit is a powerful, easy-to-use, JavaScript test suite. It's
used by the jQuery project to test its code and plugins but is
capable of testing any generic JavaScript code."
http://docs.jquery.com/Qunit
QUnit Example
test("a basic test example", function() {
ok( true, "this test is fine" );
var value = "hello";
equal( value, "hello", "We expect value to be hello" );
});

Tests
● QUnit tests of jQuery custom events
○ https://github.com/hakanson/tccc11/blob/master/tests/jquery.tests.js

○ http://jsfiddle.net/hakanson/5bgjd/
● QUnit tests of AmplifyJS Pub/Sub
○ https://github.com/hakanson/tccc11/blob/master/tests/amplify.tests.js

○ http://jsfiddle.net/hakanson/zy6Dy/

$.hub
● jQuery plug-in created for this talk
○ derived from production code
● Builds on top of OpenAjax Hub functionality
● API:
○ subscribe, unsubscribe, publish
○ guid, message
○ createVersionFilter, createIdempotentFilter
○ reply, requestReply

Problem (again)
● make all this work without app constantly breaking

Message Channel (60)
How does one application communicate with another using
messaging?

Connect the applications using a Message Channel, where one
application writes information to the channel and the other one
reads that information from the channel.

Message (66)
How can two applications connected by a message channel
exchange a piece of information?

Package the information into a Message, a data record that the
messaging system can transmit through a message channel.

Publish-Subscribe Channel (106)
How can the sender broadcast an event to all interested
receivers?

Send the event on a Publish-Subscribe Channel, which delivers
a copy of a particular event to each receiver.

channel parameter
● the name of the event to listen for
○ OpenAjax - name
○ AmplifyJS - topic
○ jQuery custom events - eventType
○ elsewhere - subject

message parameter
● An arbitrary Object holding extra information that will be
passed as an argument to the handler function
○ OpenAjax - publisherData
○ jQuery custom events - extraParameters
○ AmplifyJS - additional parameters

callback parameter
● a function object reference to be called when a subscription
receives data
● should play nice and handle:
○ own errors and not throw exceptions
○ being called synchronous and not perform long running
operations
○ being called asynchronous (depending on implementation)
● scope parameter
○ what this is when callback is invoked (default window)
○ AmplifyJS - context
○ or use callback, scope with $.proxy( function, context )

Test Excerpt
$.hub.subscribe( "A", function( channel, message ) {
countA++;
});
$.hub.subscribe( "A.*", function( channel, message ) {
countAstar++;
});
$.hub.subscribe( "A.**", function( channel, message ) {
countAstarstar++;
});
$.hub.subscribe( "A.1", function( channel, message ) {
countA1++;
});
$.hub.subscribe( "A.1.a", function( channel, message ) {
countA1a++;
});
$.hub.publish( "A", message );
$.hub.publish( "A.1", message );
$.hub.publish( "A.1.a", message );
$.hub.publish( "A.1.b", message );

Demo (Updated)
$.get("demo.html", function(data) {
data.replace( "demo.js", "demo-updated.js" );
});

Architect Talk
● If messages published on a channel carry same type of
body, then channel names identify types of messages
○ Message is channel + headers + body
● Message body carries data payload; defined by specification
○ in future could use XML or JSON schema
● Message should be serializable (JSON) and not contain
functions or logic
○ better for immutability/tampering and cross domain
usages (iframes, client to server)

Datatype Channel (111)
How can the application send a data item such that the receiver
will know how to process it?

Use a separate Datatype Channel for each data type, so that
all data on a particular channel is of the same type.

Document Message (147)
How can messaging be used to transfer data between
applications?

Use a Document Message to reliably transfer a data structure
between applications.

$.hub.message
{
"timestamp": 1317961214398,
"messageId": "31525258-8ca4-464f-96da-668e52a6cad9",
"formatVersion": "0",
"body": {
"key1": "value1",
"key2": "value2"
}
}

Test Excerpt
var channel = "name",
messageBody = { "key1": "value1", "key2": "value2"},
actualMessage,
actualChannel;
var subscription = $.hub.subscribe( channel,
function( channel, message ) {
actualChannel = channel;
actualMessage = message;
});
var message = $.hub.message( messageBody );
$.hub.publish( channel, message );
$.hub.unsubscribe( subscription );

Problem
● Two components exist on a web page
○ first component subscribes to messages from the second
component
○ second component publishes message during initialization
● When <script> tags are reordered, the functionality breaks
because second component publishes its message before
first component has a chance to subscribe
○ components on page have temporal coupling
● Need the equivalent of jQuery $.ready() for hub

Durable Subscriber (522)
How can a subscriber avoid missing messages while it’s not
listening for them?

Use a Durable Subscriber to make the messaging system save
messages published while the subscriber is disconnected.

Test Excerpt
// tell hub to save messages
$.hub({ ready: false });
// publish before any active subscribers
$.hub.publish( channelA, messageA );
var subscriptionA = $.hub.subscribe(channelA,
});
// tell hub to delivery saved messages
$.hub({ ready: true });

Architect Talk
● Need message body to be backwards compatible in both
structure and meaning
○ otherwise need versioning scheme and strategy for both
old and new versions of a message to be published
● A filter on version is important for backwards and forwards
compatibility, so your code doesn't break when a new
version of the message is published
○ using the filter parameter allows you to put the guard
outside of the callback instead of inside it (think of the filter
like an aspect)

Format Indicator (180)
How can a message’s data format be designed to allow for
possible future changes?

Design a data format that includes a Format Indicator, so that
the message specifies what format it is using.

Selective Consumer (515)
How can a message consumer select which messages it
wishes to receive?

Make the consumer a Selective Consumer, one that filteres the
messages delivered by its channel so that it only receives the
ones that match its criteria.

filter parameter
● A function that returns true or false to either match or deny a
match of the published event

Test Excerpt
var m1 = $.hub.message({
{ formatVersion: "1"
var m2 = $.hub.message({
{ formatVersion: "2"

name: "Kevin Hakanson" },
});
firstname: "Kevin", lastname: "Hakanson" },
});

function callbackV1( channel, message ) {
countV1++;
}
function callbackV2( channel, message ) {
countV2++;
}
$.hub.subscribe( "Person.*", callbackV1, null, null,
$.hub.createVersionFilter( "1" ) );
$.hub.subscribe( "Person.*", callbackV2, null, null,
$.hub.createVersionFilter( "2" ) );
$.hub.publish( "Person.Cool", m1 );

Message Translator (85)
How can systems using different data formats communicate
with each other using messaging?

Use a special filter, a Message Translator, between other filters
or applications to translate one data format into another.

Test Excerpt
var m2 = $.hub.message({ firstname: "Kevin", lastname: "Hakanson" },
{ formatVersion: "2" });
function callbackV1(channel, message ) {
countV1++;
}
function callbackV2(channel, message ) {
countV2++;
var name = message.body.firstname + " " + message.body.lastname;
var translatedMessage = $.hub.message( { name: name },
{ formatVersion: "1" } );
$.hub.publish( channel, translatedMessage );
}
subscriber1 = $.hub.subscribe(
$.hub.createVersionFilter(
subscriber2 = $.hub.subscribe(
$.hub.createVersionFilter(

"Person.*", callbackV1, null, null,
"1" ) );
"Person.*", callbackV2, null, null,
"2" ) );


Idempotent Receiver
How can a message receiver deal with duplicate messages?

Design a receiver to be an Idempotent Receiver--one that can
safely receive the same message multiple times.

Test Excerpt
var m1 = $.hub.message({}),
m2 = $.hub.message({});
function callback1( channel, message ) {
count1++;
}
function callback2( channel, message ) {
count2++;
}
$.hub.subscribe( channel, callback1, null, null, createIdempotentFilter
() );
$.hub.subscribe( channel, callback2, null, null, createIdempotentFilter
() );
$.hub.publish( channel, m1 );

Problem
● How to replace directly calling functions with messaging?
● Want to do something like below, without common coupling
(sharing a global variable).
○ var component1 = new Component1();
component1.command();
var reply = component1.request({});

Request-Reply (154)
When an application sends a message, how can it get a
response from the receiver?

Send a pair of Request-Reply messages, each on its own
channel.

Return Address (159)
How does a replier know where to send the reply?

The request message should contain a Return Address that
indicates where to send the reply message.

Correlation Identifier (163)
How does a requestor that has received a reply know which
request this is the reply for?

Each reply message should contain a Correlation Identifier, a
unique identifier that indicates which request message this
reply is for.

Test Excerpt
$.hub.subscribe(channel, function( channel, message ) {
$.hub.reply( message, replyMessage );
});
$.hub.requestReply(channel, message,
actualReplyMessage = message;
correlationId = message.correlationId;
});

Command Message (145)
How can messaging be used to invoke a procedure in another
application?

Use a Command Message to reliably invoke a procedure in
another application.

Message Endpoint (95)
How does an application connect to a messaging channel to
send and receive messages?

Connect an application to a messaging channel using a
Message Endpoint, a client of the messaging system that the
application can then use to send or receive messages.

Test Excerpt
// create a message endpoint to call counter.increment()
var subscription = $.hub.subscribe( channel,
counter.increment( message );
});
counter.increment( 2 );
// send command message to increment by 2
$.hub.publish( channel, 2 );

Summary
● Patterns help us find appropriate solutions in the software
architecture and design process.
● Using Publish/Subscribe events for browser communication
adds a level of indirection between the JavaScript
components.
● Using a common messaging structure for the events
(including naming and versioning) make things more
predictable.
● Libraries are not enough; knowledge of JavaScript and the
DOM is essential for good widget design and interaction.

Implementing Messaging Patterns in JavaScript using the OpenAjax Hub

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