Developing Revolutionary Web Applications using Comet and Ajax Push

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Doris Chen Ph.D.Staff Engineer/Technology Evangelist

Developing Revolutionary Web Applications using Comet and Ajax

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Agenda• Web 2.0• Introduction to Comet• Asynchronous HTTP and the Server• Develop a Slideshow Chat Application using

Cometd (demo)• Develop a Two Player Game using Comet

(demo)• Future, Summary and Resources

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Web 2.0A Web by the people, for the people.Documents on the web increasingly generated by users

Out of the Information Age, into the Participation Age Are web user interfaces becoming more powerful?Is the user an HTTP client?

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The Asynchronous Web RevolutionThe Web enters the Participation Age.• Ajax is still typically synchronous with user

events• Full asynchrony has updates pushed from server

any time> Update pages after they load> Send users notifications

• Allow users to communicate and collaborate within the web application

• Called “Ajax Push”, “Comet”, or “Reverse Ajax”> This is the full realization of Ajax, now fully

asynchronous

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Applications in the Participation AgeApplication-mediated communication.• Distance learning• Collaborative authoring• Auctions• Shared WebDAV filesystem• Blogging and reader comments• SIP-coordinated mobile applications• Hybrid chat/email/discussion forums• Customer assistance on sales/support pages• Multi-step business process made collaborative• Shared trip planner or restaurant selector with maps• Shared calendar, “to do” list, project plan• Games

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Server-mediated Collaboration

Server

Client 1 Client 2, 3, ..n

External Application

• User Initiated• Application Initiated

UserAction

PushPush Push

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What is Comet (Ajax Push)?Responsive, low-latency interaction for the web• A programming technique that enables web

servers to send data to the client without having any need for the client to request for it

• Allows creation of highly responsive, event-driven web applications > Keep clients up-to-date with data arriving or changing

on the server, without frequent polling• Pros

> Lower latency, not dependent on polling frequency> Server and network do not have to deal with frequent

polling requests to check for updates

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Ajax Poll vs Ajax PushBending the rules of HTTP.

• Comet Refers to both the Long Polling and Streaming methods of web programming

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Ajax Poll vs Ajax PushBending the rules of HTTP.• Poll:

> Send a request to the server every X seconds > The response is “empty” if there is no update

• Long Poll: (most popular, demo use)> Send a request to the server, wait for an event to

happen, then send the response> The response is never empty

• Http Streaming: > Send a request, wait for events, stream multi-

part/chunked response, and then wait for the events> The response is continually appended to

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Comet Examples• GMail and GTalk • Meebo• 4homemedia.com

(using GlassFish project's Comet)• JotLive• KnowNow• Many more …

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Agenda

• Web 2.0• Introduction to Comet• Asynchronous HTTP and the Server• Develop a Slideshow Chat Application using


(demo)• Summary and Resources

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How Push worksKeep an open connection.

• Deliver data over a previously opened connection

• Always “keep a connection open”> do not respond to the initiating request until event

occurs• Streaming is an option

> send response in multiple parts without closing the connection in between

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Server Architecture ChallengeCan Push scale?• Using blocking, synchronous technology will

result in a blocked thread for each open connection that is “waiting”> Every blocked thread will consume memory> This lowers scalability and can affect performance

• To get the Java Virtual Machine (JVM™) to scale to 10,000 threads and up needs specific tuning and is not an efficient way of solving this

• Servlets 2.5 are an example of blocking, synchronous technology

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Architecture ChallengesThe serious effect of blocking threads

(default thread stack size)

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Technology SolutionNIO avoids blocked threads.

• Use new I/O (NIO) non-blocking sockets to avoid blocking a thread per connection

• Use technology that supports asynchronous request processing> Release the original request thread while waiting for

an event> May process the event/response on another thread

than the original request• Advantages

> Number of clients is primarily limited by the number of open sockets a platform can support

> Could have all clients (e.g. 10’000) “waiting” without any threads processing or blocked

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Server-side Ajax Push: Who supports whatThe asynchronicity matrix

XWebLogic

Container Asynchronous IO Suspendible Request/Response

Delivery Guarantee

Jetty XTomcat X XGlassFish X X XResin X

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HTTP modules• The Grizzly Framework also have an HTTP framework

that can be used to build Web Server> This is what GlassFish™ v1|2|3 build on top of.> More specialized modules are also available like

Comet (Async HTTP).• Simple interface to allow customization of the HTTP

Protocol> GrizzlyRequest: A utility class to manipulate the HTTP

protocol request.> GrizzlyResponse: A utility class to manipulate the

HTTP protocol response.> GrizzlyAdapter: A utility class to manipulate the HTTP

request/response object.

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Introduction to GlassFish Grizzly Comet• Grizzly Comet is a framework that ship with

GlassFish v1|2|3, and can also be embedded into any application using the Grizzly Embedded interface (no GlassFish involved)

• The Grizzly Comet Framework includes a set of components that can be used for building Comet based application:> Grizzly Comet Framework, Continuation, Grizzlet,

Messages Bus, Bayeux support

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Grizzly Comet Components

Grizzly Comet FrameworkGrizzly Comet Framework

Grizzly HTTP Grizzly HTTP

Grizzly NIO FrameworkGrizzly NIO Framework

GrizzletGrizzlet BayeuxBayeux Messages BusMessages BusContinuationContinuation

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Asynchronous Ajax DemoGrizzlet with Project jMaki on GlassFish

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What Is Cometd?• HTTP-based event routing bus using

> Bayeux Protocol> Comet as underlying push technologyl

• Consists of a protocol spec called Bayeux, JavaScript technology libraries (DOJO toolkit), and an event router

• The Server uses async techniques to “park” the request without blocked threads

• Comet programming is easy, cometd is even easier> No server-side elements need to be created

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Grizzly Comet Components

Grizzly Comet FrameworkGrizzly Comet Framework

Grizzly HTTP Grizzly HTTP

Grizzly NIO FrameworkGrizzly NIO Framework

GrizzletGrizzlet BayeuxBayeux Messages BusMessages BusContinuationContinuation

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What Is the Bayeux Protocol?• Bayeux is a negotiated, multi-transport,

JSON-based protocol for publish/subscribe asynchronous messaging between:> Web browser and server, Server and web browser> Web browser and web browser> http://svn.xantus.org/shortbus/trunk/bayeux/bayeux.html

• Part of the Dojo Cometd project> http://cometdproject.dojotoolkit.org/

• Multiple server implementations> JavaScript programming language, Java™ programming

language, perl, python

http://cometdproject.dojotoolkit.org/

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Bayeux/CometdJSON Publish/Subscribe

• JSON Messages are published on specified channels• Channel operations: connect, subscribe, unsubscribe,

etc.• Multiple transports: polling, long-polling, iframe, flash• Server-side reflector with no server-side application

possible

[ { "channel": "/some/name", "clientId": "83js73jsh29sjd92", "data": { "myapp" : "specific data", value: 100 } }]

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Software and Configuration

• Glassfish Application Server (V2 or V3 TP2)> Needs to be enabled for Comet

●Configure using GlassFish V2 admin console●Configure in NetBeans + for GlassFish V3●or Command line

– asadmin set server.http-service.http-listener.http-listener-1.property.cometSupport=true

●or Edit domain.xml in GlasFish– <http-listener id="http-listener-1" port="8080"><property

name="CometSupport" value="true" />

• Grizzly Comet APIs• A browser Client, running Javascript, support Ajax

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GlassFish V3 Comet Enable (in NetBeans)

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Enable Comet in GlassFish V2 Admin Console

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Enabling Bayeux in GlassFish• Configure web.xml

web.xml<servlet-mapping> <servlet-name>Grizzly Cometd Servlet</servlet-

name> <url-pattern>/cometd/*</url-pattern></servlet-mapping>

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Step1 : Initialize

dojo.require("dojox.cometd");

//Initialize a connection to the given Comet server: the GlassFish Grizzly Bayeux servlet

dojox.cometd.init("serverURL");

Comet Server

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Step 2: Subscribe

Client 1Client 2

dojox.cometd.subscribe(“topic”, "remote topic", "callbackFunction");• Subscribes the client to the

topic channel• Any time a message is sent to

the topic channel the callback will be invoked

Comet Server

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Step 3: Publish

Comet Server

new slide url

dojox.cometd.publish("topic", {slide: url});

a client publishes the next slide url to the topic channel

JSON message

client 1

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Step 4: Update Clients: CallBack InvokeComet Server

Client 2 Client 3

callBackFunction(slideUrl){ slide.innerHTML ="<img src='" + slideUrl + "'/>"; ... }

●The callback function is called with the published message when a new message is received●This callback function updates the browser content

● html img tag with the slide url from the published message

Update BrowserUpdate Browser

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Slideshow Chat Demo

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Grizzly Comet Framework• The Framework contains the classes required to

add support for Comet in a Web Application• Main classes to interact with (details next):

> CometEngine> CometContext> CometHandler> NotificationHandler> CometReader> CometWriter

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Grizzly Comet FrameworkHow it works

CometContextCometContext

Browser 1Browser 1 CometHandlerCometHandlerCometHandlerCometHandlerCometHandlerCometHandler

NotificationHandlerNotificationHandler

ServletServlet

Push data

filters push

send

Browser 2Browser 2

send

DatabaseDatabase ClusterCometHandlerClusterCometHandler

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Grizzly Comet in Steps• Suspend:

> addCometHandler(CometHandler)• Push:

> notify(<? extends Object>) // All suspended responses

> notify(<? extends Object>, CometHandler) // Only one

• Write: Inside CometHandler.onEvent()> PrintWriter.write(CometEvent.attachment());

• Resume: > resumeCometHandler(CometHandler)

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CometContext• A CometContext is a distribution mechanism for

pushing messages that are delivered to multiple subscribers called CometHandler (a la JMS Queue/Topic)

• All http response registered to a CometContext automatically becomes suspended, waiting for an event (a push) to happens

• A browser receives only those messages published after the client suspend its response via CometContext

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Grizzly Comet Framework• CometHandler

> A CometHandler contains the business logic of what will be pushed back to the browser

> A CometHandler might be invoked by the Container:● When a push operation happens● When a I/O operations are ready to be process

(asynchronous read or write)● When the browser close the connection

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Grizzly Comet Framework (cont.)• NotificationHandler

> An inside object that you will decide to what to do with the push operation:●Throttle: if too many push occurs simultaneously,

should we delay them?●Aggregate: should we cache push operations and

aggregate them to avoid overloading the network?●Filter: should all messages by pushed back to the

client?●Should a thread pool be used to improve the push

operation? Should a JMS backed be used to deliver the message?

> The DefaultNotificationHandler push all messages

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Four Simple Steps for Comet

• Step 1: Initialize Comet• Step 2: Define your CometHandler• Step 3: Connect to Comet Servlet: Add

CometHandler to CometContext• Step 4: Advertise changes• Step 5: More details on Client

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Step 1: Initialize CometRegister a CometContext//In TTTComet Servlet class...public void init(ServletConfig config) { ServletContext context = config.getServletContext();

contextPath = context.getContextPath() + "/TTTComet1"; CometEngine engine = CometEngine.getEngine(); CometContext cometContext = engine.register(contextPath); cometContext.setExpirationDelay(120 * 1000);

//All connections registered to a CometContext automatically becomes suspended, waiting for an event (a push) to happen

//A browser receives only those messages after the client “register “ to a CometContext

//CometContex contains references to all suspended connections (encapsulated inside a CometHandler)

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Step 2: Define a CometHandler• The CometHandler<E> interface has these methods //Invoked when CometContext.notify() is called

public void onEvent(CometEvent ce);

// Invoked when the browser close a suspended

// connection or when the suspend timeout expire

public void onInterrupt(CometEvent ce);

// Invoked when the request is suspended

public void onInitialize(CometEvent ce);

// Attach an object

// most probably the HttpServletResponse

public void attach(E e);

• Long Polling, in onEvent(event),> event.getCometContext().resumeCometHandler(this);

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CometHandler: TTTHandler

//Inside the TTTComet Servlet class...private class TTTHandler implements CometHandler<HttpServletResponse> { private HttpServletResponse response;//When a push operation happens, onEvent invoked after CometContext.notify() is called public void onEvent(CometEvent event) throws IOException { ... }

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Step 3: Connect Comet Servlet: Add CometHandler to CometContext The Servlet Get (called by iframe) protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

TTTHandler handler = new TTTHandler(); handler.attach(response); CometEngine engine = CometEngine.getEngine(); CometContext context = engine.getCometContext(contextPath); context.addCometHandler(handler); }//The GET request is now suspended, awaiting further action before a

response is returned//On first load, connects to server with a GET, not POST

(Recommended by Comet)

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Step 4: Advertise ChangesPost is called (client)<table><tr><td id="cell0"><img id="img0" src="resources/0.gif" onclick=postMe("0")></td>...</table>var url = "TTTComet1";function postMe(arg) {... var xhReq = new createXMLHttpRequest(); xhReq.open("POST", url, true); xhReq.setRequestHeader("Content-Type", "application/x-www-form-urlencoded"); xhReq.send("cell="+arg);};

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Step 4: Advertise ChangesPost is called (server)protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

String cellStr = request.getParameter("cell");PrintWriter writer = response.getWriter();......game.turn(cell);......CometEngine engine = CometEngine.getEngine();CometContext<?> context = engine.getCometContext(contextPath);context.notify(null); //attachment for CometEvent

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Step 4: Advertise Changes• POST is called via a click on one client• We get the context again, and call

context.notify() on server• This sends a NOTIFY event to the handler• The handler will now update all listening clients

> Invode onEvent implemented in CometHandler> Sends a script which includes JSON formatted data> Closes all open connections when done

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CometHandler: TTTHandler (revisit)//Inside the TTTComet Servlet class...private class TTTHandler implements CometHandler<HttpServletResponse> { private HttpServletResponse response;//When a push operation happens, onEvent invoked after CometContext.notify() is called public void onEvent(CometEvent event) throws IOException { if (CometEvent.NOTIFY == event.getType()) { PrintWriter writer = response.getWriter(); writer.write( "<script type='text/javascript'>parent.chImg(" + game.getJSON() + ")</script>\n"); writer.flush(); event.getCometContext().resumeCometHandler(this); } }

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Step 5: Client Side update (called from Server Side onEvent() in CometHandler)

function chImg(args) {var data = eval(args); // redraw the board for (i = 0; i < 9; i++) { document.getElementById("img"+i).src= "resources/"+data.board[i]+".gif"; }// -1 is unfinished, 0 is tie, 1 is X win, 2 is O win var statusMsg; if (data.win == 0) { statusMsg = "It's a tie!"; } else if (data.win == 1) {.... document.getElementById("gstatus").innerHTML = statusMsg;

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Sample JSON output

{ “win”: “-1”, “board”: [“0”, “1”, “10”,

“0”, “1”, “10”, “1”, “10, “0” ],

“turn”: “10” }

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Two Player Game: Tic Tac Toe Demo

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What is Atmosphere• Atmosphere is a POJO based framework using

Inversion of Control (IoC) to bring Comet to the masses.

• A Framework which can run on any Java based Web Server.> Automatically detects native Comet API and use it.> Fallback to one thread per connection in case no

native Comet support (ex: Tomcat 4)• Run on top of Servlet 2.5 compliant Web Server• Support Servlet 3.0 Async API

> Auto-Detect Servlet 3.0 API• Easily embeddable into other framework

> Ex: Jersey, Grails, Scala, Struts, Cometd, etc.

Comet techniques aren't standardised among Web Container

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Architecture

Servlet 3.0 Grizzly Jetty Blocking WS

Comet Portable Runtime

Jersey

Atmosphere CPR

Atmosphere plug in

Application API Atmosphere Module

Atmosphere Core

External Module

JBoss WLS

Reuse experience and code...from Grizzly Comet to Jersey!

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Getting started with Atmosphere CPR• Three API to know

> AtmosphereHandler: Your POJO representing your application. Quite simple:●When a request comes in:

onEvent(AtmosphereEvent)●When a broadcast happens:

onMesage(AtmosphereEvent)> Broadcaster: Share Data between suspended

connection.> BroadcasterFilter: Manipulate broadcasted data before

they get written

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Example: AtmosphereHandler onEvent()public AtmosphereEvent onEvent( AtmosphereEvent<HttpServletRequest, HttpServletResponse> event) throws IOException {

HttpServletRequest req = event.getRequest(); HttpServletResponse res = event.getResponse(); ... if (req.getMethod().equalsIgnoreCase("GET")) { ... res.getWriter().flush(); event.suspend(); } if (req.getMethod().equalsIgnoreCase("POST")) { synchronized (game) {... Broadcaster bc = event.getBroadcaster(); String response = game.getJSON(); // broadcast the updated game state bc.broadcast(response); writer.flush(); if (game.win() != -1) { game = new TTTGame(); } } } return event; }

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Example: AtmosphereHandler onMessage() public AtmosphereEvent onMessage( AtmosphereEvent<HttpServletRequest, HttpServletResponse> event) throws IOException { // Client closed the connection. if (event.isCancelled()) { return event; }

String response = (String) event.getMessage(); response = "<script type='text/javascript'>parent.chImg(" + response + ")</script>\n"; PrintWriter writer = event.getResponse().getWriter(); writer.write(response); writer.flush();

if (!event.isResumedOnTimeout()) { event.resume(); } return event; }

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Getting started with Atmosphere Core• All the power of REST and Jersey functionality,

plus> @Suspend : suspend a response> @Resume: resume a response> @Broadcast: Broadcast events> @Cluster: Cluster your Broadcast> @BroadcasterFilter

• Subset of Atmosphere CPR available via injection> AtmosphereEvent> Broadcaster

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Example: Writing a REST application @Suspend // the returned String will be written and then response suspended @GET @Produces("text/html") public String cometGet() { return "\n"; } @Broadcast // The returned String will be broadcasted to all suspended response. @Consumes("application/x-www-form-urlencoded") @POST @Produces("text/html") public String cometPost( @FormParam("action") String action, @FormParam("name") String name, MultivaluedMap form) {

if ("login".equals(action)) { return BEGIN_SCRIPT_TAG + toJsonp("System Message", name + " has joined.") + END_SCRIPT_TAG; } else if ("post".equals(action)) { return BEGIN_SCRIPT_TAG + toJsonp(name, form.getFirst("message")) + END_SCRIPT_TAG; } else { throw new WebApplicationException(422); } }

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Atmosphere Plugs In: What is that?• Set of classes and annotation that bring new

functionality> Clustering Atmosphere

●Using @terracotta, @Shoal, @JGroup> Container specific functionality

●Tomcat and Grizzly supports asynchronous I/O> Any community provided annotation/classes

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SummaryThe Asynchronous Web Revolution is Now

• The Asynchronous Web will revolutionize human interaction

• Push can scale with Asynchronous Request Processing

• Writing Games is not that complicated• With GlassFish project and Project Grizzly, the

revolution begins with your application today!• Get ready for Atmosphere and Servlet 3.0

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For More Information• Getting Started with GlassFish and Comet

> http://weblogs.java.net/blog/jfarcand/archive/2008/04/the_hitchhikers.html

> http://weblogs.java.net/blog/jfarcand/archive/2006/10/writting_a_come.html> https://atmosphere.dev.java.net/

● [email protected] or [email protected])

• Grizzly Active’s Bloggers:> Alexey: http://blogs.sun.com/oleksiys/ > Shing Wai: http://blogs.sun.com/swchan/ > John: http://weblogs.java.net/blog/johnmann/ > Sebastien: http://www2.sebastiendionne.ca > Jeanfrancois: http://weblogs.java.net/jfarcand

• Project Grizzly> http://grizzly.dev.java.net

●[email protected] & [email protected]

http://weblogs.java.net/blog/jfarcand/archive/2006/10/writting_a_come.html

https://atmosphere.dev.java.net/

mailto:[email protected]

mailto:[email protected]

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Doris Chen [email protected] Engineer/Technology Evangelist