Prototype for Real Time Data Streaming (Data Push) Part 1

Real time data streaming from remote devices (ie. data pushing) has been a fascinating topic. In this series of blogs, I will examine how to implement a prototype to demonstrate the capability of pushing data from devices (java client) via embedded light weight web server JETTY (JETTY 7 / COMETD 2 / Bayeux Protocol) to your web browser (Javascript / JQuery). The series includes three parts:

Prototype for Real Time Data Streaming (Data Push) Part 1: maven2 generated Jetty based application

Prototype for Real Time Data Streaming (Data Push) Part 2: multi-channel subscription based web application

Prototype for Real Time Data Streaming (Data Push) Part 3: channel feeder java based application
The prerequisites for the prototype are MAVEN2, JETTY 7, JAVA SDK and JQUERY. I am doing on Ubunto 11.04. I believe it can be generalized on any linux distro. Also don’t worry too much on the minor version of all the prerequisites because MAVEN2 will take care of all the software and version dependencies. That’s why we use MVN. We just need to tell it what our goal is, and let it take care of the rest.

Maven 2 Jetty based Web Application

Get Started

I will create a server side web application for Jetty 7 by using mavern2.

 mvn archetype:generate -DarchetypeCatalog=http://cometd.org

It will present a few archetypes to choose. choose the following

4: http://cometd.org -> org.cometd.archetypes:cometd-archetype-jquery-jetty7 (2.4.3 - CometD archetype for creating a server-side event-driven web application)

Then provide some parameters like the following:

Define value for property 'groupId': : henry416      
Define value for property 'artifactId': : DeviceMonitor
Define value for property 'version': 1.0-SNAPSHOT: 
Define value for property 'package': DeviceMonitor: 
[INFO] Using property: cometdVersion = 2.4.3
[INFO] Using property: jettyVersion = 7.6.4.v20120524
[INFO] Using property: slf4jVersion = 1.6.4
....

From it, a project called DeviceMonitort is created.  We can really test drive this web application now. Here is how we start jetty embedded server:

mvn install jetty:run
....
2012-09-23 12:18:28.567:INFO:oejs.AbstractConnector:Started SelectChannelConnector@0.0.0.0:8080
[INFO] Started Jetty Server
[INFO] Starting scanner at interval of 10 seconds.

Test drive by http://localhost:8080/ from web browser:

CometD Connection Established
Server Says: Hello, World
This is just like any programming where we always start with HELLO WORLD. Don’t stop here. Let’s explore what was created:

Exploring

In web.xml, it defines two servlets:


<servlet>
<servlet-name>cometd</servlet-name>
<servlet-class>org.cometd.server.CometdServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>cometd</servlet-name>
<url-pattern>/cometd/*</url-pattern>
</servlet-mapping>

<servlet>
<servlet-name>initializer</servlet-name>
<servlet-class>DevServ.BayeuxInitializer</servlet-class>
<load-on-startup>2</load-on-startup>
</servlet>

In DevServ/BayeuxInitializer.java, it creates new HelloService(bayeux);
In DevServ/HelloService.java, it adds service : addService(“/service/hello”, “processHello”);
In processHello, it reads from input (name), writes to output (“greeting”, “Hello, ” + name), and remote.deliver(getServerSession(), “/hello”, output, null); Here /hello is the channel.

In index.jsp, it obtains context-path contextPath: ‘${pageContext.request.contextPath}’ dynamically, and pass control to application.js
In application.js, it does:

...
...
        $(window).unload(function()
        {
            cometd.disconnect(true);
        });

/// 1. configure URL for cometd protocol
        var cometURL = location.protocol + "//" + location.host + config.contextPath + "/cometd";
        cometd.configure({
            url: cometURL,
            logLevel: 'debug'
        });

/// 2. add meta listener
        cometd.addListener('/meta/handshake', _metaHandshake);
        cometd.addListener('/meta/connect', _metaConnect);

/// 3 handshake
        cometd.handshake();
...

Here is what metaHandshake do: subscribe to /hello channel, and publish { name: ‘World’ } to channel ‘/service/hello’, when the message push back, it displays $(‘#body’).append(‘<div>Server Says: ‘ + message.data.greeting + ‘</div>’);

...
 function _metaHandshake(handshake)
 {
 if (handshake.successful === true)
 {
 cometd.batch(function()
 {
 cometd.subscribe('/hello', function(message)
 {
 $('#body').append('<div>Server Says: ' + message.data.greeting + '</div>');
 });
 // Publish on a service channel since the message is for the server only
 cometd.publish('/service/hello', { name: 'World' });
 });
 }
 }
 ...

Comet / WebSocket: web server/browser support

Sometimes it is hard to keep track of which web server and browser are supporting Comet and/or WebSocket. The following is from JFarcand, the developer of Grizzly and Atmosphere framework.

Web Server Support

(LP: long-Polling HS: Http Streaming)

Server Version Native Comet Native WebSocket WebSockets LP HS JSONP
Netty 3.3.x X X X X X X
Jetty 5.x X X X
Jetty 6.x X X X X
Jetty 7.x X X X X X X
Jetty 8.x X X X X X X
GlassFish 2.x X X X X
GlassFish 3.x to 3.1.1 X X X X
GlassFish 3.1.2 X X X X X X
Tomcat 5.x X X X
Tomcat 6.x X X X X
Tomcat 7.0.26 and lower X X X X
Tomcat 7.0.27 and up X X X X X X
JBoss 5.x X X X
JBoss 6.x X X X X
JBoss 7.x X X X X
WebLogic 10.x X X X X
WebLogic 11.x and up X X X X
Resin 2.x X X X
Resin 3.x X  X X X X
WebSphere 7.x X X X
WebSphere 8.x X X X

Browser Support

Browser Version WebSockets Long-Polling Http Streaming JSONP
Firefox 3.x to 8.x X X X
Firefox 9.x to 11.x X X X X
Chrome 12.x and lower X X X
Chrome 13.x and higher X X X X
Internet Explorer 6x to 9.x X X X
Internet Explorer 10.x X X X X
Opera 10.x and lower X X
Opera 11.x X X X
Safari 4.x X X X
Safari 5.x X X X X
Android 2.x and up X X X
Safari (iOS) 1.x to 4.x X X X
Safari (iOS) 5.x X X X X

Some test on WebSocket API

Websocket is a new protocol to enable the socket-based connection between the web client and server. It will maintain some persistent connection between web client and web server. It is based HTML5.

The only problem is not all the browsers and web servers are supporting HTML5. I am using firefox 11 and chrome 18.0.1025.168 to test. It works great.

The following is a test using HTML5 and websocket:

<!DOCTYPE html>  
<meta charset="utf-8" />  
<title>WebSocket Test</title>  
<script language="javascript" type="text/javascript">  
    var wsUri = "ws://echo.websocket.org/"; var output;  
    function init() { output = document.getElementById("output"); testWebSocket(); }  
    function testWebSocket() { 
        websocket = new WebSocket(wsUri); 
        websocket.onopen = function(evt) { onOpen(evt) }; 
        websocket.onclose = function(evt) { onClose(evt) }; 
        websocket.onmessage = function(evt) { onMessage(evt) }; 
        websocket.onerror = function(evt) { onError(evt) }; }  
    function onOpen(evt) { writeToScreen("CONNECTED"); doSend("WebSocket rocks"); }  
    function onClose(evt) { writeToScreen("DISCONNECTED"); }  
    function onMessage(evt) { 
        writeToScreen('<span style="color: blue;">RESPONSE: ' + evt.data+'</span>'); websocket.close(); }
    function onError(evt) { writeToScreen('<span style="color: red;">ERROR:</span> ' + evt.data); }  
    function doSend(message) { writeToScreen("SENT: " + message);  websocket.send(message); }  
    function writeToScreen(message) { 
        var pre = document.createElement("p"); 
        pre.style.wordWrap = "break-word"; 
        pre.innerHTML = message; output.appendChild(pre); }  
    window.addEventListener("load", init, false);  
</script>  
<h2>WebSocket Test</h2>  
<div id="output"></div>  
</html>