Comet is a programming technique that enables web servers to send data to the client without having any need for the client to request it. This technique will produce more responsive applications than classic AJAX. In classic AJAX applications, web browser (client) cannot be notified in real time that the server data model has changed. The user must create a request (for example by clicking on a link) or a periodic AJAX request must happen in order to get new data fro the server.
I will now explain how to implement Comet with PHP programming language. I will demonstrate it on two demos which uses two techniques: the first one is based on hidden ”<iframe>” and the second one is based on classic AJAX non-returning request. The first demo will simply show the server date in real time on the clients and the second demo will display a mini-chat.
We need:
To understand how it works this schema should help: 
This script will do an infinite loop and will return the server time as long as the client is connected. Call it “backend.php”:
<?php header("Cache-Control: no-cache, must-revalidate");header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");flush(); ?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head> <title>Comet php backend</title><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head><body> <script type="text/javascript">// KHTML browser don't share javascripts between iframesvar is_khtml = navigator.appName.match("Konqueror") || navigator.appVersion.match("KHTML");if (is_khtml){var prototypejs = document.createElement('script');prototypejs.setAttribute('type','text/javascript');prototypejs.setAttribute('src','prototype.js');var head = document.getElementsByTagName('head');head[0].appendChild(prototypejs);}// load the comet objectvar comet = window.parent.comet; </script> <?php while(1) {echo '<script type="text/javascript">';echo 'comet.printServerTime('.time().');';echo '</script>';flush(); // used to send the echoed data to the clientsleep(1); // a little break to unload the server CPU} ?> </body></html>
This HTML document first load the prototype library in the ”<head>” tag, then it create the tag that will contains the server timer ”<div id=“content”></div>”, and finally it create a “comet” javascript object that will connect the backend script to the time container tag.
The comet object will create some invisible “iframe” tags (depends on the web browser). These iframes are in charge to create the background persistent http connection with the backend script. Notice: this script do not handle possible connection problems between client and server.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><title>Comet demo</title><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><script type="text/javascript" src="prototype.js"></script> </head><body><div id="content">The server time will be shown here</div> <script type="text/javascript">var comet = {connection : false,iframediv : false, initialize: function() {if (navigator.appVersion.indexOf("MSIE") != -1) { // For IE browserscomet.connection = new ActiveXObject("htmlfile");comet.connection.open();comet.connection.write("<html>");comet.connection.write("<script>document.domain = '"+document.domain+"'");comet.connection.write("</html>");comet.connection.close();comet.iframediv = comet.connection.createElement("div");comet.connection.appendChild(comet.iframediv);comet.connection.parentWindow.comet = comet;comet.iframediv.innerHTML = "<iframe id='comet_iframe' src='./backend.php'></iframe>"; } else if (navigator.appVersion.indexOf("KHTML") != -1) { // for KHTML browserscomet.connection = document.createElement('iframe');comet.connection.setAttribute('id', 'comet_iframe');comet.connection.setAttribute('src', './backend.php');with (comet.connection.style) {position = "absolute";left = top = "-100px";height = width = "1px";visibility = "hidden";}document.body.appendChild(comet.connection); } else { // For other browser (Firefox...)comet.connection = document.createElement('iframe');comet.connection.setAttribute('id', 'comet_iframe');with (comet.connection.style) {left = top = "-100px";height = width = "1px";visibility = "hidden";display = 'none';}comet.iframediv = document.createElement('iframe');comet.iframediv.setAttribute('src', './backend.php');comet.connection.appendChild(comet.iframediv);document.body.appendChild(comet.connection); }}, // this function will be called from backend.php printServerTime: function (time) {$('content').innerHTML = time;}, onUnload: function() {if (comet.connection) {comet.connection = false; // release the iframe to prevent problems with IE when reloading the page}}}Event.observe(window, "load", comet.initialize);Event.observe(window, "unload", comet.onUnload); </script> </body></html>
Here is the tar.gz archive of this demo.
Here is the online demo.
As on the above technique, we need:
This script will do two things:
<?php $filename = dirname(__FILE__).'/data.txt'; // store new message in the file$msg = isset($_GET['msg']) ? $_GET['msg'] : '';if ($msg != ''){file_put_contents($filename,$msg);die();} // infinite loop until the data file is not modified$lastmodif = isset($_GET['timestamp']) ? $_GET['timestamp'] : 0;$currentmodif = filemtime($filename);while ($currentmodif <= $lastmodif) // check if the data file has been modified{usleep(10000); // sleep 10ms to unload the CPUclearstatcache();$currentmodif = filemtime($filename);} // return a json array$response = array();$response['msg'] = file_get_contents($filename);$response['timestamp'] = $currentmodif;echo json_encode($response);flush(); ?>
This HTML document first load the prototype library in the ”<head>” tag, then it create the tag ”<div id=“content”></div>” that will contains the chat messages comming from “data.txt” file, and finally it create a “comet” javascript object that will call the backend script in order to watch for new chat messages.
The comet object will send AJAX requests each time a new message has been received and each time a new message is posted. The persistent connection is only used to watch for new messages. A timestamp url parameter is used to identify the last requested message, so that the server will return only when the “data.txt” timestamp is newer that the client timestamp.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><title>Comet demo</title> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><script type="text/javascript" src="prototype.js"></script></head><body> <div id="content"></div> <p><form action="" method="get" onsubmit="comet.doRequest($('word').value);$('word').value='';return false;"><input type="text" name="word" id="word" value="" /><input type="submit" name="submit" value="Send" /></form></p> <script type="text/javascript">var Comet = Class.create();Comet.prototype = { timestamp: 0,url: './backend.php',noerror: true, initialize: function() { }, connect: function(){this.ajax = new Ajax.Request(this.url, {method: 'get',parameters: { 'timestamp' : this.timestamp },onSuccess: function(transport) {// handle the server responsevar response = transport.responseText.evalJSON();this.comet.timestamp = response['timestamp'];this.comet.handleResponse(response);this.comet.noerror = true;},onComplete: function(transport) {// send a new ajax request when this request is finishedif (!this.comet.noerror)// if a connection problem occurs, try to reconnect each 5 secondssetTimeout(function(){ comet.connect() }, 5000);elsethis.comet.connect();this.comet.noerror = false;}});this.ajax.comet = this;}, disconnect: function(){}, handleResponse: function(response){$('content').innerHTML += '<div>' + response['msg'] + '</div>';}, doRequest: function(request){new Ajax.Request(this.url, {method: 'get',parameters: { 'msg' : request});}}var comet = new Comet();comet.connect();</script> </body></html>
Here is the tar.gz archive of this demo.
Here is the online demo.
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