"A port of Prototype's Ajax.PeriodicalUpdater function to jQuery. Basically, this function polls some remote service at fairly regular internvals, and (optionally) processes the result via a callback. The period of calls will decay as long as the same response keeps coming back from the server (either in the form of repeated data or in the form of a 304 Not Modified status), which reduces the load on the server naturally. The first Ajax call happens as a page 'onReady' handler $(function), so it is safe to put the PeriodicalUpdater call anywhere on the page."

"Hookbox is the best example of that philosophy I’ve yet seen—it’s a Comet server that makes WebHook requests back to your regular application stack to check if a user has permission to publish or subscribe to a given channel."

The included instructions work for the Mac, apparently: http://markmail.org/message/qfj2ydzzrkeket7v

"I think it’s going to be interesting to see how much the ability of platforms and languages to handle Comet workloads impairs or enhances their chances. Can Ruby and PHP pull off something that’ll allow them to scale? Will Twisted Python, Erlang, or full-stack Perl finally have their days in the webdev sun? Will great implementations in Java and C# hold the dynamic languages at bay?"

"So what, then is Cometd and how does it relate to this Bayeux specification? The short answer is that Cometd is a project to provide several reference implementations of Bayeux and to evolve the spec until it’s good enough for some other group to take over maintenance. Bayeux and Cometd are two complimentary parts of a plan to tackle the complexity around building and deploying comet apps, and we’re hoping for many independent Bayeux implementations outside of the Cometd project."

"Bayeux is a protocol for transporting asynchronous messages (primarily over HTTP), with low latency between a web server and web clients."

"Bayeux specification provides only a protocol definition. Cometd is a project by the Dojo Foundation to produce a specification (Bayeux) and a set of implementations of that specification (Cometd)."

"Cometd (bayeux) plugin for jQuery Supports AJAX long-polling and script callback-polling." More info from the author can be found here: http://morglog.org/?cat=13

"We recently asked our contributors to outline the maturity of their preferred Comet server implementation. The summary of those questions are outlined here."

This should get you started:

(function($) {
$(document).ready(startComet);
function startComet() {
    $.ajax({
        url: 'myCometServer.php',
        complete: onComplete
    });
};

function onComplete(xhr, status) {
    if (status == "success") {
        var json = xhr.getResponseHeader('myCometHeader');
        $.globalEval('('+json+')');
        startComet();
    }
    else {
        // handle errors
    }

"Helps you to manage AJAX requests and responses (i.e. abort requests, block requests, order requests)."

"HTML, Ajax, GWT, .NET, iPhone and Java clients can all receive data from StreamHub. Data can be streamed into StreamHub using the Java or .NET SDKs. StreamHub's highly-scalable architecture is used for streaming live stock quotes, spread betting, real-time monitoring, instant messaging/chat, live news/sports delivery, online gaming and a whole host of Rich Internet Applications."

A tutorial for a Java-based chat client on Streamhub can be found here: http://streamhub.blogspot.com/2009/07/tutorial-building-comet-chat.html

"The browser limits you to two requests at any given time. This is part of the HTTP specification."

"A port of Prototype's Ajax.PeriodicalUpdater function to jQuery. Basically, this function polls some remote service at fairly regular internvals, and (optionally) processes the result via a callback. The period of calls will decay as long as the same response keeps coming back from the server (either in the form of repeated data or in the form of a 304 Not Modified status), which reduces the load on the server naturally. The first Ajax call happens as a page 'onReady' handler $(function), so it is safe to put the PeriodicalUpdater call anywhere on the page."

More info here: http://enfranchisedmind.com/blog/posts/jquery-periodicalupdater-ajax-polling/

function listen() {
$.get("/mylongrequestfile", {}, function(data) {
$("#mydiv").html(data);
listen(); // then launch again
}));
};

I went with the second guide; "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."

"In web development, Comet is a neologism to describe a web application model in which a long-held HTTP request allows a web server to push data to a browser, without the browser explicitly requesting it. Comet is an umbrella term for multiple techniques for achieving this interaction. All these methods rely on features included by default in browsers, such as JavaScript, rather than on non-default plugins."

"Comet streaming techniques are considered to be somewhat inferior to long polling for several reasons. The Hidden IFrame technique has two major flaws: the lack of a reliable way to handle errors and the impossibility of tracking the state of the request calling process. The XMLHttpRequest Server Push technique, is even more limited in applicability, as it only works with Gecko-based browsers. Long polling, on the other hand, is easier to implement on the browser side, and works in every browser that supports JavaScript and Ajax."

Using the iframe method.

"We don't actually loop infinitely server side. You may have noticed the logic for the while loop - if we've executed for more than 30 seconds we discontinue the loop and return nothing. This nearly eliminates the possibility of substantial memory leaks. Also, if we didn't put a cap on execution time we would need to print a "space" character and flush output buffers every iteration of the loop to keep PHP abreast to the status of this process/connection. Without output being sent PHP cannot determine if the connection was lost via connection_status() or connection_aborted(). As a result this could lead to a situation where there are an increasing number of "ghost" processes eating up server resources. Not good!"

"Sounds simple enough, but the hassles are just beginning."