Comet is a World Wide Web application architecture in which a web server sends data to a client program (normally a web browser) asynchronously without any need for the client to explicitly request it. It allows creation of event-driven web applications, enabling real-time interaction otherwise impossible in a browser. Though the term Comet was coined in 2006,[a] the idea is several years older, and has been called various names, including server push, HTTP push, HTTP streaming, Pushlets, Reverse Ajax, and others.

Comet applications use long-lived HTTP connections between the client and server, which the server can respond to lazily, pushing new data to the client as it becomes available. This differs from the original model of the web, in which a browser receives a complete web page in response to each request, and also from the Ajax model, in which browsers request chunks of data, used to update the current page. The effect is similar to applications using traditional Ajax with polling to detect new information on the server, but throughput is improved and latency and server load are decreased.

Comet does not refer to any specific technique for achieving this user-interaction model, but encompasses all of them-though it implies the use of browser-native technologies such as JavaScript as opposed to proprietary plugins. Several such techniques exist, with various trade-offs in terms of browser support and side effects, latency, and throughput
Alex Russell has coined a term for a flavour of Ajax that’s been getting more attention of late. Comet describes applications where the server keeps pushing - or streaming - data to the client, instead of having the browser keep polling the server for fresh content. Alex identifies several buzzworthy examples:

* GMail’s GTalk integration
* Jot Live
* Renkoo
* cgi:irc
* Meebo

For More details please see this links
www.ajaxian.com/archives/comet-a-new-approach-to-ajax-applications
alex.dojotoolkit.org/?p=545

D.Jayapal

Tags: Google Talk, php, comet, Ajax