Comet: server pushed Ajax

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

As is illustrated above, Comet applications can deliver data to the client at any time, not only in response to user input. The data is delivered over a single, previously-opened connection. This approach reduces the latency for data delivery significantly.

The architecture relies on a view of data which is event driven on both sides of the HTTP connection. Engineers familiar with SOA or message oriented middleware will find this diagram to be amazingly familiar. The only substantive change is that the endpoint is the browser.

While Comet is similar to Ajax in that it’s asynchronous, applications that implement the Comet style can communicate state changes with almost negligible latency. This makes it suitable for many types of monitoring and multi-user collaboration applications which would otherwise be difficult or impossible to handle in a browser without plugins.

source: "Ajaxian.com"