Comet

Comet: A New Approach to Ajax Applications

New medium is arising, surging across the Web in the preferred, many-to-many way: anything flows from anyone to anyone - from anywhere to anywhere - anytime. In other words, a true network like the telephone system, rather than a radiating system like radio or TV. This new medium doesn’t wait for clicks. It doesn’t need computers. It means personalized experiences not bound by a page - think of a how-to origami video channel or a 3-D furry-muckers VR space. It means information that cascades, not just through a PC, but across all forms of communication devices - headlines sent to a pager, or a traffic map popping up on a cellular phone. And it means content that will not hesitate to find you - whether you’ve clicked on something recently or not.

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.

Comet uses a modified form of the canonical Ajax architecture:

As is illustrated below, 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.

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