Once again, Wikipedia says it best:

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. It allows creation of event-driven web applications which are hosted in the browser.


Traditionally, web pages have been delivered to the client only when the client requested it. For every client request, the browser initiates an HTTP connection to the web server, which then returns the data and the connection is closed. The drawback of this approach is that the page displayed is updated only when the user explicitly refreshes the page or moves to a new page. Since transferring entire pages takes a long time, refreshing pages introduces a long latency.

To solve this problem, Ajax can be used which allows the web browser to request only that part of the web page that has changed and update that portion accordingly. Since the overall data transferred is reduced, latency is also reduced, and overall responsiveness of the web site hosting the application increases. Further, by using asynchronous background data transfer, where the user works with partly received data as the rest of the data is being retrieved, the responsiveness of the web application can be further increased.

But this practice also suffers from the problem that the client has to request some data before it will be sent by the server. This problem becomes a major hurdle when designing applications which have to wait for some event to occur at the server side, such as some other user sending some data to the server, before it can proceed, but has no information when the event will occur.

A solution would be to design the application such that it will intermittently poll the server to find out if the event has occurred. But this is not an elegant solution as the application will waste a lot of time querying for the completion of the event, thereby directly impacting the responsiveness of the application. In addition, a lot of network bandwidth will be wasted.

A better solution would be for the server to send a message to the client when the event occurs, without the client having to ask for it. Such a client will not have to check with the server periodically; rather it can continue with other work and work on the data generated by the event when it has been pushed by the server. This is exactly what Comet sets out to achieve.

This technique has numerous applications but the main conclusion I draw from this is that DHTML has not yet reached it’s limits and has still a few good years as the main RIA platform before techologies like Fex, Silverlight and JavaFX really start to grow, gaining on it’s limitations.


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