Reverse Ajax/Comet/Polling

chameleonworks — Sat, 19 Dec 2009 12:40:01 +0000

As most of you know, Ajax is a mechanism for a client to communicate with the server. However this has always been understood to be a one way process, where by the client is always the first to initiate a conversation with the server. This is well and good in 90% of the cases, however what of the 10%?

The remaining 10% would require the server to communicate with the client first. A prime example of this is a Share monitoring application. In most traditional applications, the only way to simulate such systems, it would require the client to constantly poll the server for updates. This solution is fine assuming you have the necessary resources and able to service the millions of requests. However, the main draw back is it’s responsiveness. By polling, there will always be a delay in receiving the latest changes from the server.

This is where ‘Comet’ comes in. Also known as ‘Reverse Ajax’, it enables the server to send events to monitoring clients. Working on the same principles as publish/subscribe designs. Clients using JavaScript will subscribe to a channel established by the ‘Comet’ server. Whenever there has been publishes against the monitoring channel, its changes are automatically pushed to the clients.

That is pretty much the crux of it for now. For references, please have a look at the following Comet implementations:

- APE (Ajax Push Engine)
- Tomcat Comet API
- Comet/Server-Push Servlet

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