How does Facebook chat avoid continuous polling of the server?

Question

I am trying to understand how Facebook's chat feature receives messages without continuously poling the server.

Firebug shows me a single GET XmlHttpRequest continuously sitting there, waiting for a response from the server. After 5 minutes, this never timed out.

How are they preventing timeout?

An AJAX request can just sit there like that indefinitely, waiting for a response?

Can I do this with JSONRequest? I see this at json.org:

JSONRequest is designed to support duplex connections. This permits applications in which the server can asynchronously initiate transmissions. This is done by using two simultaneous requests: one to send and the other to receive. By using the timeout parameter, a POST request can be left pending until the server determines that it has timely data to send.

Or is there another way to let an AJAX call just sit there, waiting, besides using JSONRequest?

Answer 1

Facebook uses a technique which is now called Comet to push messages from the server to the client instead of having the client poll the server.

There are many ways that this can be implemented, with XMLHttpRequest long polling being just one option. The principle behind this method is that the client sends an ordinary XMLHttpRequest but the server doesn't respond until some event happens (such as another user sending a message), so the client is forced to wait. When the client receives a response (or if the request times out) the client simply creates a new request so that it always has one open request to the server.