How does in-browser chat work?

Question

Just curious. How exactly does chatting in a browser work? Usually if a user goes to a web page, his/her webbrowser requests the page content. A server produces output and sends it to the user's computer. But with chatting it's a little bit the other way around (well not exactly). It's not the user requesting a chat message from some server, but rather the server that sends it directly. Now this is really simple to achieve with a "normal" server, but the thing that the server sends it to a browser directly confuses me. The posting the message part is all clear, it's simple. You just post the data to the server with for example ajax or something. But how does the other computer instantly "know" that a message has been written to it? It must obviously be the server sending it to the other computer as soon as it has been written. But somehow that doesn't compute in my brain. In my brain, the browser only request things, it doesn't just get them. How exactly do you do that?

Take google talk in gmail for example. How does that work? How is it implemented?

Answer 1

There are some push technologies, such as Comet, but they're not widely implemented. Most of the time this is accomplished via polling at some small interval with AJAX and downloading any new messages that are available since the last downloaded message.