How does gmail check gtalk statuses in real time?

How is it that if I have thousands of contacts (let's suppose) all around the world and one of them changes their status to away or becomes i开发者_JS百科dol that it will change immediately in my browser?

It isn't instant really, there is a small delay, basically when you load the gmail page in your browser you also download a javascript file that refreshes the content dynamically via ajax. Similarly if a contact of yours changes gtalk status and you're using the gtalk client in gmail that change will be reflected after the next time the page you're viewing asks the server for updates. It's just constantly checking with the server for changes (the event oriented paradigm isn't really prevalent on the web).

I'm not sure of the exact mechanism gmail uses, but a fairly dumb way would be to have the page poll (via XMLHTTPRequest, aka AJAX call) the servers every X seconds for a change in contact statuses since N seconds ago... then apply those changes.

Google chat system is based upon XMPP protocol and Gmail chat block is just like another XMPP client (similar to gtalk,pidgin,psi for desktops). XMPP runs over browser using Bosh extension. Though i m sure google must have hacked to get it working in their own way, but underlying idea is still the same.

In short, when one of your contact update his/her status, it is being pushed to the google chat xmpp servers which in turn pushes that information to your gmail chat client.