How does incoming mail notification on Gmail works?

Question

I'm wondering how it's implemented in Gmail, that every time you receive e-mail, the list of mails is automatically refreshed. It looks like the server is sending some kind of event to the browser, but how is it possible? Or maybe it’s simle: the browser ask the server for new messages every let’s say 2 seconds? But it would probably kill the performance…

Anyone have some ideas?

EDIT: OK, so if it's the simple answer, how do they manage performance? When I send an email from an other account to the gmail account the view is "refreshed" almost instantly. You were saying about a simple function that returns true / false, but it must have some logic (db connection or reads some files). How they manage it?

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See also: How is GMail Chat able to make AJAX requests without client interaction?

Answer 1

Dont know exactly which technoloy Gmail uses, but the concept is to open a channel - using reverse AJAX, comet or sprocket based techniques.

Think of it as the client requesting the server for data, but the server does not return for one minute unless it has new mail. Using this technique, the client can almost show the results in a real time manner and it does not have to poll every 2 secs. Makes sense?

Answer 2

gmail is, in fact, polling the server for updates. Not as often as every two seconds, though. That would be madness. A bit of testing with Tamper Data makes it look like maybe every 20 seconds, though there seem to be multiple events going through that confuse it a bit.

Regarding your edit, I imagine they might have a last-activity timestamp on the account tracking in their database, with the client polling query retrieving that via Ajax and comparing with its last sync to determine whether it needs to do a full update.