A: 

AppEngine only supports HTTP and limits the response time (which makes Comet a no-go).

The only (supported) way is XMPP, but it only supports BOSH protocol, which does not facilitate push from server to clients.

It seems that your only way would be polling.

Peter Knego  2010-10-30 09:22:03
Comet is coming; it's currently available to trusted testers.
Wooble  2010-10-31 01:38:35
That's great news. Websockets maybe?
Peter Knego  2010-10-31 17:14:02
+1  A: 

Partychat an example of an XMPP-based chatroom running on App Engine. And it's open-source.

Jason Hall  2010-10-30 11:53:25

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