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Jetty includes a servlet implementation of the Bayuex Bayeux protocol of cometd the CometD project from the Dojo Foundation.
Cometd CometD is a message bus for Ajax web applications that allows multi channel messaging between client and server - and more importantly - between server and client. The paradigm is publish/subscribe to named channels.
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There is more information on the cometd.org website, but here's a few links to get you started:
Read the page on Stress Testing Cometd for more information on how to tune and scale jetty for Bayeux.
hassle gregw(at)mortbay.com for more details (and to write them up here).