Tonight I went to a very interesting BOF given by Alex Russell and Joe Walker about Comet. Also known as ‘Reverse Ajax’, Comet is a way of implementing ‘push’ technology in the browser. Basically, it is just long-lived or delayed-response HTTP connections; so a client will open an HTTP connection and then this is kept open to push data to the client. It is needed because now that many sites use Ajax it means page refreshes don’t happen as much as they used to so while parts of a page may be refreshed through ajax, other parts of the page may become out of date. It is used for things like asynchronous updates in GMail, chat in Meebo and collaborative editing in Google Docs.

It looks really cool and seems to finally be the push technology that was hyped in the 90’s.

You can’t use a normal web server like Apache as it would collapse under all the connections it would need to keep open so comet must be run on a server like Jetty that supports the techonology. However, it is possible to retrofit an existing app using an external cometd server. The original server serves a page but the clients open long-lived connections to this external cometd server (eg jetty) which then routes ‘events’ from the app server to the clients. Apparently it is quite easy to convert an existing app to use this external cometd approach.

JavaOne was the first I’d heard of this technology and it sounds really cool. They showed a fancy graph that suggested there weren’t problems with scaling the technology to many thousands of concurrent connections.

It seems like this could be used to deliver ads to pages that are heavily ajax-based and so don’t see many page refreshes and therefore ad views. Maybe we need a new ‘Comet’ tag type in OpenX.

By the way, ‘Comet’ doesn’t mean anything, it is just a bad pun.