Jetty 6’s Continuation Mechanism for Ajax

Dietrich Kappe, Thursday, April 20, 2006 @ 9:00 am

I've touched on the topic of updates and asynchronous processing before. My preferred method of performing updates between the browser and server is via a polling mechanism that returns quickly. An alternative is to open up an XHR connection and keep it open to wait for a response (or a timeout). I don't like this method because it is wasteful in terms of sockets and threads; also, it is likely to stress stateful firewalls, load balancers, etc., and may break in lots of client environments.

Nevertheless, if you want to keep a connection open for notification initiated by the server, this is the way for now. And the Jetty 6 server has at least addressed the thread issue with Continuations.

Behind the scenes, Jetty has to be a bit sneaky to work around Java
and the Servlet specification as there is no mechanism in Java to
suspend a thread and then resume it later. The first time the request
handler calls continuation.getEvent(timeoutMS) a RetryReqeuest runtime
exception is thrown. This exception propogates out of all the request
handling code and is caught by Jetty and handled specially. Instead of
producing an error response, Jetty places the request on a timeout
queue and returns the thread to the thread pool.

When the timeout expires, or if another thread calls
continuation.resume(event) then the request is retried. This time, when
continuation.getEvent(timeoutMS) is called, either the event is
returned or null is returned to indicate a timeout. The request handler
then produces a response as it normally would.

Sockets are still consumed, though. Hopefully the next servlet specification will address some of these issues. Until that time, this may be a good workaround.

Still, my preference is to keep everything except for the display logic on the server side, and that includes handling complex communication with async processing.

Update 1: ActiveMQ can make use of Jetty 6's continuation mechanism.

Update 2: Greg Wilkins has some more extensive thoughts on using Jetty 6 to scale Ajax apps.

agile ajax

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