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This presentation (in PowerPoint) covers why we'd want to take web request processing asynchronous, what problems it solves, and two methods for implementing asynchronous processing.
WW:Moving Web Apps From Synchronous to Asynchronous Processing
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The example app is a simple application showing the use of the "execAndWait" interceptor to provide the user an intermediate "in progress" page as well as a web request which is bridged into a JMS message to be handled asynchronously in a "fire and forget" mode.