Asynchronous Servlets: Suspendable Requests.

Most non trivial web applications need to wait at some stage during the processing of a HTTP request, examples of waiting request handling include:

• waiting for a resource (eg. thread, JDBC Connection) to be available before processing the request.
• waiting for an application event in an Ajax Comet application (eg Chat message, price change, etc.)
• waiting for a response from a remote service (eg RESTful or SOAP call to a web service).

The current servlet API (<=2.5) supports only a synchronous call style, so that any waiting that a servlet needs to do must be with blocking. Unfortunately this means that the thread allocated to the request must be held during that wait with all its resources including kernal thread, stack memory and often pooled buffers, character converters, EE authentication context, etc. It is wasteful of system resources to be held while waiting.