Pattern-Based Design of an Asynchronous Invocation Framework for Web Services

Asynchronous invocations are needed in the context of distributed object frameworks to prevent clients from blocking during remote invocations. Popular Web Service frameworks offer only synchronous invocations (over HTTP). An alternative are messaging protocols but these implement a different communication paradigm. When client asynchrony is not supported, client developers have to build asynchronous invocations on top of the synchronous invocation facility. But this is tedious, error-prone, and might result in different remote invocation styles used within the same application. We present a number of patterns for asynchronous invocations and explain how these patterns can be used to build asynchronous invocation facilities for Web Service frameworks. We exemplify this approach by explaining the design and implementation of an asynchronous invocation framework for Apache Axis.