BPEL is a rich XML based language for describing the assembly of a set of existing
web services into either a composite service or a business process. Once deployed,
a BPEL process itself is actually invoked as a web service.
Thus anything that can call a web service, can also call a BPEL process, including
of course other BPEL processes. This allows you to take a nested approach to
writing BPEL processes, giving you a lot of flexibility.
In this chapter we first introduce the basic structure of a BPEL process, its key
constructs, and the difference between a synchronous and asynchronous service.
We then demonstrate through the building and refinement of two example BPEL
processes, one synchronous the other asynchronous, how to use BPEL to invoke
external web services (including other BPEL processes) to build composite services.
During this procedure we also take the opportunity to introduce the reader to many
of the key BPEL activities in more detail.
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