BPEL processes specify the order in which participating web services should be
invoked. This can be done sequentially or in parallel. With BPEL, we can express
conditional behavior, like a web service invocation, that can depend on the result of a
previous invocation. We can also construct loops, declare variables, copy and assign
values, define fault handlers, and so on.
A BPEL process consists of steps, where each step is called an activity. BPEL
supports primitive and structured activities.
For its clients, a BPEL process looks like any other web service. When we define a
BPEL process, we actually define a new web service that is a composition of existing
services. The interface of the new BPEL composite web service uses a set of port
types, through which it provides operations like any other web service. To invoke a
business process described in BPEL, we have to invoke the resulting composite web
service also known as a composite application, as defined by JBI.
BPEL also supports compensation by undoing steps in the business process that have
already completed successfully. The goal of compensation is to reverse the effects
of previous activities that have been carried out as part of a business process that is
being revoked.
Chapter 5
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Compensation is required in most business processes, which are long running
and use asynchronous communication with heterogeneous partner web services.
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