Understanding sequential invocation

Within the <process> element, a BPEL process will usually have the top-level <sequence> element. Within the sequence, the process will first wait for the incoming message to start the process. This wait is usually modeled with the <receive> construct. It could also be modeled with the <pick> activity, if we expect more than one incoming message. We will explain the <pick> activity in Chapter 10, Events and Event Handlers.

Then, the process will invoke the related services using the <invoke> construct. Such invocations can be done sequentially or in parallel. If we want to make them sequential, we simply write an <invoke> activity for each invocation, and the services will be invoked in that order. Finally, the process will return a response using <reply>. This is shown in the following code excerpt:

<process ...> 
  ... 
  <sequence> 
    <!-- Wait for the incoming request to start the process...