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 --> 
    <receive ... /> 
    <!-- Invoke a set of related services, one by one --> 
    <invoke ... /> 
    <invoke ... /> 
    <invoke ... /> 
    ... 
    <!-- Return the response --> 
    <reply ... /> 
  </sequence> 
</process> 

A closer look at <invoke>

With <invoke>, the BPEL process invokes operations on other services. We have to specify the following attributes:

• partnerLink: This specifies which partner link will be used
• portType: This specifies the used port type
• operation: This specifies the name of the operation to invoke (<invoke>)

When the business process invokes an operation on the service, it sends a set of parameters. These parameters are modeled as input messages with services. To specify the input message for the invocation, we use the inputVariable attribute and specify a variable of the corresponding type.

If we invoke a synchronous request/response operation, it returns a result. This result is again a message, modeled as an output message. To store it in a variable, <invoke> provides another attribute, called outputVariable.

We can also specify a name attribute. We use the name attribute to provide names for activities.

The following code excerpt shows an example of the <invoke> activity. We specify that the BPEL process should invoke the synchronous operation getBookData on port type ns2:bookstoreABPEL using the BookstoreABPEL partner link, providing the input from the BookstoreARequest variable and storing the output in the BookstoreAResponse variable, as shown in the following screenshot: