Chapter 2. Service Invocation

Services are self-contained units of functionality. BPEL processes specify the exact order in which participating services should be invoked—this is called orchestration. We can do this sequentially or in parallel. With BPEL, we can also express conditional behavior; for example, a service invocation can depend on the return value of a previous invocation. We will see that BPEL processes are essentially graphs of activities.

In this chapter, we will learn how to use BPEL for service invocations using sequential flows. We will develop a BPEL process, which will invoke other services and orchestrate them. This way, the BPEL process will consume services, thus building a composite application.

In traditional programming languages, we used to invoke operations sequentially, one by one. In business processes, however, we often need to execute activities in parallel. We will see that BPEL provides a relatively easy way to invoke services and execute other activities in parallel.

In this chapter, we will extend our example process from the previous chapter and learn how to invoke and compose services in a sequential and parallel manner. In this chapter, we shall:

• Understand service invocation and orchestration of services
• Develop a book warehousing process that will invoke and orchestrate services
• Understand sequential invocation and partner links
• Understand parallel invocation, parallel flows, and develop a process using parallel invocation

So let's get started.