BPEL uses an XML-based vocabulary that allows us to specify and describe business processes. BPEL is a programming language. Most development environments that support BPEL, such as Oracle JDeveloper, IBM WebSphere Integration Developer, or Eclipse usually provide a visual editor, where we can compose BPEL processes by dragging and dropping the BPEL activities in a visual way. However, the majority of tools also allow a switch to the source view, where you can enter the BPEL code directly. A BPEL visual representation is generated out of BPEL code. In this chapter we will look at the BPEL code.

With BPEL, you can describe business processes in two distinct ways:

This chapter focuses on executable business processes. Abstract business processes are covered in the next chapter.

Executable business processes are processes that comprise a set of services. When we describe a business process in BPEL, we actually define a new service that is a composition of existing services. The interface (WSDL) of the new BPEL composite service uses a set of port types, through which it provides operations like any other service. To invoke a business process described in BPEL, we must invoke the resulting composite service.

In a typical scenario, the BPEL business process receives a request. To fulfill it, the process then invokes the involved services and finally responds to the original caller. Because the BPEL process communicates with other services, it relies heavily on the WSDL description of the services invoked by the composite BPEL service.

Anyone developing BPEL processes requires a good understanding of WSDL and other related technologies. BPEL introduces WSDL extensions, which enable us to accurately specify relations between several services in the business process. These relations are called partner links. The following figure shows a BPEL process and its relation to services (partner links):

Any BPEL process specifies the exact order in which participating services should be invoked. This can be done sequentially or in parallel. With BPEL, we can express conditional behavior; for example, a service invocation can depend on the value of a previous invocation. We can also construct loops, declare variables, copy, assign values, define fault handlers, and so on. By combining all these constructs, we can define complex business processes in an algorithmic manner. We can describe deterministic as well as non-deterministic flows. Because business processes are essentially graphs of activities, it is sometimes useful to express them using a modeling notation, such as BPMN (Business Process Modeling Notation) or UML (Unified Modeling Language) activity diagrams. BPEL is not a modeling language for processes, but an execution language for processes and orchestration of services. To understand how business processes are defined in BPEL, we look at the core concepts in the next section.