Web Service Composition

Web Services are often invoked in isolation, meaning, a Web Service operation provided by a Web Service provider is invoked by a Web Service requester. Based on the input message, the operation provides the result in form of an output message. However, in many cases an individual invocation is insufficient to achieve a more complicated communication or execution scenario that a Web Service requester needs to achieve, especially in the case of long-running communication as discussed in Section 8.3. In this case, several Web Service operations have to be invoked in a specified order to achieve the final outcome. For example, when an invoice for a patient is to be put together, several services are invoked to retrieve the patient’s address, to obtain the line items that require payment, to compute the total amount the patient is liable for, and so on. The end state of the invoice is therefore the result of a series of service invocations. Web Service composition is the concept that provides a means for defining and executing explicitly long-running communication involving several Web Service invocations in an explicitly specified way.