Structure of a Service-Oriented Application

A service-oriented application is an application composed largely of services. Often, the invoked services are in a hierarchy, as Figure 1.1 illustrates.

Figure 1.1: Service-oriented application

The topmost level contains one or more integration services, each of which controls a flow of activities such as processing an applicant's request for insurance coverage. Each integration service invokes one or more business services.

The second level is composed of services that each fulfill a relatively low-level business task. For example, an integration service might invoke such business services to verify the details provided by an insurance-policy applicant. If the business services return values that are judged to mean "issue a policy," the integration service invokes yet another business service, which calculates a quote and returns the quote to the software (for example, a Web application) that invoked the service-oriented application.

The third level consists of data-access services, each of which handles the relatively technical task of reading from and writing to data-storage areas such as databases and message queues. A data-access service is most often invoked from the business layer.

Great complexity is possible. Some integration services, for example, provide different operations to different requesters, and some invoke other integration services and are said to be composed of those services. Many applications, however, fulfill the three-level model described here.