One of the main goals of service-oriented design is to derive the physical service interface definitions based on the service candidates that were determined during service-oriented analysis. Service interface design is significant because the design phase is the first time that real technology is identified.
If you recall the key principles for service orientation, they need to be applied to the design of the service interfaces. Service contracts need to be standardized with each other and within a service inventory. They should be loosely coupled from their implementations, with design decisions abstracted out so that the interfaces only contain what is essential for service consumers.
Service interface design identifies internal and external exposure of the services. For example, an order posting service may need to be used externally as well as internally. This is an example of a service that may need more than one interface published for the same service. Each interface to the service may expose different operations and will require different levels of security and authentication. The differences must be determined and each interface must be designed prior to the design of the implementations.
In addition to developers, a service interface plays an important role in testing and quality assurance. Testers need the service interface to design their tests. Once the service interface is known, a test harness can be created that is capable of invoking the service that needs to be tested. Services need to be tested independently of other services as well as within the services that will be consuming it.