To those outside the application server team, an application server is just a set of services that support the user interface programs. The user interface collects data and then sends it to the application server, where the data is processed. Depending on the result, the application server returns either the requested data or an error message. The user interface programmers do not need to know how the service interface does its job—only that it works according to the specifications.

This is the goal of a good service interface design. The implementation details should be irrelevant to those working with the services. The services are well defined and documented and the results are understood, but only the application server programmers need to know how the results are obtained.

This chapter will examine how the service interfaces are designed, from use case analysis through design specifications. The topics covered will include:

• What is a service interface?

• Design by interface

• More on JAD: developing use cases

• Turning use cases into services

• Building services out of business objects

What Is a Service Interface?

A service interface is more than just a list of function calls specified by the user interface programmers. Each interface should contain a set of standardized services that not only make sense within the context of a single application, but conform to an organization's standard application architecture. This requires each service to conform to standard naming conventions and use consistent parameter-passing and exception-handling protocols. The user interface programmer should be able to take a new service interface and quickly and easily integrate it into an application with a minimum of research and testing.