Requirements Modelling and Specification for Service Oriented Architecture

2.6. Practical Principles for Developing, Maintaining and Exploiting SOA

To convert existing systems to SOA involves identifying the services that are needed by the business (the real users) and then providing new interfaces that supply those services. This means that a wrapper or adapter will need to be developed that exposes the business service at the outside interface and maps the operations of that service into low level calls to the legacy API. It may well mean that a service call with the associated request message will involve taking the message apart and making multiple calls to the API. Thus, great care must be taken when wrapping existing legacy systems.

When wrapping an existing application, to use it as a service, just wrapping the existing API will not provide business services; it will just give existing systems a very minor face-lift. They may look sexier, but there is no new substance. All that is usually done is to turn the system's parameters into XML messages and the RPC-style calls to calls done using SOAP. If the original interface supported the direct user, then just exposing it as a set of operations that are part of a service will not lead to the right level of granularity for the real user.

To develop services successfully, you must understand the business, not just the computer systems. The idea is to provide services for people to do tasks that deliver value to them. These could be employees, customers, regulators, partners, suppliers, in fact any stakeholders. ...