Interface Composition for Web Service Intermediaries

The use of XML as a format for message exchange makes Web services well suited for composition of heterogeneous components. However, since clients must manage differences in message schemas between services, interoperability is still a significant problem. Interoperability currently can be supported through the use of transformations provided by a Web service intermediary. However, intermediary technologies do not provide a way for clients to reason about the composition of services and intermediaries. We propose an approach to provide clients with an interface composed of schema information from a Web service and an intermediary. Composition is performed by applying rewriting rules, defined by the intermediary, to the server interface schema. This new interface takes into account what transformations are available at an intermediary. The advantage of the approach is that clients can continue to benefit from codegeneration and static type-checking offered by interface definition languages such as WSDL; while still making use of the flexibility offered by intermediary transformations. We provide the algorithmic details of composition, including a proof of correctness and an upper bound on complexity. We demonstrate the approach in the context of a Web service composition of three publicly available Web services.