Chapter 7 - The Service Interface— Contract

Abstract

To facilitate a service interaction, an agreement must exist. This agreement (often referred to as the “contract”) is defined by the service interface, and noncompliance results in an inability to process a request and possibly a fault message returned to consumers. The service interface follows legal contract analogy, with a buyer (the consumer), and a seller (the service). The contract (WSDL) also includes terms and conditions (XML Schemas). As the consumer and service execute in compliance with the contract, they exchange messages (XML). The design of the Web service interface is possibly the single most important activity for service design. The designer or architect will need to identify the type of service interaction and the message exchange patterns (MEPs), determine the needed service functionality as service operations, and then formalize the result of those operations as message content. Once the service interface has been designed, design of the service behavior can be matched to the service interface. MEPs can vary from request-reply to an asynchronous fire-and-forget, but the interaction and exchange of information is constant. The consumer request is expressed in the body of a request message, and the service response is expressed in a reply message (assuming a request-reply MEP).