Data and behavioral contracts for web services

The recent trend in Web services is fostering a computing scenario where loosely coupled parties interact in a distributed and dynamic environment. Such interactions are sequences of XML messages and in order to assemble parties – either statically or dynamically – it is important to verify that the “contracts” of the parties are “compatible”. The Web Service Description Language (WSDL) is a standard used for describing one-way (asynchronous) and request/response (synchronous) interactions. Web Service Conversation Language extends WSCL contracts by allowing the description of arbitrary, possibly cyclic sequences of exchanged messages between communicating parties. Unfortunately, neither WSDL nor WSCL can effectively define a notion of compatibility, for the very simple reason that they do not provide any formal characterization of their contract languages. We define two contract languages for Web services. The first one is a data contract language and allow us to describe a Web service in terms of messages (XML documents) that can be sent or received. The second one is a behavioral contract language and allow us to give an abstract definition of the Web service conversation protocol. Both these languages are equipped with a sort of “sub-typing” relation and, therefore, they are suitable to be used for querying Web services repositories. In particular a query for a service compatible with a given contract may safely return services with “greater” contract.