Model-­driven Quality Engineering of Service-­based Systems

Web services are the building blocks of systems based on service-oriented architectures. A web service is a self-describing, open component that supports rapid composition of distributed applications. Web service definitions are used to describe the service capabilities in terms of the operations of the service and the input and output messages for each operation. Such definitions are expressed in XML by use of the Web Service Definition Language (WSDL). Unfortunately, a WSDL description only addresses the functional aspects of a web service without containing any useful description of non functional or quality of service (QoS) characteristics. This paper introduces a lightweight WSDL extension for the description of QoS characteristics of a web service. The extension is carried out as a metamodel transformation, according to principles and standards recommended by the Model Driven Architecture (MDA). The WSDL metamodel is introduced and then transformed into the Q-WSDL (QoS-enabled WSDL) metamodel. As an example application of Q-WSDL, the paper illustrates a model-driven method that exploits Q-WSDL in order to automatically predict and describe the QoS of systems based on composite web services specified by use of the Business Process Execution Language for Web Services (BPEL). The paper specifically addresses the prediction of the reliability attribute of QoS and is illustrated by use of an example application to a composite web service for travel planning.