Supporting service composition and real-time execution throught characterization of QoS properties

Complex service-oriented networked applications can be built in a flexible way by appropriate composition of individual services. Services are self-contained pieces of functionality that reside in remote nodes in the network and that communicate only via the exchange of messages. Service composition must be done in compliance with the application-level requirements i.e., quality of service (QoS) constraints. In this way, not only data dependencies must be satisfied but also other non-functional execution requirements must be considered (i.e., resource limitations and timing properties). This paper describes the structure of service-based applications, and it deepens in the characterization of the QoS properties of individual services. It explores the implications of application-level QoS properties in order to select particular services. The proposed characterization presents two dimensions of QoS properties, separating data-related information (functional properties) from physical resources information (non-functional properties). This scheme is proposed and used in the iLAND project where target applications are networked systems for which QoS informaiton is relevant; the execution in these systems must comply with functional and non-functional requirements, especially timing requirements. In this project, the communication middleware performs QoS-based service composition in a time-deterministic manner. To validate the proposed characterization, a service based video application demonstrator is implemented following this scheme, and it shows to be fully compliant with the requirements of the application. It has also proved to be a simple and easy-to-use scheme.