What Constitutes a Service on the Web?

Abstract

There are many approaches to modelling and development of service-oriented systems, but there is still no convincing answer to what a (software) service is. In this chapter, we discuss the various attempts to develop a theory of services, identify aspects that have already been addressed and extract gaps. This leads us to propose the BDCM2 framework capturing behaviour, description, contracting, monitoring and mediation. For the behavioural model, we refer to the two-layer model of Abstract State Services (AS2s) capturing functional aspects of data-intensive Web services. The model of service mediators permits building complex applications, in which parts are realised by services. Furthermore, we highlight the decisive role of service ontologies for supporting the location of services as well as the capture of contractual aspects by means of service-level agreements (SLAs). Finally, we conclude that a contract is only as good as the means to monitor the agreements. While part of the research has already reached a promising level of maturity, some aspects are still in an infant state.