Web services have become popular as a vehicle for the design, integration, composition, reuse, and deployment of distributed and heterogeneous software. However, although industry standards for the description, composition, and orchestration of Web services have been under development, their conceptual underpinnings are not fully understood. Conceptual models for service specification are rare, as are investigations based on them. This paper presents and studies a multi-level service composition model that perceives service specification as going through several levels of abstraction. It starts from transactional operations at the lowest level and abstracts into activities at higher levels that are close to the service provider or end user. The authors treat service composition from a specification and execution point of view, where the former is about composition logic and the latter about transactional guarantees. Consequently, the model allows for the specification of a number of transactional properties, such as atomicity and guaranteed termination, at all levels. Different ways of achieving the composition properties and implications of the model are presented. The authors also discuss how the model subsumes practical proposals like the OASIS Business Transaction Protocol, Sun’s WS-TXM, and execution aspects of the BPEL4WS standard.
