A pi-calculus based semantics for WS-BPEL

Recently, the term Web services orchestration has been introduced to address some issues related to Web services composition, that is the way of defining a complex service out of simpler ones. Several proposals for describing orchestration for business processes have been presented in the last years and many of these languages make use of concepts as long-running transactions and compensations for coping with error handling. WS-BPEL 2.0, the most credited candidate for becoming a standard, provides three different mechanisms allowing to cope with abnormal situations: exception, event and compensation handling. This complexity makes it difficult to formally define the framework, thus limiting the formal reasoning about the designed applications. In this paper we advocate that three different mechanisms for error handling are not necessary and we formalize a novel orchestration language based on the idea of event notification as the unique error handling mechanism. To this end, we formally define the three BPEL mechanisms in terms of our calculus. It is possible to take advantages of this formal description in two ways. Firstly, this language represents by itself a proposal of simplification for WS-BPEL 2.0 including an unambiguous specification. Secondly, an implementor of an actual WS-BPEL 2.0 orchestration engine could implement simply this single mechanism providing all the remaining ones by compilation. With this attempt we intend to give a concrete contribute towards the improvement of the quality of the BPEL specification, the applicability of BPEL itself and the implementation of real orchestration engines. Finally, as a case study we consider some of the hundreds of open issues met by the WS-BPEL designers and we propose a solution making use of the experience gained developing our algebra. 1 Corresponding author. E-mail: [email protected]. Preprint submitted to Elsevier Preprint 10th June 2005