Process-Level Composition of Executable Web Services: "On-the-fly" Versus "Once-for-all" Composition

Most of the work on automated composition of web services has focused so far on the problem of composition at the functional level, i.e., composition of atomic services that can be executed in a single request-response step. In this paper, we address the problem of automated composition at the process level, i.e., a composition that takes into account that executing a web service requires interactions that may involve different sequential, conditional, and iterative steps.We define two kinds of process-level composition problems: on-the-fly compositions that satisfy one-shot user requests specified as composition goals, and a more general form, called once-for-all compositions, whose goal is to build a general composed web service that is able to interact directly with the users, receive requests from them, and propose suitable answers. We propose a solution to these two kinds of process-level compositions, and apply the solution to the case of web services described in OWL-S. As a result, we automatically generate process-level compositions as executable OWL-S process models. We show that, while executable on-the-fly compositions can be described as standard OWL-S process models, once-for-all compositions need OWL-S process models to be extended with receive and reply constructs.