On Design Principles for Realizing Adaptive Service Flows with BPEL

Web service technology offers a promising approach for realizing enterprisewide and cross-organizational business applications. With the Business Process Execution Language for Web Services (BPEL, also known as WS-BPEL or BPEL4WS) a powerful language for the process-oriented composition and orchestration of Web services exists. However, BPEL flow specifications tend to be too complex, and current BPEL engines do not provide the flexibility needed to cover the broad spectrum of business processes we have to deal with in practice. Neither ad-hoc deviations form the pre-specified BPEL schema (e.g., to add or move activities for single flow instances) nor the propagation of BPEL schema changes to a collection of flow instances have been supported by BPEL engines yet. This limits their applicability to only wellstructured, rigid service flows. In this paper we address fundamental issues, which arise when enriching BPEL engines with dynamic change capabilities. We focus on the question how such changes can be realized in a correct and consistent manner and which prerequisites must be met in this context. In particular, we restrict BPEL to a reasonable subset of language elements in order to decide on these fundamental questions. By offering flexibility and adaptability in a controlled and reliable way the so promising Web Service technology will broaden its application scope significantly.