Dependability and Flexibility Centered Approach for Composite Web Services Modeling Neila Ben Lakhal 1 , Takashi Kobayashi 2 , and Haruo Yokota 12 1 Tokyo Institute of Technology, Department of Computer Science 2 Tokyo Institute of Technology, Global Scientific Information, and Computing Center Abstract. The interest surrounding the Web services (WS) composition issue has been growing tremendously. In the near future, it is awaited to prompt a ver- itable shift in the distributed computing history, by making the Service-Oriented Architecture (SOA) a reality. Yet, the way ahead is still long. A careful investi- gation of a major part of the solutions proposed so far reveals that they follow a workflow-like composition approach and that they view failures as exceptional situations that need not to be a primary concern. In this paper, we claim that obey- ing these assumptions in the WS realm may constrain critically the chances to achieve a high-dependability level and may hamper significantly flexibility. Mo- tivated with these arguments, we propose a WS composition modeling approach that accepts failures inevitability and enriches the composition with concepts that can add flexibility and dependability but that are not part from the WS architec- ture pillars, namely, the state, the transactional behavior, the vitality degree, and the failure recovery. In addition, we describe a WS composition in terms of defi- nition rules, composability rules, and ordering rules, and we introduce a graphical and a formal notation to ensure that a WS composition is easily and dynamically adaptable to best suit the requirements of a continuously changing environment. Our approach can be seen as a higher level of abstraction of many of the cur- rent solutions, since it extends them with the required support to achieve higher flexibility, dependability, and expressiveness power. 1 Introduction The interest surrounding the Web services composition (WSC) issue has been growing tremendously. To date, it has triggered a substantial amount of research e orts. A sig- nificant progress exemplified with the emergence of a myriad of specification languages (e.g., BPEL [1] and OWL-S [2]) and of a whole panoply of WSC strategies including dynamic WSC models (e.g., eFlow [3]), declarative WSC models (e.g., SELF-SERV [4]), and semantic WSC models (e.g., SHOP2 [5]), has been noticed. Nevertheless, the WSC technology is still regarded as not mature enough and much more must happen before it reaches its apogee. A careful investigation of major part of the available solutions reveals that they share two limitations that preclude them from being solutions that meet modern IT environment requirements (i.e., high dependability and flexibility, automated discovery, composition, and enactment). The first limitation is that the Workflow-based WSC approach is at the core of the majority of the current solutions. Representative examples are eFlow platform [3], R. Meersman, Z. Tari et al. (Eds.): OTM 2006, LNCS 4275, pp. 163–182, 2006. c Springer-Verlag Berlin Heidelberg 2006