In context of pervasive environment, the availability and reliability of Web services cannot be always guaranteed due to the dynamic and uncertain nature of these environments. Therefore, ensuring reliable and flexible service composition is one of the most difficult tasks. To overcome this problem, we propose an Event-B driven approach to easily design reliable and flexible Services compositions. We introduce a new paradigm, called dynamic transactional pattern. This new paradigm is a convergence concept of dynamic workflow patterns and advanced transactional model. It combines dynamic Control-Flow flexibility and transactional processing reliability. A composite Service is defined as a set of dynamic transactional patterns instances properly connected together. We show how the transactional behavior of a composite Service is automatically defined in an efficient way. In this work, we combine the modeling and verification activities related to formal development process supported by Event-B. The verification activity is based on theorems proving and model-checker. Afterwards, the model is validated using ProB animator in order to ensure that it behaves as expected.

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