Abstract: Composite services ought to be adaptable to changing environments and business rules. Existing service composition languages and execution engines lack variability and adaptability needed to cater for dynamic changes. In spite of many extensions,how to ensure the correctness for the dynamic adaptation of composite services remains a challenge.We proposed a mechanism for flexible evolution of composite services based on conditional Pi calculus. We extended classical Pi calculus through introducing the conditional control operator and the belonging operator, which suits the Event Condition-Action(ECA) pattern better. A method to describe composite service processes was proposed based on conditional Pi calculus and ECA. I}hrough scenario analysis of various composite service changes, eleven basic dynamic scenarios and corresponding flexible evolution patterns were proposed. Each pattern was formalized and analyzed to ensure the variability and adaptation correctness of composite services.

Key words: Conditional Pi calculus, Service adaptation, Flexible evolution pattern