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Dealing with changes in processes has become unavoidable in deploying workflow. To improve the ability of workflow management in dealing with changes has emerged as a hot research topic in the area of workflow management. According to the changeable characteristics, workflow processes are categorized into dynamic, adaptive and flexible processes. In this paper, we primarily deal with the flexible processes. Flexibility is the ability of workflow process to execute on the basis of a loosely, or partially specified model, where the full specification of the model is made at runtime, and may be unique to each instance. To provide full support for flexibility, we propose a simple but applicable flexible workflow model based on ECA rules and composition of activities at runtime. Furthermore, selection constraints and composition constraints are put forward to ensure the valid selection and composition of activities at runtime. We also design two algorithms, one is to automate the composition with the best concurrence which is defined as the ability of activities to be executed in parallel and the other is to verify the validness of the manual selection and composition.