Different from traditional software applications, Web services are defined independently from any execution context. Their consequent inherent autonomy and heterogeneity fostered by a continuous evolution in business context and requirements make ensuring the execution of a composite service as intended a challenging task. This chapter presents a reengineering approach to ensure transactional reliability of composite services. Contrary to previous approaches which check correctness properties based on the composition model, the authors start from executions' log to improve services' recovery mechanisms. Basically, the chapter proposes a set of mining techniques to discover the transactional behavior from an event based log. Then, based on this mining step, the authors use a set of rules in order to improve services' reliability.