Service compositions, e.g., Internet of Things (IoT) service compositions or RESTful service compositions are widely used in the industry to enhance the interoperability and integration of their systems and applications. Testing service compositions is considered as a long and difficult activity as each service may be deployed on different servers and often requires specialised testing tools. This paper proposes an automated approach to help developers generate test cases for experimenting every service in isolation. These test cases can be later adapted or used for regression testing. This approach is based upon 4 steps that aim to: 1. extract traces from event logs, 2. gather similar behaviours to reduce the final number of test cases and to extract knowledge, which will be used while the test case generation, 3. produce generic test cases given under the form of IOTS (Input Output Transition Systems) that encode the use of mock components and provide test verdicts, 4. generate test scripts and mock components for every service. We evaluate our approach on 4 Web service compositions and show that our algorithms build effective test cases and scale well with the event log size.