Abstract: A system for testing of an application programming interface (API) is disclosed. The system includes a usage data collection source to track a trace identity associated with at least one message exchange pattern of the API using an end to end tracing (E2E) technique, captures usage data of an application in one or more modes, wherein the usage data corresponds to the trace identity. A usage data selection subsystem selects a predefined strata of usage data. A test case generation learning subsystem generates multiple test cases. A mocking subsystem use the trace identity to simulate the at least one message pattern associated with the usage data from a current version of the application against an updated version of the application. A comparison subsystem records an actual result, compares the actual result with an expected result and generates a test result upon comparison of the actual result with the expected result.