Kafka streams testing: A deep dive

Even though Kafka streams "come bundled" with TopologyTestDriver, which makes streaming applications testing quite enjoyable, it's not so simple: there are limitations and certain classes of defects that this technology doesn't match. For integration tests, you can check the behavior of a real cluster but it's not simple: we're entering a slippery road of asynchronous tests with timeouts, flakiness, and unsolved questions. Conclusion: you need to use both approaches + head.