Kafka streams testing: A deep dive

Even though Kafka Streams "come bundled" with TopologyTestDriver, which makes streaming applications unit-testing quite enjoyable, it's not so simple: there are limitations and certain classes of defects that this technology is blind to. With integration tests, you can check the behavior of a real cluster but it's even less simple: we're entering a slippery road of asynchronous tests with timeouts, flakiness, and unsolvable questions.

Conclusion: you need to use both approaches and your head.