Stream processing topologies which are configured to use EXACTLY_ONCE processing guarantee cannot be tested with the TopologyTestDriver. Tests usually crash with java.lang.IllegalStateException: MockProducer hasn't been initialized for transactions within the second call to TopologyTestDriver.pipeInput(), the first call works fine.
Changing the processing guarantee to AT_LEAST_ONCE makes tests pass.
This is a problem because it is expected that proper processor topologies can be successfully tested using TopologyTestDriver, however TopologyTestDriver can't handle EXACTLY_ONCE and crashes during tests. To a developer, this usually means that there is something wrong with their processor topologies.
Kafka developpers can reproduce this by adding:
to line 88 of TopologyTestDriverTest: streams/test-utils/src/test/java/org/apache/kafka/streams/TopologyTestDriverTest.java
Originally reported on the ML.