Kafka Streams doesn't always spread the tasks across all available instances/threads
I have a topology which consumes a single partition topic and goes .through() a 12 partition topic. The makes 13 partitions.
I then started 2 instances of the application. I would have expected the 13 partitions to be split across the 2 instances roughly evenly (7 partitions on one, 6 partitions on the other).
Instead, one instance gets 12 partitions, and the other instance gets 1 partition.
Repro case attached. I ran it a couple times, and it was fairly repeatable.
Setup for the repro: