Kafka Streams - How to better control partitioning of internally created state store topic?

Question

State stores in Kafka Streams are created internally. State stores are partitioned by key, but do not allow to provide partitioning other than by key (to my knowledge)

Answer 1

The number of changelog topic partitions depends in the number of input topic partitions and you cannot change it, because the state is shared based on this number (ie, there is one partitions by shard).

If you want to partition changelog topic by some attribute, you must set it as key. In your case, you must set SpecificKey as message key. It's not allowed to change the partitioning because it would "break" Kafka Streams leading to incorrect result.