Hey there! My team and I have run across a bit of a jam in our application where, given a particular setup, our Kafka Streams application never seems to start successfully, instead just getting stuck in the REBALANCING state. We've been able to get the crucial factors that cause this behavior down to a particular combination of (1) grouping, (2) windowing, (3) aggregating, and (4) foreign-key joining, with some of those steps specifying Serdes besides the default.
It's probably more useful to see a minimal example, so there's one here <https://github.com/ajablonski/streams-issue-demo/blob/master/build.gradle>. The underlying Kafka Streams version is 2.5.1. The first test should show the application eventually transition to running state, but it doesn't within the 30 second timeout I've set. Interestingly, getting rid of the 'Grouped.with' argument to the 'groupBy' function and the 'Materialized.with' in 'aggregate' in the 'StreamsConfiguration' lets the application transition to "RUNNING", though without the correct Serdes that's not too valuable. There might be a cleaner way to organize the particular flow in the toy example, but is there something fundamentally wrong with the approach laid out in that application that would cause Streams to be stuck in REBALANCING? I'd appreciate any advice folks could give! Thanks! Alex Jablonski
