Major Description
StreamPartitionAssigner tries to follow a sticky assignment policy to avoid expensive task migration. Currently, it does this in a best-effort approach.
We could observe a case, for which tasks did migrate for no good reason, thus we assume that the current implementation could be improved to be more sticky.
The concrete scenario is as follows:
assume we have topology with 3 tasks, A, B, C
assume we have 3 threads, each executing one task: 1-A, 2-B, 3-C
for some reason, thread 1 goes down and a rebalance gets triggered
thread 2 and 3 get their partitions revoked
sometimes (not sure what the exact condition for this is), the new assignment flips the assignment for task B and C (task A is newly assigned to either thread 2 or 3)
> possible new assignment 2(A,C) and 3-B
There is no obvious reason (like load-balancing) why the task assignment for B and C does change to the other thread resulting in unnecessary task migration.
