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Major Description
If a task that uses a state store is in the restoring state or in a running state and the task gets rebalanced to a separate thread on the same instance, the newly created task will attempt to lock the state store director while the first thread is continuing to use it. This is totally normal and expected behavior when the first thread is not yet aware of the rebalance. However, that newly created task is effectively running a while loop with no backoff waiting to lock the directory:
- TaskManager tells the task to restore in `tryToCompleteRestoration`
- The task attempts to lock the directory
- The lock attempt fails and throws a `org.apache.kafka.streams.errors.LockException`
- TaskManager catches the exception, stops further processing on the task and reports that not all tasks have restored
- The StreamThread `runLoop` continues to run.
I've seen some documentation indicate that there is supposed to be a backoff when this condition occurs, but there does not appear to be any in the code. The result is that if this goes on for long enough, the lock-loop may dominate CPU usage in the process and starve out the old stream thread task processing.
When in this state, the DEBUG level logging for TaskManager will produce a steady stream of messages like the following:
2021-03-30 20:59:51,098 DEBUG --- [StreamThread-10] o.a.k.s.p.i.TaskManager : stream-thread [StreamThread-10] Could not initialize 0_34 due to the following exception; will retry org.apache.kafka.streams.errors.LockException: stream-thread [StreamThread-10] standby-task [0_34] Failed to lock the state directory for task 0_34
I've attached a git formatted patch to resolve the issue. Simply detect the scenario and sleep for the backoff time in the appropriate StreamThread.
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