[GitHub] [druid] kfaraz commented on pull request #13076: Independent Supervisor Creation/Updation/Termination

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kfaraz commented on PR #13076:
URL: https://github.com/apache/druid/pull/13076#issuecomment-1243746717

   @AmatyaAvadhanula , thanks for taking a stab at this. Could you share some more details regarding why these changes are needed? I see that this would increase the possible concurrency of supervisor update operations, but some more details would be helpful.
   
   If the intention is higher concurrency, I feel that the approach can be improved. Maps such as `autoscalers` etc are already `ConcurrentHashMap`s and don't really need separate locks. You could just perform atomic updates on them using methods like `computeIfAbsent()`.
   
   But the real concern is the purpose of the original global `lock`. AFAICT from the code, that `lock` is there less to protect the supervisors from each other and more to ensure that while the manager itself is being started or stopped, no other action should occur. Changing the usage of this `lock` can have negative effects.
   
   For example, what would happen when the `SupervisorManager` is stopping and a new supervisor is submitted? You could try using `globalLock` on some of the operations inside `createOrUpdateSupervisorInternal()` but that could still lead to a weird state where the stopping thread is say, removing stuff from the `autoscalers` map but another thread keeps adding to it.
   
   A better implementation would be to maintain states of the `SupervisorManager` rather than a single `started` boolean. And while it is in `STARTING` or `STOPPING` state, every other action would have to be failed (or maybe just blocked?).


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