[GitHub] [druid] hqx871 opened a new pull request #11938: Fix load rule and balance concurrent conflict

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hqx871 opened a new pull request #11938:
URL: https://github.com/apache/druid/pull/11938


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   ### Description
   
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   When LoadRule drops segment or BalanceSegments moves segment, the actual work will be done by asking LoadQueuePeon to load or drop segment asynchronous. This surely cause concurrent conflict problem. 
   
   For example, when BalanceSegments moves segment, it will first mark the segment to drop on fromServer and ask toServer to load, when load work done, the fromServer does drop work. If the LoadRule happens to check segment replica count after the load work done and before the drop occur, the  LoadRule will find there is  one segment to drop, it will then pick one server to drop the segment, mostly the fromServer will not picked as it marked the segment to drop and so has smaller cost -- if you use CostBalancerStrategy. Then when the fromServer does drop, two replica will totally dropped! Especially when the tier config to one replica, there will be none segment replica serving!
   
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   ### Approach
   To solve the concurrent problem, we can make a snapshot of the unloading segments. If there one segment is unloading in tier, the LoadRule should not drop the segment in the tier at this time.
   
   <hr>
   
   ##### Key changed/added classes in this PR
    * `LoadRule`
    * `UpdateCoordinatorStateAndPrepareCluster`
   
   <hr>
   
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   This PR has:
      - [x] using the [concurrency checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md) (Remove this item if the PR doesn't have any relation to concurrency.)
   - [x] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for [code coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md) is met.
   - [x] been tested in a test Druid cluster.
   


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