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https://issues.apache.org/jira/browse/FLINK-6612?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16017093#comment-16017093
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ASF GitHub Bot commented on FLINK-6612:
---------------------------------------

Github user StefanRRichter commented on the issue:

    https://github.com/apache/flink/pull/3939
  
    Thanks for your work @tillrohrmann ! I merged this manually. Please close 
this PR and the jira.


> ZooKeeperStateHandleStore does not guard against concurrent delete operations
> -----------------------------------------------------------------------------
>
>                 Key: FLINK-6612
>                 URL: https://issues.apache.org/jira/browse/FLINK-6612
>             Project: Flink
>          Issue Type: Bug
>          Components: Distributed Coordination, State Backends, Checkpointing
>    Affects Versions: 1.3.0, 1.4.0
>            Reporter: Till Rohrmann
>            Assignee: Till Rohrmann
>            Priority: Blocker
>             Fix For: 1.3.0, 1.4.0
>
>
> The {{ZooKeeperStateHandleStore}} does not guard against concurrent delete 
> operations which could happen in case of a lost leadership and a new 
> leadership grant. The problem is that checkpoint nodes can get deleted even 
> after they have been recovered by another 
> {{ZooKeeperCompletedCheckpointStore}}. This corrupts the recovered checkpoint 
> and thwarts future recoveries.
> I propose to add reference counting to the {{ZooKeeperStateHandleStore}}. 
> That way, we can monitor how many concurrent processes have a hold on a given 
> checkpoint node. Only if the reference count reaches {{0}}, we are allowed to 
> delete the checkpoint node and dispose the checkpoint data.
> Stephan proposed to use ephemeral child nodes to track the reference count of 
> a checkpoint node. That way we are sure that locks on the a checkpoint node 
> are released in case of {{JobManager}} failures.



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