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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-8533) Support MasterTriggerRestoreHook state reinitialization
Date Thu, 15 Feb 2018 08:27:00 GMT

    [ https://issues.apache.org/jira/browse/FLINK-8533?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16365234#comment-16365234

ASF GitHub Bot commented on FLINK-8533:

Github user StephanEwen commented on the issue:

    I think we are talking about two different things here:
      1. We DO need a new method on the hook interface to reinitialize the reader group. I
think the one suggested by Eron works.
      2. We DO NOT need a new method on the CheckpointCoordinator, but calling `Hook.initializeState()`
within `CC.restoreLatestCheckpointedState()` (whenever there is no checkpoint to be restored)
should work.
    ==> Let's add the Hook method, but not add an additional method to the CheckpointCoordinator.

> Support MasterTriggerRestoreHook state reinitialization
> -------------------------------------------------------
>                 Key: FLINK-8533
>                 URL: https://issues.apache.org/jira/browse/FLINK-8533
>             Project: Flink
>          Issue Type: Bug
>          Components: State Backends, Checkpointing
>    Affects Versions: 1.3.0
>            Reporter: Eron Wright 
>            Assignee: Eron Wright 
>            Priority: Major
> {{MasterTriggerRestoreHook}} enables coordination with an external system for taking
or restoring checkpoints. When execution is restarted from a checkpoint, {{restoreCheckpoint}}
is called to restore or reinitialize the external system state. There's an edge case where
the external state is not adequately reinitialized, that is when execution fails _before the
first checkpoint_. In that case, the hook is not invoked and has no opportunity to restore
the external state to initial conditions.
> The impact is a loss of exactly-once semantics in this case. For example, in the Pravega
source function, the reader group state (e.g. stream position data) is stored externally.
In the normal restore case, the reader group state is forcibly rewound to the checkpointed
position. In the edge case where no checkpoint has yet been successful, the reader group state
is not rewound and consequently some amount of stream data is not reprocessed.
> A possible fix would be to introduce an {{initializeState}} method on the hook interface.
Similar to {{CheckpointedFunction::initializeState}}, this method would be invoked unconditionally
upon hook initialization. The Pravega hook would, for example, initialize or forcibly reinitialize
the reader group state.    

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