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https://issues.apache.org/jira/browse/FLINK-8533?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16454651#comment-16454651
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ASF GitHub Bot commented on FLINK-8533:
---------------------------------------
Github user StephanEwen commented on a diff in the pull request:
https://github.com/apache/flink/pull/5427#discussion_r184477785
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/checkpoint/CheckpointCoordinator.java
---
@@ -1009,6 +1013,11 @@ public boolean restoreLatestCheckpointedState(
LOG.debug("Status of the shared state registry after
restore: {}.", sharedStateRegistry);
+ // Instruct the master hooks to initialize their state
(unconditionally)
+ LOG.debug("Initializing the master hooks.");
--- End diff --
Can you elaborate a bit why this initialization is happening in all cases?
An alternative would be to have a `reset()` method or so on the master hook
that is called further below, in the `if (latest == null)` code block.
Initializing the state seems a tad bit unituitive to me here - I somehow
assume that an init function is called once, while this one here is called on
every restore.
> 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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