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https://issues.apache.org/jira/browse/FLINK-2296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14608255#comment-14608255
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Gyula Fora commented on FLINK-2296:
-----------------------------------
The reason we made these changes for the Checkpoint committing are the
following:
Storing uncommitted changes with ids in the operator like before makes no sense
at all. If the committing is actually important this will not work because if
the operator fails before it receives the commit the state to be committed is
lost.
State committing usually involves committing some actual state, which you
either store or provide a way to access it. Since storing locally doesnt work
for the specific reasons I mentioned before, we need to send a handle to it.
Also if we abstract the actual checkpointing away like we do in the
OperatorState implementations the user is not aware when the checkpointing
happens, so he would need to mess around with the checkpointing logic as well.
Caching local states in the task managers could be an option to increase
performance I agree.
> Checkpoint committing broken
> ----------------------------
>
> Key: FLINK-2296
> URL: https://issues.apache.org/jira/browse/FLINK-2296
> Project: Flink
> Issue Type: Bug
> Components: Streaming
> Affects Versions: 0.10
> Reporter: Robert Metzger
> Assignee: Robert Metzger
> Priority: Blocker
>
> While working on fixing the failing {{PersistentKafkaSource}} test, I
> realized that the recent changes introduced in "New operator state
> interfaces" https://github.com/apache/flink/pull/747 (sadly, there is no JIRA
> for this huge change) introduced some changes that I was not aware of.
> * The {{CheckpointCooridnator}} is now sending the StateHandle back to the
> TaskManager when confirming a checkpoint. For the non-FS case, this means
> that for checkpoint committed operators, the state is send twice over the
> wire for each checkpoint.
> For the FS case, this means that for every checkpoint commit, the state needs
> to be retrieved from the file system.
> Did you conduct any tests on a cluster to measure the performance impact of
> that?
> I see three approaches for fixing the aforementioned issue:
> - keep it this way (probably poor performance)
> - always keep the state for uncommitted checkpoints in the TaskManager's
> memory. Therefore, we need to come up with a good eviction strategy. I don't
> know the implications for large state.
> - change the interface and do not provide the state to the user function
> (=old behavior). This forces users to think about how they want to keep the
> state (but it is also a bit more work for them)
> I would like to get some feedback on how to solve this issue!
> Also, I discovered the following bugs:
> * Non-source tasks didn't get {{commitCheckpoint}} calls, even though they
> implemented the {{CheckpointCommitter}} interface. I fixed this issue in my
> current branch.
> * The state passed to the {{commitCheckpoint}} method did not match with the
> subtask id. So user functions were receiving states from other parallel
> instances. This lead to faulty behavior in the KafkaSource (thats also the
> reason why the KafkaITCase was failing more frequently ...). I fixed this
> issue in my current branch.
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