[
https://issues.apache.org/jira/browse/FLINK-2296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14608264#comment-14608264
]
Stephan Ewen commented on FLINK-2296:
-------------------------------------
I think there is a misunderstanding on what the "commit" messages currently
are. They are currently a "best effort" notification of completed checkpoints.
There are no guarantees in the delivery in the presence of any failures after a
completed checkpoint.
Also, and form of serious distributed commit activity would need a 2-phase or
consensus protocol, so these messages are not really suitable for that.
Committing does not require the state in many cases. For many interactions with
the outside world, the actual checkpointing would handle the state, and the
"post commit" notification would require mainly a something like a transaction
ID.
We may want to rename them from "commit" to "notifyCheckpointComplete", though.
I think we have a classical case here of inappropriate communication and
description. Both documentation on what the model of the "commit" messages is,
as well as and clear description of the changes performed to the checkpointing
mechanism.
> 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 {{CheckpointCoordinator}} 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.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
