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https://issues.apache.org/jira/browse/FLINK-3257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15743344#comment-15743344
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ASF GitHub Bot commented on FLINK-3257:
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Github user StephanEwen commented on the issue:
https://github.com/apache/flink/pull/1668
To suggest some way to fix the guarantees: To my mind, the crux lies in the
way that the feedback channel is implemented - a simple blocking queue just
does not cut it for that case. To make this proper, I think we need to do the
following:
- Have an elastic feedback channel (unbounded) with a certain memory
budget, that can spill if needed. I think it would be best implemented holding
data serialized.
- On checkpoint, one simply adds the feedback channel data (already
bytes) to the checkpoint
- The source task should probably prioritize reading from the feedback
channel, to keep it always as small as possible.
> Add Exactly-Once Processing Guarantees in Iterative DataStream Jobs
> -------------------------------------------------------------------
>
> Key: FLINK-3257
> URL: https://issues.apache.org/jira/browse/FLINK-3257
> Project: Flink
> Issue Type: Improvement
> Reporter: Paris Carbone
> Assignee: Paris Carbone
>
> The current snapshotting algorithm cannot support cycles in the execution
> graph. An alternative scheme can potentially include records in-transit
> through the back-edges of a cyclic execution graph (ABS [1]) to achieve the
> same guarantees.
> One straightforward implementation of ABS for cyclic graphs can work as
> follows along the lines:
> 1) Upon triggering a barrier in an IterationHead from the TaskManager start
> block output and start upstream backup of all records forwarded from the
> respective IterationSink.
> 2) The IterationSink should eventually forward the current snapshotting epoch
> barrier to the IterationSource.
> 3) Upon receiving a barrier from the IterationSink, the IterationSource
> should finalize the snapshot, unblock its output and emit all records
> in-transit in FIFO order and continue the usual execution.
> --
> Upon restart the IterationSource should emit all records from the injected
> snapshot first and then continue its usual execution.
> Several optimisations and slight variations can be potentially achieved but
> this can be the initial implementation take.
> [1] http://arxiv.org/abs/1506.08603
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