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https://issues.apache.org/jira/browse/FLINK-18706?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17169377#comment-17169377
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Kostas Kloudas commented on FLINK-18706:
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[~Yumeng] if you are using an exactly-once sink, then the sink will only send
data to the outside world upon completion of the last savepoint. So the
semantics will be exactly-once end-to-end. But this of course holds only for
exactly-once sinks.
> Stop with savepoint cannot guarantee exactly-once for kafka source
> ------------------------------------------------------------------
>
> Key: FLINK-18706
> URL: https://issues.apache.org/jira/browse/FLINK-18706
> Project: Flink
> Issue Type: Bug
> Components: Connectors / Kafka, Runtime / Checkpointing
> Affects Versions: 1.10.1, 1.11.1
> Reporter: Yumeng Zhang
> Priority: Major
> Labels: pull-request-available
>
> When I run stop-with-savepoint command with my old job and submit a new job
> with the previous sync-savepoint, I find sometimes my new job will consume a
> few duplicate data. Here is my case. I have a data generation job with
> parallelism 1, which will generate long number incrementally and send the
> data to Kafka topicA which only has one partition. Then I have another
> consumer job with parallelism 1, which reads data from topicA and does
> nothing processing, just print these numbers to system out. For example,
> after doing stop-with-savepoint, my consumer job has printed sequence
> 0,1,2,3...40,41,42,43. Then I start the consumer job again from that
> sync-savepoint. It prints 41,42,43,44..., which means it has consumed some
> duplicate data.
> I think the reason is that we fail to guarantee the mutual exclusion between
> canceling source task and sending data to downstream by checkpoint lock. It
> may send some data to downstream first before sync-savepoint completed and
> then cancel the task. Therefore, We need to keep the source operator running
> in the synchronous savepoint mailbox loop for triggerCheckpoint method before
> synchronous savepoint completed and keep checking running state before
> sending data to downstream for Kafka connector.
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