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https://issues.apache.org/jira/browse/BEAM-12654?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17476492#comment-17476492
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Kenneth Knowles commented on BEAM-12654:
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It is complex: without doing this, you don't actually have exactly once. But if
you do it properly, you have the latency of waiting for a Flink checkpoint to
write a file.
> FileIO can produce duplicates in output files
> ---------------------------------------------
>
> Key: BEAM-12654
> URL: https://issues.apache.org/jira/browse/BEAM-12654
> Project: Beam
> Issue Type: Bug
> Components: beam-model, io-java-files, runner-flink, runner-spark,
> sdk-java-core
> Reporter: Jozef Vilcek
> Priority: P2
>
> FileIO can produce duplicates in output files - depending on a runner.
> Concrete example for Spark when executing as batch:
> When using FileIO with specific number of shards, it will use default
> sharding function which is a round robin shard assignment with random seed.
> In multistage pipeline, data between stages are hold by shuffle service until
> downstream stage request it for further computations. If shuffle results
> computed with this seeded shard function are lost - e.g. shuffle service
> fails because of HW error - then Spark will attempt to recover data by
> computing them again from source data. As a result of a random seed sharding,
> this will assign different shard - and therefore key to the element.
> More details are discussed in this thread:
> https://lists.apache.org/thread.html/r5e91d1996479defbf5e896dca3cf237ee2d9b59396cb3c4edf619df1%40%3Cdev.beam.apache.org%3E
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