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https://issues.apache.org/jira/browse/BEAM-68?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15219353#comment-15219353
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Daniel Halperin edited comment on BEAM-68 at 3/31/16 5:08 AM:
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Eugene: I disagree with the premise that even the group by key trick works
without runner support. Fusion breaks and dynamic work rebalancing violate all
such assumptions. Model changes are all that will guarantee anything here,
without completely removing modularity from the system
was (Author: dhalp...@google.com):
Eugene: I disagree with the premise that even the group by key trick works
without runner support. Fusion breaks and dynamic work rebalancing violate all
such assumptions. Model changes are all that will guarantee anything here.
> Support for limiting parallelism of a step
> ------------------------------------------
>
> Key: BEAM-68
> URL: https://issues.apache.org/jira/browse/BEAM-68
> Project: Beam
> Issue Type: New Feature
> Components: beam-model
> Reporter: Daniel Halperin
>
> Users may want to limit the parallelism of a step. Two classic uses cases are:
> - User wants to produce at most k files, so sets
> TextIO.Write.withNumShards(k).
> - External API only supports k QPS, so user sets a limit of k/(expected
> QPS/step) on the ParDo that makes the API call.
> Unfortunately, there is no way to do this effectively within the Beam model.
> A GroupByKey with exactly k keys will guarantee that only k elements are
> produced, but runners are free to break fusion in ways that each element may
> be processed in parallel later.
> To implement this functionaltiy, I believe we need to add this support to the
> Beam Model.
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