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https://issues.apache.org/jira/browse/BEAM-68?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15219126#comment-15219126
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Eugene Kirpichov commented on BEAM-68:
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I think the two use cases are quite different and need different APIs. The
first use case is about the expected outputs of the pipeline (i.e.
runner-agnostic); the second is about the execution (implemented in a
runner-specific way).
I think for the first case we just need to come up with a convenient way to
encode the sharding needs of custom sinks (e.g. output filename for a
file-based sink, or say, shard of a pubsub topic for a sink that outputs to
some pubsub system), and it can probably be done as a PTransform. I think it
can actually be done using the "data-dependent sinks" API (route the data to a
destination shard, one sink per shard), BEAM-92.
The second case requires support in the Beam model.
I'm inclined to reopen BEAM-159, let me know what you think.
> 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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