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https://issues.apache.org/jira/browse/KAFKA-12540?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17308091#comment-17308091
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Antony Stubbs commented on KAFKA-12540:
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Ah yes! Totally - thanks! Now we have the same thing explained three ways :) Go
go SEO! ;)
> Sub-key support to avoid unnecessary rekey operations with new key is a
> compound key of the original key + sub-field
> --------------------------------------------------------------------------------------------------------------------
>
> Key: KAFKA-12540
> URL: https://issues.apache.org/jira/browse/KAFKA-12540
> Project: Kafka
> Issue Type: New Feature
> Components: streams
> Affects Versions: 2.8.0
> Reporter: Antony Stubbs
> Priority: Major
>
> If I am, for example, wanting to aggregate by an account, and by a metric,
> and the input topic is keyed by account (and let’s say there’s massive amount
> of traffic), this will have have to rekey on account+metric, which will cause
> a repartition topic, then group by and aggregate.
> However because we know that all the metrics for an account will already
> exist on the same partition, we ideally don’t want to have to repartition -
> causing a large unneeded overhead.
>
> Ideally a new `#selectSubkey` sort of method could be introduced, which would
> force a compound key with the original.
>
> {{var subKeyStream = stream#selectSubKey(x,v->v.getField(“metric”)) <— under
> the hood this appends the returned key to the existing key, so actual key of
> next stream in memory will be (account+metric)}}
>
> Although this might break key->partition strategy, the topology shouldn’t be
> dirty at this stage still as we know we’re still co-partitioned. What can
> happen next in the topology may need to be restricted however. In this case
> we would then do a:
>
> {{subKeyStream.groupByKey().aggregate(<snip>)}}
>
> Functions other than aggregate, may need a repartition still, or maybe not -
> not sure.
>
> Similarly described quite well in this forum here:
> [http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Re-keying-sub-keying-a-stream-without-repartitioning-td12745.html]
>
> I can achieve what I want with a custom processor and state store, but this
> seems something that might be common and useful to have supported at DSL
> level.
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