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https://issues.apache.org/jira/browse/KAFKA-8770?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17008118#comment-17008118
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John Roesler commented on KAFKA-8770:
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Thanks for the comment, Richard. Caching is an important consideration.
Fortunately, it will happen automatically if caching is enabled for the
stateful operator in question (via Materialized).
> Either switch to or add an option for emit-on-change
> ----------------------------------------------------
>
> Key: KAFKA-8770
> URL: https://issues.apache.org/jira/browse/KAFKA-8770
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: John Roesler
> Priority: Major
> Labels: needs-kip
>
> Currently, Streams offers two emission models:
> * emit-on-window-close: (using Suppression)
> * emit-on-update: (i.e., emit a new result whenever a new record is
> processed, regardless of whether the result has changed)
> There is also an option to drop some intermediate results, either using
> caching or suppression.
> However, there is no support for emit-on-change, in which results would be
> forwarded only if the result has changed. This has been reported to be
> extremely valuable as a performance optimizations for some high-traffic
> applications, and it reduces the computational burden both internally for
> downstream Streams operations, as well as for external systems that consume
> the results, and currently have to deal with a lot of "no-op" changes.
> It would be pretty straightforward to implement this, by loading the prior
> results before a stateful operation and comparing with the new result before
> persisting or forwarding. In many cases, we load the prior result anyway, so
> it may not be a significant performance impact either.
> One design challenge is what to do with timestamps. If we get one record at
> time 1 that produces a result, and then another at time 2 that produces a
> no-op, what should be the timestamp of the result, 1 or 2? emit-on-change
> would require us to say 1.
> Clearly, we'd need to do some serious benchmarks to evaluate any potential
> implementation of emit-on-change.
> Another design challenge is to decide if we should just automatically provide
> emit-on-change for stateful operators, or if it should be configurable.
> Configuration increases complexity, so unless the performance impact is high,
> we may just want to change the emission model without a configuration.
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