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From "John Roesler (JIRA)" <j...@apache.org>
Subject [jira] [Created] (KAFKA-8770) Either switch to or add an option for emit-on-change
Date Thu, 08 Aug 2019 15:36:00 GMT
John Roesler created KAFKA-8770:
-----------------------------------

             Summary: 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


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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