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https://issues.apache.org/jira/browse/KAFKA-3101?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15375673#comment-15375673
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Guozhang Wang commented on KAFKA-3101:
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This LGTM, also we want to get some information on how accurate the jamm
library (or any such util libraries that are compatible with Apache 2.0 license
to be included as dependency) estimate object size, i.e. whether it is safe to
use this for limiting the cache size in bytes for flushing for memory
management.
Thanks!
> Optimize Aggregation Outputs
> ----------------------------
>
> Key: KAFKA-3101
> URL: https://issues.apache.org/jira/browse/KAFKA-3101
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: Guozhang Wang
> Labels: architecture
> Fix For: 0.10.1.0
>
>
> Today we emit one output record for each incoming message for Table /
> Windowed Stream Aggregations. For example, say we have a sequence of
> aggregate outputs computed from the input stream (assuming there is no agg
> value for this key before):
> V1, V2, V3, V4, V5
> Then the aggregator will output the following sequence of Change<newValue,
> oldValue>:
> <V1, null>, <V2, V1>, <V3, V2>, <V4, V3>, <V5, V4>
> where could cost a lot of CPU overhead computing the intermediate results.
> Instead if we can let the underlying state store to "remember" the last
> emitted old value, we can reduce the number of emits based on some configs.
> More specifically, we can add one more field in the KV store engine storing
> the last emitted old value, which only get updated when we emit to the
> downstream processor. For example:
> At Beginning:
> Store: key => empty (no agg values yet)
> V1 computed:
> Update Both in Store: key => (V1, V1), Emit <V1, null>
> V2 computed:
> Update NewValue in Store: key => (V2, V1), No Emit
> V3 computed:
> Update NewValue in Store: key => (V3, V1), No Emit
> V4 computed:
> Update Both in Store: key => (V4, V4), Emit <V4, V1>
> V5 computed:
> Update NewValue in Store: key => (V5, V4), No Emit
> One more thing to consider is that, we need a "closing" time control on the
> not-yet-emitted keys; when some time has elapsed (or the window is to be
> closed), we need to check for any key if their current materialized pairs
> have not been emitted (for example <V5, V4> in the above example).
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