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https://issues.apache.org/jira/browse/BEAM-6332?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ismaël Mejía resolved BEAM-6332.
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Resolution: Fixed
Fix Version/s: 2.11.0
> Avoid unnecessary serialization steps when executing combine transform
> ----------------------------------------------------------------------
>
> Key: BEAM-6332
> URL: https://issues.apache.org/jira/browse/BEAM-6332
> Project: Beam
> Issue Type: Improvement
> Components: runner-spark
> Reporter: Vaclav Plajt
> Assignee: Vaclav Plajt
> Priority: Major
> Fix For: 2.11.0
>
> Time Spent: 20m
> Remaining Estimate: 0h
>
> Combine transformation is translated into Spark's RDD API in
> [GroupCombineFunctions|https://github.com/apache/beam/blob/master/runners/spark/src/main/java/org/apache/beam/runners/spark/translation/GroupCombineFunctions.java]
> `combinePerKey` and `combineGlobally` methods. Both methods use byte arrays
> as intermediate state of aggregation so they can be transferred over network.
> That leads to serialization and de-serialization of intermediate aggregation
> value every time new element is added to aggregation. That is unnecessary and
> should be avoided.
> _(copied from BEAM-6214 because it explains the approach):_
> We can do much better by letting accumulators to work on user defined java
> types and only serialize accumulators when we need to send them over the
> network.
> In order to do this, we need following:
> * Acummulator wrapper -> contains transient `T` value + byte payload, that
> is filled in during serialization
> * JavaSerialization: accumulator (beam wrapper) needs to implement
> Serializable and override writeObject and readObject methods and use beam
> coder
> * KryoSerialization: we need a custom kryo serializer for accumulator wrapper
> This should be enough to hook into all possible spark serialization
> interfaces.
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