[jira] [Resolved] (BEAM-6332) Avoid unnecessary serialization steps when executing combine transform

["Ismaël Mejía (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/BEAM-6332?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Ismaël Mejía resolved BEAM-6332.
--------------------------------
       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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