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https://issues.apache.org/jira/browse/BEAM-11048?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17549076#comment-17549076
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Danny McCormick commented on BEAM-11048:
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This issue has been migrated to https://github.com/apache/beam/issues/20684
> Add alternate Sorting transform as an implementation of CombineFn
> -----------------------------------------------------------------
>
> Key: BEAM-11048
> URL: https://issues.apache.org/jira/browse/BEAM-11048
> Project: Beam
> Issue Type: Improvement
> Components: extensions-java-sorter
> Reporter: Claire McGinty
> Priority: P3
> Labels: Clarified
> Time Spent: 1h
> Remaining Estimate: 0h
>
> My team has been using the
> [SortValues|https://github.com/apache/beam/blob/master/sdks/java/extensions/sorter/src/main/java/org/apache/beam/sdk/extensions/sorter/SortValues.java]
> transform in `extensions-java-sorter` to sort pre-grouped values by a
> secondary sorter key. However, for large key groups, we've run into many OOM
> issues and have to increase disk size quite a bit to accommodate the larger
> key groups spilling to disk, even if there are only a few large key groups
> and most fit in memory.
> I drafted a new iteration of a Sorter that's a distributed merge-sort
> implemented as a `CombineFn`: each Accumulator maintains an always-sorted
> list of elements, and those Accumulators can be merged simply by zipping
> their lists together. This has the extra advantage that `extractOutput` can
> be lazily evaluated as a merging Iterator rather than as a fully materialized
> list. I also observed that this implementation is able to scale more
> effectively than the old SortValues, and for several use cases where
> `SortValues` ran OOM, the CombineFn-based implementation was able to complete
> using only the default Dataflow disk specs.
> Finally, from an API perspective, I think it's a little easier to use,
> because the user doesn't have to extract the sortKey out into the PCollection
> itself, but instead provide a function mapping each element type T to its
> sort key K, which will be evaluated inside the combiner. So I think in that
> sense it's more intuitive and similar to a Comparator-style sort.
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