[jira] [Created] (BEAM-9824) Multiple reshuffles are ignored in some cases on Flink batch runner.

["David Morávek (Jira)" <ji...@apache.org>]

David Morávek created BEAM-9824:
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             Summary: Multiple reshuffles are ignored in some cases on Flink batch runner.
                 Key: BEAM-9824
                 URL: https://issues.apache.org/jira/browse/BEAM-9824
             Project: Beam
          Issue Type: Bug
          Components: runner-flink
    Affects Versions: 2.19.0, 2.20.0
            Reporter: David Morávek
            Assignee: David Morávek
             Fix For: 2.21.0


Multiple reshuffles are ignored in some cases on Flink batch runner. This may lead to huge performace penalty in IO connectors (when reshuffling splits).

In flink optimizer, when we `.rebalance()` dataset, is output channel is marked as `FORCED_REBALANCED`. When we chain this with another `.rebalance()`, the latter is ignored because it's source is already `FORCED_REBALANCED`, thus requested property is met. This is correct beaviour because rebalance is idempotent.

When we include `flatMap` in between rebalances -> `.rebalance().flatMap(...).rebalance()`, we need to reshuffle again, because dataset distribution may have changed (eg. you can possibli emit unbouded stream from a single element). Unfortunatelly `flatMap` output is still incorrectly marked as `FORCED_REBALANCED` and the second reshuffle gets ignored.

This especially affects IO connectors -> `FileIO.match()` returns reshuffled list of matched files -> we split each file into ranges -> **reshuffle** -> read. Ignoring the second reshuffle leads to huge perf. degradation (5m -> 2h in one of our production pipelines)



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