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https://issues.apache.org/jira/browse/FLINK-21936?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17307386#comment-17307386
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Arvid Heise commented on FLINK-21936:
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Alternatively or additionally, we might want to add the keyed context to
forward channels. Then users could also use state in these tasks. For that, we
would need to encode the keygroup in the data stream.
Note that we would also need to find a way to encode splits at least for UC
recovery (maybe the source coordinator can assign unique numbers to splits?).
> Disable checkpointing of inflight data in pointwise connections for unaligned
> checkpoints
> -----------------------------------------------------------------------------------------
>
> Key: FLINK-21936
> URL: https://issues.apache.org/jira/browse/FLINK-21936
> Project: Flink
> Issue Type: Improvement
> Components: Runtime / Checkpointing
> Affects Versions: 1.13.0
> Reporter: Arvid Heise
> Assignee: Arvid Heise
> Priority: Major
>
> We currently do not have any hard guarantees on pointwise connection
> regarding data consistency. However, since data was structured implicitly in
> the same way as any preceding source or keyby, some users relied on this
> behavior to divide compute-intensive tasks into smaller chunks while relying
> on ordering guarantees.
> As long as the parallelism does not change, unaligned checkpoints (UC)
> retains these properties. With the implementation of rescaling of UC
> (FLINK-19801), that has changed. For most exchanges, there is a meaningful
> way to reassign state from one channel to another (even in random order). For
> some exchanges, the mapping is ambiguous and requires post-filtering.
> However, for point-wise connections, it's impossible while retaining these
> properties.
> Consider, {{source -> keyby -> task1 -> forward -> task2}}. No if we want to
> rescale from parallelism p = 1 to p = 2, suddenly the records inside the
> keyby channels need to be divided into two channels according to the
> keygroups. That is easily possible by using the keygroup ranges of the
> operators and a way to determine the key(group) of the record (independent of
> the actual approach). For the forward channel, we completely lack the key
> context. No record in the forward channel has any keygroup assigned; it's
> also not possible to calculate it as there is no guarantee that the key is
> still present.
> The root cause for this limitation is the conceptual mismatch between what we
> provide and what some users assume we provide (or we assume that the users
> assume). For example, it's impossible to use (keyed) state in task2 right
> now, because there is no key context, but we still want to guarantee
> orderness in respect to that key context.
> For 1.13, the easiest solution is to disable channel state in pointwise
> connections. For any non-trivial application with at least one shuffle, the
> number of pointwise channels (linear to p) is quickly dwarfed by all-to-all
> connections (quadratic to p). I'd add some alternative ideas to the
> discussion.
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