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https://issues.apache.org/jira/browse/FLINK-35076?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17931219#comment-17931219
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Piotr Nowojski commented on FLINK-35076:
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Hi, selecting {{mean}} instead of {{min}} doesn't sound like a good solution.
Watermark alignment's purpose is to slow down records processing if processing
of those records doesn't have any affect on the result. In your case those
partitions that are not emitting records are also blocking the watermark
progression for the downstream operators so it shouldn't matter that we are
blocking other partitions. It's actually what we want to happen...
... unless you have a backlog of messages that needs to be processed. In that
case you might be experiencing
https://issues.apache.org/jira/browse/FLINK-37399 .
Either way, you probably should just extend the max allowed watermark drift.
> Watermark alignment will cause data flow to experience serious shake
> --------------------------------------------------------------------
>
> Key: FLINK-35076
> URL: https://issues.apache.org/jira/browse/FLINK-35076
> Project: Flink
> Issue Type: Improvement
> Components: Runtime / Coordination
> Affects Versions: 1.16.1
> Reporter: elon_X
> Priority: Major
> Attachments: image-2024-04-10-20-15-05-731.png,
> image-2024-04-10-20-23-13-872.png, image-2024-04-10-20-25-59-387.png,
> image-2024-04-10-20-29-13-835.png
>
>
> In our company, there is a requirement scenario for multi-stream join
> operations, we are making modifications based on Flink watermark alignment,
> then I found that the final join output would experience serious shake.
> and I analyzed the reasons: an upstream topic has more than 300 partitions.
> The number of partitions requested for this topic is too large, causing some
> partitions to frequently experience intermittent writes with QPS=0. This
> phenomenon is more serious between 2 am and 5 am.However, the overall topic
> writing is very smooth.
> !image-2024-04-10-20-29-13-835.png!
> The final join output will experience serious shake, as shown in the
> following diagram:
> !image-2024-04-10-20-15-05-731.png!
> Root cause:
> # The {{SourceOperator#emitLatestWatermark}} reports the
> lastEmittedWatermark to the SourceCoordinator.
> # If the partition write is zero during a certain period, the
> lastEmittedWatermark sent by the subtask corresponding to that partition
> remains unchanged.
> # The SourceCoordinator aggregates the watermarks of all subtasks according
> to the watermark group and takes the smallest watermark. This means that the
> maxAllowedWatermark may remain unchanged for some time, even though the
> overall upstream data flow is moving forward. until that minimum value is
> updated, only then will everything change, which will manifest as serious
> shake in the output data stream.
> I think choosing the global minimum might not be a good option. Using min/max
> could more likely encounter some edge cases. Perhaps choosing a median value
> would be more appropriate? Or a more complex selection strategy?
> If replaced with a median value, it can ensure that the overall data flow is
> very smooth:
> !image-2024-04-10-20-23-13-872.png!
>
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