elon_X created FLINK-35076:
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Summary: 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
Attachments: image-2024-04-10-20-13-14-752.png,
image-2024-04-10-20-15-05-731.png, image-2024-04-10-20-23-13-872.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.
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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