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https://issues.apache.org/jira/browse/BEAM-11366?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kenneth Knowles updated BEAM-11366:
-----------------------------------
    Status: Open  (was: Triage Needed)

> Dataflow and KafkaIO has erratic watermark progress due to ReaderCache 
> timeouts
> -------------------------------------------------------------------------------
>
>                 Key: BEAM-11366
>                 URL: https://issues.apache.org/jira/browse/BEAM-11366
>             Project: Beam
>          Issue Type: Bug
>          Components: runner-dataflow
>            Reporter: Sam Whittle
>            Assignee: Sam Whittle
>            Priority: P2
>
> The ReaderCache has a default timeout of 1 minute. If the source is not 
> polled within that time, the UnboundedReader is destroyed and will be 
> reinitilized from the checkpoint when next polled. This is too short for runs 
> where the source is not polled for a while due to other keys to process. In 
> particular this seems to affect Streaming engine pipelines with backlog.
> From an initial look, recovery from the Kafka UnboundedReader checkpoint does 
> not seem to use the previous watermark for initializing the timestamp policy 
> and perhaps the timestamp policy will return unknown watermark if it has not 
> yet observed a record.
> So possible fixes would be:
> - make cache timeout configurable, or maybe dynamic, and increase it
> - ensure that KafkaIO recovery from checkpoints initializes the watermark 
> properly so that cache eviction does not matter for watermark smoothness



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