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https://issues.apache.org/jira/browse/BEAM-591?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15438443#comment-15438443
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Daniel Halperin commented on BEAM-591:
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May be worth checking out PubSubIO. It has two modes -- processing-time
timestamps, and message-time timestamps. But it keeps watermark tracking to
itself.
> Better handling watermark in KafkaIO
> ------------------------------------
>
> Key: BEAM-591
> URL: https://issues.apache.org/jira/browse/BEAM-591
> Project: Beam
> Issue Type: Bug
> Components: sdk-java-extensions
> Reporter: Raghu Angadi
> Assignee: Raghu Angadi
>
> Right now default watermark in KafkaIO is same as timestamp of the record.
> The main problem with this is that watermark does not change if there n't any
> new records on the topic. This can hold up many open windows.
> The record timestamp by default is set to processing time (i.e. when the
> runner reads a record from Kafka reader).
> A user can provide functions to calculate watermark and record timestamps.
> There are a few concerns with current design:
> * What should happen when a kafka topic is idle:
> ** in default case, I think watermark should advance to current time.
> ** What should happen when user has provided a function to calculate record
> timestamp?
> *** Should the watermark stay same as record timestamp?
> *** same when user has provided own watermark function?
> * Are the current semantics of user provided watermark function correct?
> ** -it is run once for each record read-.
> ** -Should it instead be run inside {{getWatermark()}} called by the runner
> (we could still provide the last user record, and its timestamp)-.
> ** It does run inside {{getWatermark()}}. should we pass current record
> timestamp in addition to the record?
>
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