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https://issues.apache.org/jira/browse/FLINK-28719?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17578890#comment-17578890
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Chesnay Schepler commented on FLINK-28719:
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> As far as I understand, op1 and op2 should have watermark 1 and 2
> respectively, because those subtasks don't have any events in them (besides 1
> and 2, of course, which create those watermarks). Then, why do they get the
> maximum watermark of 7 after first step?
Watermarks are broadcasted to all downstream operators. So, if a source emits
watermark 7, then all maps get it as an input.
> Also, why after op1 and op2, op4 and op5 get consumed? Is there any strategy
> that dictates in which order to process subtasks?
Whichever arrives first at the downstream operator. You have 7 map subtasks all
sending data to the window operators at the same time, and the order in which
they arrive is not deterministic. So they _may_ arrive in a perfect round-robin
pattern, or sequentially, or in any other pattern. The only guarantee is that
the elements from a particular map subtask arrive in the same order that they
were sent in.
> Also, why op3 to op7 have such watermarks: W2, W5, W6, W6, W6, after first
> step? I thought those subtasks should have watermark of Long.MinValue,
> because there were no elements before?
The above 2 comments should answer it; all watermarks are sent to all map
subtasks, and are consumed by the window operator in any order.
> Mapping a data source before window aggregation causes Flink to stop handle
> late events correctly.
> --------------------------------------------------------------------------------------------------
>
> Key: FLINK-28719
> URL: https://issues.apache.org/jira/browse/FLINK-28719
> Project: Flink
> Issue Type: Bug
> Components: API / DataStream
> Affects Versions: 1.15.1
> Reporter: Mykyta Mykhailenko
> Priority: Major
>
> I have created a
> [repository|https://github.com/mykytamykhailenko/flink-map-with-issue] where
> I describe this issue in detail.
> I have provided a few tests and source code so that you can reproduce the
> issue on your own machine.
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