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https://issues.apache.org/jira/browse/FLINK-10050?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16590245#comment-16590245
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Aljoscha Krettek commented on FLINK-10050:
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I think {{DataStream.join()}} and {{DataStream.coGroup()}} are a bit of a dead 
end because they don't allow getting any information about what window the 
result is in, or other meta information about the window that you would get 
from a {{ProcessWindowFunction}}.

I'm interested if you have a use case for this, where you don't need to know 
what window your result is in. 

> Support 'allowedLateness' in CoGroupedStreams
> ---------------------------------------------
>
>                 Key: FLINK-10050
>                 URL: https://issues.apache.org/jira/browse/FLINK-10050
>             Project: Flink
>          Issue Type: Improvement
>          Components: Streaming
>    Affects Versions: 1.5.1, 1.6.0
>            Reporter: eugen yushin
>            Priority: Major
>              Labels: ready-to-commit, windows
>
> WindowedStream has a support of 'allowedLateness' feature, while 
> CoGroupedStreams are not. At the mean time, WindowedStream is an inner part 
> of CoGroupedStreams and all main functionality (like evictor/trigger/...) is 
> simply delegated to WindowedStream.
> There's no chance to operate with late arriving data from previous steps in 
> cogroups (and joins). Consider the following flow:
> a. read data from source1 -> aggregate data with allowed lateness
> b. read data from source2 -> aggregate data with allowed lateness
> c. cogroup/join streams a and b, and compare aggregated values
> Step c doesn't accept any late data from steps a/b due to lack of 
> `allowedLateness` API call in CoGroupedStreams.java.
> Scope: add method `WithWindow.allowedLateness` to Java API 
> (flink-streaming-java) and extend scala API (flink-streaming-scala).



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