[jira] [Commented] (FLINK-2577) Watermarks Stall When a Source Finishes Prematurely

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-2577?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14738693#comment-14738693 ] 

ASF GitHub Bot commented on FLINK-2577:
---------------------------------------

Github user fhueske commented on the pull request:

    https://github.com/apache/flink/pull/1060#issuecomment-139226385
  
    @aljoscha, @rmetzger Can this PR be merged?


> Watermarks Stall When a Source Finishes Prematurely
> ---------------------------------------------------
>
>                 Key: FLINK-2577
>                 URL: https://issues.apache.org/jira/browse/FLINK-2577
>             Project: Flink
>          Issue Type: Bug
>          Components: Streaming
>    Affects Versions: 0.10
>            Reporter: Aljoscha Krettek
>            Assignee: Aljoscha Krettek
>             Fix For: 0.10
>
>
> The problem with a streaming source that closes is that downstream operations never notice that it is not running anymore and keep waiting for watermarks from all upstream operations (including the source). This has the effect that watermarks just stop propagating through the topology.
> I think an easy fix is to change sources to emit a last watermark of +Inf before closing. Because watermarks are always the minimum of all watermarks on the inputs this would have the effect of advancing only depending on the other inputs.
> The added benefit would be that once all sources emit a +Inf watermark the operator also get's a last +Inf watermark which tells it that all sources are done. Right now, streaming operators (and user code) have no way of telling if there are going to come elements in the future. This is especially problematic in Co-Map (Co-FlatMap) operations where you have one input that feeds a hash-table and the other input is elements that you want to stream by this hash-table.



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