[jira] [Commented] (FLINK-3428) Add fixed time trailing timestamp/watermark extractor

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

    [ https://issues.apache.org/jira/browse/FLINK-3428?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15222140#comment-15222140 ] 

ASF GitHub Bot commented on FLINK-3428:
---------------------------------------

Github user StephanEwen commented on the pull request:

    https://github.com/apache/flink/pull/1764#issuecomment-204508742
  
    The code looks good now.
    
      - How about we move all the convenience timestamp extractors / watermark generators into a separate packe `.functions.timestamps`?
    
    **Docs**
    I am a bit unsure about the docs. They seem to get confusing now. What is a mechanism (periodic / punctuated), what is just a convenience implementation (Ascending, BoundedOutOfOrderness).
    
    How about leaving the docs page on watermark generation as it is and add another sub-page that lists pre-defined watermark generators. We would list all the future watermark generators there:
      - ascending timestamps
      - bounded out-of-orderness
      - historgam based


> Add fixed time trailing timestamp/watermark extractor
> -----------------------------------------------------
>
>                 Key: FLINK-3428
>                 URL: https://issues.apache.org/jira/browse/FLINK-3428
>             Project: Flink
>          Issue Type: Improvement
>            Reporter: Robert Metzger
>            Assignee: Kostas Kloudas
>
> Flink currently provides only one build-in timestamp extractor, which assumes strictly ascending timestamps. In real world use cases, timestamps are almost never strictly ascending.
> Therefore, I propose to provide an utility watermark extractor which is generating watermarks with a fixed-time trailing.
> The implementation should keep track of the highest event-time seen so far and subtract a fixed amount of time from that event time.
> This way, users can for example specify that the watermarks should always "lag behind" 10 minutes.



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