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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-6969) Add support for deferred computation for group window aggregates
Date Wed, 28 Jun 2017 08:14:00 GMT

    [ https://issues.apache.org/jira/browse/FLINK-6969?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16066115#comment-16066115
] 

ASF GitHub Bot commented on FLINK-6969:
---------------------------------------

Github user fhueske commented on the issue:

    https://github.com/apache/flink/pull/4183
  
    Hi @wuchong, that's an interesting idea but I think it has the drawback that it might
add latency. 
    
    A timestamp extractor is only called with records and doesn't see watermarks. Since an
operator might emit multiple records for the same timestamp, a timestamp extractor would always
have to emit a watermark of last timestamp - 1 (we can be sure that the records are emitted
in timestamp order) because it does not know which record is the last for a timestamp. So,
we would add a latency of one window length (until the next window is processed).
    
    Custom operators are a low level interface but it shouldn't be too hard to implement one
that holds watermarks back.


> Add support for deferred computation for group window aggregates
> ----------------------------------------------------------------
>
>                 Key: FLINK-6969
>                 URL: https://issues.apache.org/jira/browse/FLINK-6969
>             Project: Flink
>          Issue Type: New Feature
>          Components: Table API & SQL
>            Reporter: Fabian Hueske
>            Assignee: sunjincheng
>
> Deferred computation is a strategy to deal with late arriving data and avoid updates
of previous results. Instead of computing a result as soon as it is possible (i.e., when a
corresponding watermark was received), deferred computation adds a configurable amount of
slack time in which late data is accepted before the result is compute. For example, instead
of computing a tumbling window of 1 hour at each full hour, we can add a deferred computation
interval of 15 minute to compute the result quarter past each full hour.
> This approach adds latency but can reduce the number of update esp. in use cases where
the user cannot influence the generation of watermarks. It is also useful if the data is emitted
to a system that cannot update result (files or Kafka). The deferred computation interval
should be configured via the {{QueryConfig}}.



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