[jira] [Commented] (BEAM-2535) Allow explicit output time independent of firing specification for all timers

["Kenneth Knowles (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/BEAM-2535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16951180#comment-16951180 ] 

Kenneth Knowles commented on BEAM-2535:
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[~rohdesam]

> Allow explicit output time independent of firing specification for all timers
> -----------------------------------------------------------------------------
>
>                 Key: BEAM-2535
>                 URL: https://issues.apache.org/jira/browse/BEAM-2535
>             Project: Beam
>          Issue Type: New Feature
>          Components: beam-model, sdk-java-core
>            Reporter: Kenneth Knowles
>            Assignee: Shehzaad Nakhoda
>            Priority: Major
>          Time Spent: 4.5h
>  Remaining Estimate: 0h
>
> Today, we have insufficient control over the event time timestamp of elements output from a timer callback.
> 1. For an event time timer, it is the timestamp of the timer itself.
>  2. For a processing time timer, it is the current input watermark at the time of processing.
> But for both of these, we may want to reserve the right to output a particular time, aka set a "watermark hold".
> A naive implementation of a {{TimerWithWatermarkHold}} would work for making sure output is not droppable, but does not fully explain window expiration and late data/timer dropping.
> In the natural interpretation of a timer as a feedback loop on a transform, timers should be viewed as another channel of input, with a watermark, and items on that channel _all need event time timestamps even if they are delivered according to a different time domain_.
> I propose that the specification for when a timer should fire should be separated (with nice defaults) from the specification of the event time of resulting outputs. These timestamps will determine a side channel with a new "timer watermark" that constrains the output watermark.
>  - We still need to fire event time timers according to the input watermark, so that event time timers fire.
>  - Late data dropping and window expiration will be in terms of the minimum of the input watermark and the timer watermark. In this way, whenever a timer is set, the window is not going to be garbage collected.
>  - We will need to make sure we have a way to "wake up" a window once it is expired; this may be as simple as exhausting the timer channel as soon as the input watermark indicates expiration of a window
> This is mostly aimed at end-user timers in a stateful+timely {{DoFn}}. It seems reasonable to use timers as an implementation detail (e.g. in runners-core utilities) without wanting any of this additional machinery. For example, if there is no possibility of output from the timer callback.



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