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https://issues.apache.org/jira/browse/BEAM-3776?focusedWorklogId=80098&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-80098
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ASF GitHub Bot logged work on BEAM-3776:
----------------------------------------

                Author: ASF GitHub Bot
            Created on: 13/Mar/18 22:28
            Start Date: 13/Mar/18 22:28
    Worklog Time Spent: 10m 
      Work Description: scwhittle commented on a change in pull request #4793: 
[BEAM-3776] Fix issue with merging late windows where a watermark hold could be 
added behind the input watermark.
URL: https://github.com/apache/beam/pull/4793#discussion_r174305062
 
 

 ##########
 File path: 
runners/core-java/src/main/java/org/apache/beam/runners/core/WatermarkHold.java
 ##########
 @@ -247,8 +249,26 @@ private Instant addGarbageCollectionHold(
   /**
    * Prefetch watermark holds in preparation for merging.
    */
-  public void prefetchOnMerge(MergingStateAccessor<?, W> state) {
-    StateMerging.prefetchWatermarks(state, elementHoldTag);
+  public void prefetchOnMerge(MergingStateAccessor<?, W> context) {
 
 Review comment:
   I moved the merging code out of StateMerging.java so that I could reuse 
WatermarkHold.addElementHold instead of passing in the watermarks to 
StateMerging and duplicating the late detection logic.  To keep everything 
located together I moved the other uses of WatermarkHoldState from StateMerging 
to WatermarkHold.  AFAIK the StateMerging class is a leftover from lazy state 
merging and doesn't provide much benefit any more.

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Issue Time Tracking
-------------------

    Worklog Id:     (was: 80098)
    Time Spent: 1h 40m  (was: 1.5h)

> StateMerging.mergeWatermarks sets a late watermark hold for late merging 
> windows that depend only on the window
> ---------------------------------------------------------------------------------------------------------------
>
>                 Key: BEAM-3776
>                 URL: https://issues.apache.org/jira/browse/BEAM-3776
>             Project: Beam
>          Issue Type: Bug
>          Components: runner-core
>    Affects Versions: 2.1.0, 2.2.0, 2.3.0
>            Reporter: Sam Whittle
>            Assignee: Sam Whittle
>            Priority: Critical
>          Time Spent: 1h 40m
>  Remaining Estimate: 0h
>
> WatermarkHold.addElementHold and WatermarkHold.addGarbageCollectionHold take 
> to not add holds that would be before the input watermark.
> However WatermarkHold.onMerge calls StateMerging.mergeWatermarks which if the 
> window depends only on window, sets a hold for the end of the window 
> regardless of the input watermark.
> Thus if you have a WindowingStrategy such as:
> WindowingStrategy.of(Sessions.withGapDuration(gapDuration))
>  .withMode(AccumulationMode.DISCARDING_FIRED_PANES)
>  .withTrigger(
>  Repeatedly.forever(
>  AfterWatermark.pastEndOfWindow()
>  .withLateFirings(AfterPane.elementCountAtLeast(10))))
>  .withAllowedLateness(allowedLateness))
> and you merge windows that are late, you might end up holding the watermark 
> until the allowedLateness has passed.



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