[GitHub] [beam] zhoufek commented on a change in pull request #15603: [BEAM-9487] Various Trigger.may_lose_data fixes

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zhoufek commented on a change in pull request #15603:
URL: https://github.com/apache/beam/pull/15603#discussion_r718524059



##########
File path: sdks/python/apache_beam/transforms/trigger.py
##########
@@ -161,12 +159,42 @@ def with_prefix(self, prefix):
 
 
 class DataLossReason(Flag):
-  """Enum defining potential reasons that a trigger may cause data loss."""
+  """Enum defining potential reasons that a trigger may cause data loss.
+
+  These flags should only cover when the trigger is the cause, though windowing
+  can be taken into account. For instance, AfterWatermark may not flag itself
+  as finishing if the windowing doesn't allow lateness.
+  """
+
+  # Trigger will never be the source of data loss.
   NO_POTENTIAL_LOSS = 0
+
+  # Trigger may finish. In this case, data that comes in after the trigger may
+  # be lost. Example: AfterCount(1) will stop firing after the first element.
   MAY_FINISH = auto()
+
+  # Trigger has a condition that is not guaranteed to ever be met. In this
+  # case, data that comes in may be lost. Example: AfterCount(42) will lose
+  # 20 records if only 20 come in, since the condition to fire was never met.
   CONDITION_NOT_GUARANTEED = auto()

Review comment:
       Something that still isn't clear to me:
   
   In Beam documentation, we make a  note:
   
   ```
   It is important to note that if, for example, you specify .elementCountAtLeast(50)
   and only 32 elements arrive, those 32 elements sit around forever. If the 32 elements 
   are important to you, consider using composite triggers to combine multiple conditions.
   This allows you to specify multiple firing conditions such as “fire either when I receive
   50 elements, or every 1 second”.
   ```
   ([link](https://beam.apache.org/documentation/programming-guide/#data-driven-triggers))
   
   Based on discussion, though, it sounded like those 32 records would become available at GC time. Is this warning in the documentation only for those who  want the data before GC?
   
   Or, putting it in the context of GBK where this is actually used, an [existing test](https://github.com/apache/beam/blob/0111cff88025f0dc783a0890078b769139c8ae36/sdks/python/apache_beam/transforms/ptransform_test.py#L500) shows that `AfterCount(5)` won't emit the data. If I change to `Repeatedly(AfterCount(5))` it has the same behavior, so practically, it seems `Repeatedly(AfterCount(5))` is still unsafe. Is this something to do with the runner or windowing that wouldn't affect a real pipeline?




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