robertwb commented on code in PR #27891:
URL: https://github.com/apache/beam/pull/27891#discussion_r1293898964
##########
runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/DataflowPipelineTranslator.java:
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@@ -936,8 +937,9 @@ private <K, V> void groupByKeyHelper(
&& windowingStrategy.getWindowFn().assignsToOneWindow();
if (isStreaming) {
allowCombinerLifting &= transform.fewKeys();
- // TODO: Allow combiner lifting on the non-default trigger, as
appropriate.
- allowCombinerLifting &= (windowingStrategy.getTrigger()
instanceof DefaultTrigger);
+ // We don't currently have a good way to safely lift combiners
in the face of count triggers, as we will
+ // lose track of how many elements have been combined.
+ allowCombinerLifting &=
!containsCountTrigger(windowingStrategy.getTrigger());
Review Comment:
On this note, we should probably have a policy about this. Historically I
think we've generally required a flag for new behavior, such that we're
backwards compatible by default. (But perhaps this doesn't impact as many
users.)
I'm on the fence here. Is it worth a query to dev@?
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