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Kenneth Knowles commented on BEAM-696: -------------------------------------- Ben points out (in person) that this latter spec is already the spec, because Combine is not a model primitive transform: Combine.perKey expands to "GroupByKey and then combine the groups with ParDo over the iterable". So semantically, it occurs in one single call to {{processElement}}, over an iterable that is in a single main input window, and reads the side input at just once point in its evolution. > Side-Inputs non-deterministic with merging main-input windows > ------------------------------------------------------------- > > Key: BEAM-696 > URL: https://issues.apache.org/jira/browse/BEAM-696 > Project: Beam > Issue Type: Bug > Components: beam-model > Reporter: Ben Chambers > Assignee: Pei He > > Side-Inputs are non-deterministic for several reasons: > 1. Because they depend on triggering of the side-input (this is acceptable > because triggers are by their nature non-deterministic). > 2. They depend on the current state of the main-input window in order to > lookup the side-input. This means that with merging > 3. Any runner optimizations that affect when the side-input is looked up may > cause problems with either or both of these. > This issue focuses on #2 -- the non-determinism of side-inputs that execute > within a Merging WindowFn. > Possible solution would be to defer running anything that looks up the > side-input until we need to extract an output, and using the main-window at > that point. Specifically, if the main-window is a MergingWindowFn, don't > execute any kind of pre-combine, instead buffer all the inputs and combine > later. > This could still run into some non-determinism if there are triggers > controlling when we extract output. -- This message was sent by Atlassian JIRA (v6.3.4#6332)