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Renamed Exactly Once to Effectively Once #2254
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nlu90
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Aug 29, 2017
nicknezis
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* Renamed Exactly Once to Effectively Once * Substitute exactly once with effectively once * Replaced exactly once with effectively once
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The stateful processing in Heron relies on Distributed snapshotting method where state is periodically check pointed and restored when a failure occurs. However once a failure occurs and the last globally consistent checkpoint is restored, tuples which were already processed(but not check pointed) could be replayed. Thus it is possible that a tuple is processed more than once, however because all state information is stored in the state variable, the effects are applied once once. In other words, this mechanism is effectively once.
To achieve true exactly once processing where one needs to guarantee that every tuple is processed only once, one needs to adopt MillWheel Style approach where each incoming and outgoing tuple is stored and deduped.
This pr thus renames exactly once wordings with effectively once to reflect the thought above.