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1. For each event source event sequence Source, the last processed position Pos. For a system with multiple inputs, this can be represented as  [StreamSource, Pos]
2. The stream processing engine internal state S_SYS
3. The state that has already been emitted: S_OUT

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A stream processing system snapshot is represented by {[SeqSource, Pos], S_SYS, S_OUT} at a given time.

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Also note that in this particular case, n2 does not need to be reset to Snapshot_n2_1 because its input stream S2 does not contain any duplicates duplicate S_OUT.

Conclusion 2: Exactly once delivery requires a global snapshot for all the nodes in the same processing tree.

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Condition 1: For all the messages produced by processing node N  and are before the marker for snapshot version V from N, the state change introduced by those messages should all be all included in snapshot version V.

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