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Let's now prove that the sync-mode is achieved.:

  • For any operator in the IterationBody, we define its output for the Nth epoch as the output emitted by the Nth invocation of onEpochWatermarkIncremented(). This definition is well-defined because operators only emit records in onEpochWatermarkIncremented().
  • At the time an operator OpB computes its output for the Nth epoch, this operator must have received exactly the following records from its input edges:
    • Suppose an edge is a non-feedback input edge from OpA and this edge is part of a feedback loop. It follows that OpA has emitted records for its Nth epoch (by lemma-1) and has not started to emit records for its (N+1)th epoch (by lemma-2).
    • Suppose an edge is a feedback input edge from OpA and this edge is part of a feedback loop. It follows that OpA has emitted records for its (N-1)th epoch (by lemma-1) and has not started to emit records for its Nth epoch (by lemma-2).

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