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• Goes into Logging Mode. That means that from that point on every record it receives from its Iteration Sink is buffered in its own operator state (log) and not forwarded further until it goes back to normal mode.

• Forwards the barrier to its downstream nodes (this guarantees liveness, otherwise we have a deadlock) and feedback records as usual since their effects are part of a future checkpoint.

(2) Eventually, the Iteration Head receives a barrier back from its Iteration Sink. At that point:

• It checkpoints the log as its operator state.Flushes to its outputs all pending records from the log and resets itclears the backup log.

• Goes back to normal forwarding mode.

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