Watermarks

Bounded out-of-orderness, per-partition generation, the minimum rule, idleness — who decides that a minute is over

A watermark is a claim that no earlier event will arrive. It is generated per source partition from the data seen, and an operator's event time is the minimum of its inputs. One slow partition holds every window in the job hostage; you diagnose it from the watermark metrics and fix it with idleness and alignment.

Estimated time: 90 minutes

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