Data engineering·Time and correctness in streams
the job announces that it believes nothing older than 14:30 is still coming, and closes every window that ended before then.
Watermark
Also calledlow watermark, progress marker
The stream's assertion about how far event time has advanced — a claim that records older than this are no longer expected. It is what lets an unbounded stream ever produce a finished answer, because a window cannot close until something declares the input for it complete. It is a heuristic, not a fact: set it aggressively and you drop real data, set it conservatively and every result waits, which is the central tuning decision in any streaming job.