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watermark
The time you believe the stream is complete up to. Late events after that need a rule.
What is watermark?
A watermark is a threshold: events with time before it are assumed arrived. Windows can close and emit results. Late events after the watermark are dropped, side-lined, or they reopen the window. “Wait forever” is not a watermark.
Why does watermark matter when vibe coding?
Models group events by minute and emit at the end of the minute on the processing clock. A slow producer’s events land in the next day. Name event time versus processing time.
How do you do watermark?
Window on event time. Set allowed lateness. Say what happens to a late event. Do not use the wall clock of the worker as the event’s time.
How do you ask a model for watermark?
Window (stream) by event time with a watermark. Allow lateness of (duration). Late events (drop or correct the window — pick one). Do not close the window on the worker’s wall clock alone.
What goes wrong with watermark?
A watermark that stalls because one partition is idle. An idle source must still advance, or the whole job waits forever.