Watermark.
also: watermarksA stream processor's declaration that all events up to event time W have (probably) arrived, so windows ending before W can close.
the two-minute refresher
- Often 'max event time seen − allowed delay'.
- Too eager: many events count as late. Too cautious: results are delayed.
- Events older than the watermark are late: drop them, send them to a side output, or update the result.
- Pick the lateness tolerance from measured delays, not hope.