Watermark.

also: watermarks

A 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.

also: watermarks

A stream processor's declaration that all events up to event time W have (probably) arrived, so windows ending before W can close.

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.