Event-Time Watermark

An event-time watermark expresses how far processing has progressed in event time under a source or engine policy. Operators can use it with triggers and lateness rules to emit results or retire state. It is distinct from an incremental extraction watermark, which records a position used to fetch subsequent source changes.

In a toy policy, the maximum observed timestamp is 09:08 and the delay is three minutes, giving watermark 09:05. If windows close when their end is at or before that watermark, [09:00, 09:05) closes. This is a policy example, not every engine’s boundary convention. The delay is not a timer attached to each event; no newer timestamp means this particular watermark stops.

A heuristic threshold can be passed by a later arrival. Specify whether that arrival revises a result, goes to a correction route, or is excluded. Multiple inputs often advance at different speeds; a minimum-based policy can stall on an idle input. Excluding idle inputs changes the treatment needed when they resume. A bad future timestamp can also move a maximum-based watermark too far, so validate the timestamp before using it.

Reference: Flink watermark generation and idleness.