Watermarks
==========

Another concept closely related to windows and event time in Apache Flink® is *watermarks*. Flink uses watermarks as a mechanism to measure progress in event time; they flow as part of the data stream, carrying a timestamp that declares the minimum event time reached in the data stream.

This allows Flink to set points in the stream when all events up to a certain timestamp should have arrived, so that operators can set their internal event time to the value of the watermarks that reach them.

Flink uses *watermark strategies* and *watermark generators* to define how the watermark logic is implemented. For example, you can set Flink to generate watermarks either periodically at specific intervals or when triggered by an event or element with a specific marker.

For more information on watermarks, see the `Apache Flink® documentation on generating watermarks <https://ci.apache.org/projects/flink/flink-docs-release-1.15/docs/dev/datastream/event-time/generating_watermarks/>`_.