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course: Apache Flink® 101

Event Time and Watermarks

12 min
David Anderson

David Anderson

Software Practice Lead

Event Time and Watermarks


Flink jobs can measure time using either the system clock (processing time), or timestamps in the events (event time). Operations that use event time need some way to determine how long to wait for the stream to be complete before taking an action, such as closing a window. This video explains how Flink uses watermarks to address this need.


  • Event time vs. processing time
  • Out-of-order event streams
  • What watermarks are, and the problem they solve
  • How watermarks are created
  • How watermarks are propagated
  • The idle source problem, and how to solve it


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