Learning Apache Apex
by Ananth Gundabattula, Thomas Weise, Munagala V. Ramanath, David Yan, Kenneth Knowles
Watermarks
A Watermark is a tuple that contains a timestamp that indicates all events before the watermark timestamp are expected to have arrived and influences how results are emitted from windows.
After the arrival of a watermark, if there are events with timestamps before the watermark timestamp, those events are considered late. Applications can have their Windowed Operator configured to fire late triggers for arrival of such events. We will discuss triggers later in this section.
Likewise, windows with a time period that lie before the watermark timestamp are considered late. Otherwise, they are considered early.
Watermarks can be generated by the input operator that generates the events, or it can be heuristically generated by the WindowedOperator. ...
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