Learning Apache Apex
by Ananth Gundabattula, Thomas Weise, Munagala V. Ramanath, David Yan, Kenneth Knowles
Triggering
Triggering is a way for the WindowedOperator to emit its accumulated state to downstream. A trigger is fired at the watermark for a window by default, that is, when a window is flipped from being early to being late as a result of the arrival of a watermark.
The application developer can configure the WindowedOperator to fire timed-based triggers (triggers that get fired at time intervals) or count-based triggers (triggers that get fired at tuple count intervals) when the window is early or when the window is late. A typical setup is time-based early triggers and count-based late triggers; in such a case, the WindowedOperator would periodically update the downstream the state of windows that are still expecting events and would ...
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