**Windowing** defines time boundaries: tumbling (non-overlapping), sliding (overlapping), session (gap-based). Enables aggregations over event time. **Triggering** controls when output is emitted: processing-time, event-time watermark, count-based, or composite (early + final for late data). **Why both**: Windowing defines *what* to aggregate; triggering defines *when* to emit. Example: 1-hour tumbling window with 5-min trigger = emit partial results every 5 mins, final at watermark....
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