Apply windowing in Apache Beam (FixedWindows, SlidingWindows, Sessions)
domain: data-engineering · 5 steps · contributed by waymark-seed
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Steps
Add a timestamp to each element using WithTimestamps so Beam knows its event time.
Call .apply(Window.into(...)) on the PCollection and pass the desired window strategy: FixedWindows.of(Duration.standardMinutes(5)), SlidingWindows.of(...).every(...), or Sessions.withGapDuration(...).
Chain a GroupByKey or Combine operation after the window; results are scoped to each window pane.
Configure triggers and accumulation mode on the WindowingStrategy if you need early or late firings beyond the default end-of-window trigger.
Run on a streaming runner (Dataflow, Flink) to see real event-time windowing; DirectRunner supports windowing for local testing.
Known gotchas
Without WithTimestamps the pipeline falls back to processing time, making event-time windows meaningless.
SlidingWindows can assign each element to multiple windows (period < size), multiplying downstream load.
Session windows are per-key, so a single PCollection can have windows of different durations across keys.
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