Cheatsheet: Triggers

Topic Overview

Control exactly when window results are materialized and sent downstream.

Syntax Snapshot

from apache_beam.transforms.trigger import AfterWatermark, AfterCount, Repeatedly
from apache_beam.transforms.trigger import AccumulationMode

triggered = elements | beam.WindowInto(
    beam.transforms.window.FixedWindows(60),
    trigger=Repeatedly(AfterWatermark(
        early=AfterCount(10),
        late=AfterCount(1)
    )),
    allowed_lateness=1800,
    accumulation_mode=AccumulationMode.ACCUMULATING
)

Key Points

• Triggers dictate when partial or final window panes emit results.
• Early triggers provide speculative updates before the watermark passes.
• Late triggers handle elements arriving after the watermark but within allowed lateness.
• Accumulation mode: ACCUMULATING (keeps past state) vs DISCARDING (deltas only).

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