Triggers
Interactive Syntax Reference
TriggersApache Beam Cheatsheet: Triggers
Recommended reading: 4 minsCopy-paste ready Python code snippets
Core Architecture Overview
Control exactly when window results are materialized and sent downstream.
WindowInto Triggers
returns: PCollectionPurpose & Description
Defines triggering rules (early, on-time, late) for window pane materialization.
Syntax Signature
beam.WindowInto(windowfn, trigger=trigger_fn, accumulation_mode=mode)Executable Example
from apache_beam.transforms.trigger import AfterWatermark, AfterCount, Repeatedly
from apache_beam.transforms.trigger import AccumulationMode
import apache_beam as beam
triggered = stream | beam.WindowInto(
beam.window.FixedWindows(60),
trigger=Repeatedly(AfterWatermark(early=AfterCount(10))),
accumulation_mode=AccumulationMode.ACCUMULATING
)Used In
Managing latency and speculative result rendering.
Comparison Note
ACCUMULATING retains state across panes; DISCARDING emits delta values only.
Pro Tip
Pair triggers with a reasonable allowed_lateness duration to prevent infinite state storage leaks.
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