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Allow sliding windows + combine with hot key fanout #37899

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9f9686d
init
claudevdm Mar 20, 2026
0b2e98c
Dont assign windows in sliding windows case
claudevdm Mar 20, 2026
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78 changes: 60 additions & 18 deletions sdks/python/apache_beam/transforms/combiners_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -584,26 +584,68 @@ def has_expected_values(actual):

assert_that(result, has_expected_values)

def test_combining_with_sliding_windows_and_fanout_raises_error(self):
def test_combining_with_sliding_windows_and_fanout(self):
# SlidingWindows + fanout now works (fixed in 2.73.0).
# Previously this raised ValueError due to WindowInto re-evaluating
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@tvalentyn tvalentyn Mar 20, 2026

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line 589-582 - let's move this to PR description

# window assignments. The fix sets windowing strategy metadata directly
# on the PCollection without re-assigning windows, mirroring Java's
# setWindowingStrategyInternal().
options = PipelineOptions()
options.view_as(StandardOptions).streaming = True
with self.assertRaises(ValueError):
with TestPipeline(options=options) as p:
_ = (
p
| beam.Create([
window.TimestampedValue(0, Timestamp(seconds=1666707510)),
window.TimestampedValue(1, Timestamp(seconds=1666707511)),
window.TimestampedValue(2, Timestamp(seconds=1666707512)),
window.TimestampedValue(3, Timestamp(seconds=1666707513)),
window.TimestampedValue(5, Timestamp(seconds=1666707515)),
window.TimestampedValue(6, Timestamp(seconds=1666707516)),
window.TimestampedValue(7, Timestamp(seconds=1666707517)),
window.TimestampedValue(8, Timestamp(seconds=1666707518))
])
| beam.WindowInto(window.SlidingWindows(10, 5))
| beam.CombineGlobally(beam.combiners.ToListCombineFn()).
without_defaults().with_fanout(7))
with TestPipeline(options=options) as p:

def has_expected_values(actual):
from hamcrest.core import assert_that as hamcrest_assert
from hamcrest.library.collection import only_contains
ordered = sorted(actual)
hamcrest_assert(
ordered,
only_contains([0, 1, 2, 3], [0, 1, 2, 3, 5, 6, 7, 8], [5, 6, 7, 8]))

result = (
p
| beam.Create([
window.TimestampedValue(0, Timestamp(seconds=1666707510)),
window.TimestampedValue(1, Timestamp(seconds=1666707511)),
window.TimestampedValue(2, Timestamp(seconds=1666707512)),
window.TimestampedValue(3, Timestamp(seconds=1666707513)),
window.TimestampedValue(5, Timestamp(seconds=1666707515)),
window.TimestampedValue(6, Timestamp(seconds=1666707516)),
window.TimestampedValue(7, Timestamp(seconds=1666707517)),
window.TimestampedValue(8, Timestamp(seconds=1666707518))
])
| beam.WindowInto(window.SlidingWindows(10, 5))
| beam.CombineGlobally(beam.combiners.ToListCombineFn()).
without_defaults().with_fanout(7))
assert_that(result, has_expected_values)

def test_combining_with_session_windows_and_fanout(self):
options = PipelineOptions()
options.view_as(StandardOptions).streaming = True
with TestPipeline(options=options) as p:

def has_expected_values(actual):
from hamcrest.core import assert_that as hamcrest_assert
from hamcrest.library.collection import only_contains
ordered = sorted(actual)
hamcrest_assert(ordered, only_contains([0, 1, 2, 3], [5, 6, 7, 8]))

result = (
p
| beam.Create([
window.TimestampedValue(0, Timestamp(seconds=1666707510)),
window.TimestampedValue(1, Timestamp(seconds=1666707511)),
window.TimestampedValue(2, Timestamp(seconds=1666707512)),
window.TimestampedValue(3, Timestamp(seconds=1666707513)),
window.TimestampedValue(5, Timestamp(seconds=1666707515)),
window.TimestampedValue(6, Timestamp(seconds=1666707516)),
window.TimestampedValue(7, Timestamp(seconds=1666707517)),
window.TimestampedValue(8, Timestamp(seconds=1666707518))
])
| beam.WindowInto(window.Sessions(2))
| beam.CombineGlobally(beam.combiners.ToListCombineFn()).
without_defaults().with_fanout(7))
assert_that(result, has_expected_values)

def test_MeanCombineFn_combine(self):
with TestPipeline() as p:
Expand Down
38 changes: 26 additions & 12 deletions sdks/python/apache_beam/transforms/core.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3271,10 +3271,7 @@ def expand(self, pcoll):
combine_fn = self._combine_fn
fanout_fn = self._fanout_fn

if isinstance(pcoll.windowing.windowfn, SlidingWindows):
raise ValueError(
'CombinePerKey.with_hot_key_fanout does not yet work properly with '
'SlidingWindows. See: https://github.com/apache/beam/issues/20528')
use_direct_windowing = isinstance(pcoll.windowing.windowfn, SlidingWindows)

class SplitHotCold(DoFn):
def start_bundle(self):
Expand Down Expand Up @@ -3344,14 +3341,31 @@ def StripNonce(nonce_key_value):

cold, hot = pcoll | ParDo(SplitHotCold()).with_outputs('hot', main='cold')
cold.element_type = typehints.Any # No multi-output type hints.
precombined_hot = (
hot
# Avoid double counting that may happen with stacked accumulating mode.
| 'WindowIntoDiscarding' >> WindowInto(
pcoll.windowing, accumulation_mode=AccumulationMode.DISCARDING)
| CombinePerKey(PreCombineFn())
| Map(StripNonce)
| 'WindowIntoOriginal' >> WindowInto(pcoll.windowing))

if use_direct_windowing:
# For SlidingWindows, swap windowing strategy metadata directly on the
# PCollection without re-assigning windows. This mirrors Java's
# setWindowingStrategyInternal(). Using WindowInto would call
# windowfn.assign() which re-evaluates window assignments from
# timestamps — with SlidingWindows, this causes accumulators to leak
# into adjacent overlapping windows.
if pcoll.windowing.accumulation_mode == AccumulationMode.ACCUMULATING:
discarding_windowing = copy.copy(pcoll.windowing)
discarding_windowing.accumulation_mode = AccumulationMode.DISCARDING
hot._windowing = discarding_windowing
precombined_hot = (hot | CombinePerKey(PreCombineFn()) | Map(StripNonce))
precombined_hot._windowing = pcoll.windowing
else:
precombined_hot = (
hot
# Avoid double counting that may happen with stacked accumulating
# mode.
| 'WindowIntoDiscarding' >> WindowInto(
pcoll.windowing, accumulation_mode=AccumulationMode.DISCARDING)
| CombinePerKey(PreCombineFn())
| Map(StripNonce)
| 'WindowIntoOriginal' >> WindowInto(pcoll.windowing))

return ((cold, precombined_hot)
| Flatten()
| CombinePerKey(PostCombineFn()))
Expand Down
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