Martijn Visser created FLINK-40504:
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Summary: FLIP-27 source never re-emits WatermarkStatus.ACTIVE
after idleness unless its watermark strictly advances
Key: FLINK-40504
URL: https://issues.apache.org/jira/browse/FLINK-40504
Project: Flink
Issue Type: Bug
Components: API / Core, Runtime / Task
Reporter: Martijn Visser
Two compounding mechanisms leave a resumed FLIP-27 source announced-IDLE
downstream:
# {{WatermarkOutputMultiplexer.updateCombinedWatermark()}} calls
{{underlyingOutput.markIdle()}} when combined-idle but has no
{{underlyingOutput.markActive()}} counterpart; the per-output {{markActive}}
implementations only mutate internal partial state.
# {{WatermarkToDataOutput.emitWatermark}} returns at the monotonicity guard
({{newWatermark <= maxWatermarkSoFar}}) *before* {{markActiveInternally()}}
({{WatermarkToDataOutput.java:77-85}}).
The only path back to ACTIVE is therefore a strictly larger watermark.
Scenario: all
splits of a subtask go idle; the combined watermark is flushed to the max M
across
splits (FLINK-38454). A split then resumes with backlog whose watermarks are <=
M
(e.g. a recovered Kafka partition catching up). Result — proven by red unit
tests on
both layers: *zero* calls reach the underlying output (no watermark, no
ACTIVE), even
via {{onPeriodicEmit}}; the edge stays IDLE downstream indefinitely while
records flow.
Downstream min-watermark is then driven by the other inputs, past the resumed
backlog,
and its records are dropped as late.
Every sibling path re-activates eagerly on resume: {{StatusWatermarkValve}}
(explicitly,
since FLINK-40475), the legacy {{StreamSourceContexts}} (ACTIVE on every
collect), the
table {{WatermarkAssignerOperator}} (ACTIVE on first record), and DataStream V2
(explicit idle=false watermark on next record). The FLIP-27 multiplexer path is
the only
one without a re-activation signal.
FLINK-22926 (open since 2021) describes the new-split registration special case
of this
same defect; it should be closed into this ticket.
Note: the fix is a semantics decision (emit ACTIVE eagerly on
record/markActive, aligning
with all other paths) rather than a local patch
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