Raymond van den Bedum created NIFI-16515:
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Summary: Cron Scheduling Executes Twice After Daylight Saving Time
Transition (NiFi 2.9.0)
Key: NIFI-16515
URL: https://issues.apache.org/jira/browse/NIFI-16515
Project: Apache NiFi
Issue Type: Bug
Affects Versions: 2.9.0
Reporter: Raymond van den Bedum
A GenerateFlowFile processor configured with CRON-driven scheduling executed
twice on the first Monday following the daylight saving time (DST) transition
from summer time to standard time.
The processor was already running before the DST transition and had not been
stopped or restarted.
The configured CRON expression should execute exactly once per weekday at 20:30
local time. However, two executions occurred one hour apart.
h2. Environment
* Apache NiFi version: 2.9.0
* Processor: GenerateFlowFile
* Scheduling strategy: CRON driven
* CRON expression: {{0 30 20 ? * MON-FRI}}
* Timezone: Europe/Amsterdam
* DST transition: October 25, 2026 (CEST UTC+02:00 to CET UTC+01:00)
* Processor state: Running continuously before and after the DST transition
h2. Observed Behavior
On Monday, October 26, 2026, the processor generated two FlowFiles:
||Date||Time||Timezone||Event||
|2026-10-26|19:30:02|CET|Unexpected execution|
|2026-10-26|20:30:00|CET|Expected execution|
Both events originated from the same GenerateFlowFile processor.
h2. Expected Behavior
The processor should execute only once per weekday at 20:30 Europe/Amsterdam
local time, regardless of DST transitions.
The DST transition should not result in an additional execution on the
following day.
h2. Steps to Reproduce
# Configure a GenerateFlowFile processor with CRON-driven scheduling.
# Set the CRON expression to {{{}0 30 20 ? * MON-FRI{}}}.
# Use the Europe/Amsterdam timezone.
# Start the processor before the autumn DST transition.
# Leave the processor running continuously through the transition.
# Observe the processor executions on the first Monday after the DST
transition.
h2. Possible Root Cause
The {{CronSchedulingAgent}} implementation appears to use
{{java.time.OffsetDateTime}} for calculating scheduled execution times.
{{OffsetDateTime}} represents a fixed UTC offset rather than a timezone with
daylight saving transition rules.
A schedule calculated before the DST transition may retain the previous UTC
offset, causing the first execution after the transition to occur one hour
earlier than intended.
After that execution, the scheduler may calculate the next occurrence using the
new UTC offset, resulting in a second execution on the same day.
This would explain the two observed executions:
* 20:30 CEST (UTC+02:00), corresponding to 19:30 CET after the transition.
* 20:30 CET (UTC+01:00), corresponding to the intended local execution time.
This is a suspected root cause based on the available source code and observed
behavior. It has not yet been confirmed against the exact NiFi 2.9.0 release
implementation.
Relevant source file:
{{nifi-framework-core/src/main/java/org/apache/nifi/controller/scheduling/CronSchedulingAgent.java}}
h2. Impact
This behavior may cause CRON-driven processors to execute more than once on the
first scheduled day after an autumn DST transition.
For scheduled workflows, duplicate executions may result in repeated data
processing or duplicate downstream operations.
h2. Additional Notes
The processor remained running across the DST transition. The issue was
observed through two distinct FlowFile creation events approximately one hour
apart.
No independent reproduction in a test environment has yet been performed.
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