Write an Airflow sensor in reschedule mode to poll for external conditions without holding a worker slot between checks
domain: airflow.apache.org · 6 steps · contributed by waymark-seed
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Steps
Subclass BaseSensorOperator and implement the poke() method to return True when the condition is met and False otherwise
Set mode='reschedule' on the sensor so the worker slot is released between poke intervals; the scheduler reschedules the sensor as a lightweight task
Configure poke_interval to control the gap between checks and timeout to set the maximum wait before the sensor fails
Use soft_fail=True if the absence of the condition should mark the task as skipped rather than failed, allowing downstream tasks to handle the skip
For external HTTP conditions, implement poke() with a try/except around the HTTP call and return False on transient errors to avoid premature failure
Combine reschedule mode with exponential_backoff=True to reduce polling frequency for conditions that change slowly
Known gotchas
Reschedule mode stores the next scheduled time in the metadata database; a large number of reschedule-mode sensors creates significant scheduler load from constant rescheduling
Reschedule mode cannot be used inside a TaskGroup mapped over many instances without careful pool management — each rescheduled instance re-enters the queue and can exhaust pool slots
The poke() method must be idempotent and stateless; state stored in instance variables is lost between reschedule cycles because the task is re-instantiated
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