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Sensors

Sensors are a special type of :doc:Operator <operators> that are designed to do exactly one thing - wait for something to occur. It can be time-based, or waiting for a file, or an external event, but all they do is wait until something happens, and then succeed so their downstream tasks can run. Or, in the case when that thing does not happen within the configured timeout, fails so that you can be alerted to the failure through the usual mechanisms.

Because they are primarily idle, Sensors have two different modes of running so you can be a bit more efficient about using them:

• poke (default): The Sensor takes up a worker slot for its entire runtime
• reschedule: The Sensor takes up a worker slot only when it is checking, and sleeps for a set duration between checks

The poke and reschedule modes can be configured directly when you instantiate the sensor; generally, the trade-off between them is latency. Something that is checking every second should be in poke mode, while something that is checking every minute should be in reschedule mode.

Much like Operators, Airflow has a large set of pre-built Sensors you can use, both in core Airflow as well as via our providers system.

.. seealso:: :doc:../authoring-and-scheduling/deferring

BaseSensorOperator parameters

All sensors in Airflow ultimately inherit from BaseSensorOperator (directly or indirectly). This base class defines the common behavior and parameters that control how a sensor waits, retries, and manages worker resources.

As of the Task SDK refactor, BaseSensorOperator is implemented in the Task SDK. Because provider documentation is generated separately, these parameters may not always be directly visible on individual provider sensor API pages. However, they apply to all sensors.

Common parameters ^^^^^^^^^^^^^^^^^

The following parameters are provided by BaseSensorOperator and are available on all sensors:

poke_interval Time in seconds between successive checks. In poke mode, the sensor sleeps between checks while occupying a worker slot. In reschedule mode, the task is deferred and rescheduled after this interval.

timeout Maximum time in seconds the sensor is allowed to run before failing. This timeout is measured from the first execution attempt, not per poke.

mode Determines how the sensor occupies worker resources.

* ``poke`` (default): occupies a worker slot for the entire duration
* ``reschedule``: releases the worker slot between checks

soft_fail If set to True, the sensor will be marked as SKIPPED instead of FAILED when the timeout is reached.

exponential_backoff If enabled, the time between checks increases exponentially up to max_wait. This is useful when polling external systems with unpredictable availability.

max_wait Upper bound (in seconds) for the delay between checks when exponential_backoff is enabled.

For the authoritative API reference, see the Task SDK documentation for BaseSensorOperator:

https://airflow.apache.org/docs/task-sdk/stable/api.html#airflow.sdk.BaseSensorOperator

Example ^^^^^^^

.. code-block:: python

BashSensor(
    task_id="wait_for_file",
    bash_command="test -f /data/input.csv",
    poke_interval=60,
    timeout=60 * 60,
    mode="reschedule",
)

.. tip::

For waiting on files specifically, consider using
:class:`~airflow.providers.standard.sensors.filesystem.FileSensor` from the
standard provider. It is purpose-built for detecting files in your local
filesystem and supports deferrable mode for better resource efficiency.
See :doc:`apache-airflow-providers-standard:sensors/file` for details.

Common sensors

Airflow provides many sensors out of the box. Here are some commonly used ones from the standard provider:

• :class:~airflow.providers.standard.sensors.filesystem.FileSensor - Wait for a file to appear in a filesystem
• :class:~airflow.providers.standard.sensors.bash.BashSensor - Wait for a bash command to return true
• :class:~airflow.providers.standard.sensors.python.PythonSensor - Wait for a Python callable to return true
• :class:~airflow.providers.standard.sensors.time.TimeSensor - Wait until a specified time of day
• :class:~airflow.providers.standard.sensors.time_delta.TimeDeltaSensor - Wait for a specified time duration
• :class:~airflow.providers.standard.sensors.external_task.ExternalTaskSensor - Wait for a task in another DAG to complete

For a complete list of available sensors, see the :doc:../operators-and-hooks-ref.