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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`.
