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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.

Because they are primarily idle, Sensors have three 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
* ``smart sensor``: There is a single centralized version of this Sensor that batches all executions of it

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.

Smart Sensors take a bit more setup; for more information on them, see :doc:`smart-sensors`.

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.