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    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at

 ..   http://www.apache.org/licenses/LICENSE-2.0

 .. Unless required by applicable law or agreed to in writing,
    software distributed under the License is distributed on an
    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    KIND, either express or implied.  See the License for the
    specific language governing permissions and limitations
    under the License.


Callbacks
=========

A valuable component of logging and monitoring is the use of task callbacks to act upon changes in state of a given task, or across all tasks in a given DAG.
For example, you may wish to alert when certain tasks have failed, or have the last task in your DAG invoke a callback when it succeeds.

.. note::

    Callback functions are only invoked when the task state changes due to execution by a worker.
    As such, task changes set by the command line interface (:doc:`CLI <../usage-cli>`) or user interface (:doc:`UI <../ui>`) do not
    execute callback functions.

Callback Types
--------------

There are five types of task events that can trigger a callback:

=========================================== ================================================================
Name                                        Description
=========================================== ================================================================
``on_success_callback``                     Invoked when the task :ref:`succeeds <concepts:task-instances>`
``on_failure_callback``                     Invoked when the task :ref:`fails <concepts:task-instances>`
``sla_miss_callback``                       Invoked when a task misses its defined :ref:`SLA <concepts:slas>`
``on_retry_callback``                       Invoked when the task is :ref:`up for retry <concepts:task-instances>`
``on_execute_callback``                     Invoked right before the task begins executing.
=========================================== ================================================================


Example
-------

In the following example, failures in any task call the ``task_failure_alert`` function, and success in the last task calls the ``dag_success_alert`` function:

.. code-block:: python

    import datetime
    import pendulum

    from airflow import DAG
    from airflow.operators.empty import EmptyOperator


    def task_failure_alert(context):
        print(f"Task has failed, task_instance_key_str: {context['task_instance_key_str']}")


    def dag_success_alert(context):
        print(f"DAG has succeeded, run_id: {context['run_id']}")


    with DAG(
        dag_id="example_callback",
        schedule_interval=None,
        start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
        dagrun_timeout=datetime.timedelta(minutes=60),
        catchup=False,
        on_success_callback=None,
        on_failure_callback=task_failure_alert,
        tags=["example"],
    ) as dag:

        task1 = EmptyOperator(task_id="task1")
        task2 = EmptyOperator(task_id="task2")
        task3 = EmptyOperator(task_id="task3", on_success_callback=dag_success_alert)
        task1 >> task2 >> task3