 .. Licensed to the Apache Software Foundation (ASF) under one
    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.


Lineage
=======

.. note:: Lineage support is very experimental and subject to change.

Airflow can help track origins of data, what happens to it and where it moves over time. This can aid having
audit trails and data governance, but also debugging of data flows.

Airflow tracks data by means of inlets and outlets of the tasks. Let's work from an example and see how it
works.

.. code-block:: python

    from airflow.operators.bash import BashOperator
    from airflow.operators.dummy import DummyOperator
    from airflow.lineage import AUTO
    from airflow.lineage.entities import File
    from airflow.models import DAG
    from airflow.utils.dates import days_ago
    from datetime import timedelta

    FILE_CATEGORIES = ["CAT1", "CAT2", "CAT3"]

    args = {
        'owner': 'airflow',
        'start_date': days_ago(2)
    }

    dag = DAG(
        dag_id='example_lineage', default_args=args,
        schedule_interval='0 0 * * *',
        dagrun_timeout=timedelta(minutes=60))

    f_final = File(url="/tmp/final")
    run_this_last = DummyOperator(task_id='run_this_last', dag=dag,
        inlets=AUTO,
        outlets=f_final)

    f_in = File(url="/tmp/whole_directory/")
    outlets = []
    for file in FILE_CATEGORIES:
        f_out = File(url="/tmp/{}/{{{{ execution_date }}}}".format(file))
        outlets.append(f_out)

    run_this = BashOperator(
        task_id='run_me_first', bash_command='echo 1', dag=dag,
        inlets=f_in,
        outlets=outlets
    )
    run_this.set_downstream(run_this_last)

Inlets can be a (list of) upstream task ids or statically defined as an attr annotated object
as is, for example, the ``File`` object. Outlets can only be attr annotated object. Both are rendered
at run time. However the outlets of a task in case they are inlets to another task will not be re-rendered
for the downstream task.

.. note:: Operators can add inlets and outlets automatically if the operator supports it.

In the example DAG task ``run_this`` (task_id=``run_me_first``) is a BashOperator that takes 3 inlets: ``CAT1``, ``CAT2``, ``CAT3``, that are
generated from a list. Note that ``execution_date`` is a templated field and will be rendered when the task is running.

.. note:: Behind the scenes Airflow prepares the lineage metadata as part of the ``pre_execute`` method of a task. When the task
          has finished execution ``post_execute`` is called and lineage metadata is pushed into XCOM. Thus if you are creating
          your own operators that override this method make sure to decorate your method with ``prepare_lineage`` and ``apply_lineage``
          respectively.

Shorthand notation
------------------

Shorthand notation is available as well, this works almost equal to unix command line pipes, inputs and outputs.
Note that operator precedence_ still applies. Also the ``|`` operator will only work when the left hand side either
has outlets defined (e.g. by using ``add_outlets(..)`` or has out of the box support of lineage ``operator.supports_lineage == True``.

.. code-block:: python

    f_in > run_this | (run_this_last > outlets)

.. _precedence: https://docs.python.org/3/reference/expressions.html