contributing-docs/14_metadata_database_updates.rst

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Metadata Database Updates
=========================

When developing features, you may need to persist information to the metadata
database. Airflow has `Alembic <https://github.com/sqlalchemy/alembic>`__ built-in
module to handle all schema changes. Alembic must be installed on your
development machine before continuing with migration. If you had made changes to the ORM,
you will need to generate a new migration file. This file will contain the changes to the
database schema that you have made. To generate a new migration file, run the following:


.. code-block:: bash

    # starting at the root of the project
    $ breeze --backend postgres
    $ cd airflow-core/src/airflow
    $ alembic revision -m "add new field to db" --autogenerate

       Generating
    ~/airflow-core/src/airflow/migrations/versions/a1e23c41f123_add_new_field_to_db.py

Note that migration file names are standardized by prek hook ``update-migration-references``, so that they sort alphabetically and indicate
the Airflow version in which they first appear (the alembic revision ID is removed). As a result you should expect to see a prek failure
on the first attempt.  Just stage the modified file and commit again
(or run the hook manually before committing).

After your new migration file is run through prek hook it will look like this:

.. code-block::

    1234_A_B_C_add_new_field_to_db.py

This represents that your migration is the 1234th migration and expected for release in Airflow version A.B.C.

How to Resolve Conflicts When Rebasing
--------------------------------------

When rebasing your branch onto the latest ``main``, you may encounter conflicts in certain files. This often happens when another PR updates the Metadata Database and is merged before yours.

The affected files may include:

- ``docs/apache-airflow/img/airflow_erd.sha256``
- ``docs/apache-airflow/img/airflow_erd.svg``
- ``docs/apache-airflow/migrations-ref.rst``
- ``airflow/migrations/versions/1234_A_B_C_<your_migration_name>.py``

    There should be another file, ``1234_A_B_C_<other_migration_name>.py``, with the same ``1234_A_B_C`` prefix.

To resolve these conflicts:

1. First, resolve all conflicts **except** those in the files listed above. This includes conflicts in other ``.py`` files within the ``airflow/`` or ``tests/`` directories.
2. Then, run the following commands to automatically update the affected files:

.. code-block:: bash

    prek update-migration-references --all-files
    prek update-er-diagram --all-files

3. Add the updated files to the staging area and continue with the rebase.

How to hook your application into Airflow's migration process
-------------------------------------------------------------

Airflow 3.0.0 introduces a new feature that allows you to hook your application into Airflow's migration process.
This feature is useful if you have a custom database schema that you want to migrate along with Airflow's schema.
This guide will show you how to hook your application into Airflow's migration process.

Subclass the BaseDBManager
==========================
To hook your application into Airflow's migration process, you need to subclass the ``BaseDBManager`` class from the
``airflow.utils.db_manager`` module. This class provides methods for running Alembic migrations.

Create Alembic migration scripts
================================
At the root of your application, run "alembic init migrations" to create a new migrations directory. Set the
``version_table`` variable in the ``env.py`` file to the name of the table that stores the migration history. Specify this
version_table in the ``version_table`` argument of the alembic's ``context.configure`` method of the ``run_migration_online``
and ``run_migration_offline`` functions. This will ensure that your application's migrations are stored in a separate
table from Airflow's migrations.

Next, define an ``include_object`` function in the ``env.py`` that ensures that only your application's metadata is included in the application's
migrations. This too should be specified in the ``context.configure`` method of the ``run_migration_online`` and ``run_migration_offline``.

Next, set the config_file not to disable existing loggers:

.. code-block:: python

    if config.config_file_name is not None:
        fileConfig(config.config_file_name, disable_existing_loggers=False)

Replace the content of your application's ``alembic.ini`` file with Airflow's ``alembic.ini`` copy.

If the above is not clear, you might want to look at the FAB implementation of this migration.

After setting up those, and you want Airflow to run the migration for you when running ``airflow db migrate`` then you need to
add your DBManager to the ``[core] external_db_managers`` configuration.

--------

You can also learn how to setup your `Node environment <15_node_environment_setup.rst>`__ if you want to develop Airflow UI.