db: Enable auto-generation of database migrations

Alembic does lots of new things. Provide docs for how to use this. This
doesn't actually work that well at the moment because it appears our
database migrations are not in sync with the models. Those issues will
have to be resolved separately.

Change-Id: I6645ca951114b94ce9ed5a83e5e11d53879a7cd5
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

.gitignore

@@ -31,13 +31,14 @@ mypy-report
 tools/lintstack.head.py
 tools/pylint_exceptions
 tags
+
 # Files created by Sphinx build
 doc/build
 doc/source/_static/cinder.conf.sample
 doc/source/_static/cinder.policy.yaml.sample
 doc/source/drivers.rst
 
-#Files created for API reference
+# Files created for API reference
 api-ref/build
 
 # Files created by releasenotes build
@@ -45,3 +46,6 @@ releasenotes/build
 contrib/block-box/.db_data
 RELEASENOTES.rst
 releasenotes/notes/reno.cache
+
+# Files created by alembic
+cinder.db

cinder/db/alembic.ini

@@ -35,9 +35,7 @@ script_location = %(here)s/migrations
 # are written from script.py.mako
 # output_encoding = utf-8
 
-# Uncomment and update to your sql connection string if wishing to run
-# alembic directly from command line
-# sqlalchemy.url = driver://user:pass@localhost/dbname
+sqlalchemy.url = sqlite:///cinder.db
 
 [post_write_hooks]
 # post_write_hooks defines scripts or Python functions that are run

cinder/db/migrations/env.py

@@ -16,6 +16,8 @@ from alembic import context
 from sqlalchemy import engine_from_config
 from sqlalchemy import pool
 
+from cinder.db.sqlalchemy import models
+
 # this is the Alembic Config object, which provides
 # access to the values within the .ini file in use.
 config = context.config
@@ -25,11 +27,13 @@ config = context.config
 if config.attributes.get('configure_logger', True):
     fileConfig(config.config_file_name)
 
-# add your model's MetaData object here
-# for 'autogenerate' support
-# from myapp import mymodel
-# target_metadata = mymodel.Base.metadata
-target_metadata = None
+target_metadata = models.BASE.metadata
+
+
+def include_name(name, type_, parent_names):
+    # if there are any columns or tables that should be excluded from
+    # auto-generation, include them here
+    return True
 
 
 def run_migrations_offline():
@@ -45,6 +49,7 @@ def run_migrations_offline():
     context.configure(
         url=url,
         target_metadata=target_metadata,
+        include_name=include_name,
         literal_binds=True,
         dialect_opts={"paramstyle": "named"},
     )
@@ -80,7 +85,9 @@ def run_migrations_online():
 
     with connectable.connect() as connection:
         context.configure(
-            connection=connection, target_metadata=target_metadata
+            connection=connection,
+            target_metadata=target_metadata,
+            include_name=include_name,
         )
 
         with context.begin_transaction():

cinder/db/sqlalchemy/models.py

@@ -37,8 +37,7 @@ CONF = cfg.CONF
 BASE = declarative_base()
 
 
-class CinderBase(models.TimestampMixin,
-                 models.ModelBase):
+class CinderBase(models.TimestampMixin, models.ModelBase):
     """Base class for Cinder Models."""
 
     __table_args__ = {'mysql_engine': 'InnoDB'}

doc/source/admin/upgrades.rst

@@ -72,8 +72,8 @@ the background until it tells you no more migrations are needed. Note that you
 won't be able to apply N+1's schema migrations before completing N's online
 data migrations.
 
-For information on developing your own schema migrations as part of a feature
-or bugfix, refer to **TODO**.
+For information on developing your own schema or data migrations as part of a
+feature or bugfix, refer to :doc:`/contributor/database-migrations`.
 
 API load balancer draining
 ~~~~~~~~~~~~~~~~~~~~~~~~~~

doc/source/contributor/database-migrations.rst

@@ -0,0 +1,135 @@
+===================
+Database migrations
+===================
+
+.. note::
+
+   This document details how to generate database migrations as part of a new
+   feature or bugfix. For info on how to apply existing database migrations,
+   refer to the documentation for the :program:`cinder-manage db sync`
+   command in :doc:`/cli/cinder-manage`.
+   For info on the general upgrade process for a cinder deployment, refer to
+   :doc:`/admin/upgrades`.
+
+Occasionally the databases used in cinder will require schema or data
+migrations.
+
+
+Schema migrations
+-----------------
+
+.. versionchanged:: 24.0.0 (Xena)
+
+   The database migration engine was changed from ``sqlalchemy-migrate`` to
+   ``alembic``.
+
+The `alembic`__ database migration tool is used to manage schema migrations in
+cinder. The migration files and related metadata can be found in
+``cinder/db/migrations``. As discussed in :doc:`/admin/upgrades`, these can be
+run by end users using the :program:`cinder-manage db sync` command.
+
+.. __: https://alembic.sqlalchemy.org/en/latest/
+
+.. note::
+
+   There are also legacy migrations provided in the
+   ``cinder/db/legacy_migrations`` directory . These are provided to facilitate
+   upgrades from pre-Xena (24.0.0) deployments and will be removed in a future
+   release. They should not be modified or extended.
+
+The best reference for alembic is the `alembic documentation`__, but a small
+example is provided here. You can create the migration either manually or
+automatically. Manual generation might be necessary for some corner cases such
+as renamed tables but auto-generation will typically handle your issues.
+Examples of both are provided below. In both examples, we're going to
+demonstrate how you could add a new model, ``Foo``, to the main database.
+
+.. __: https://alembic.sqlalchemy.org/en/latest/
+
+.. code-block:: diff
+
+   diff --git cinder/db/sqlalchemy/models.py cinder/db/sqlalchemy/models.py
+   index 7eab643e14..8f70bcdaca 100644
+   --- cinder/db/sqlalchemy/models.py
+   +++ cinder/db/sqlalchemy/models.py
+   @@ -73,6 +73,16 @@ def MediumText():
+            sqlalchemy.dialects.mysql.MEDIUMTEXT(), 'mysql')
+
+
+   +class Foo(BASE, models.SoftDeleteMixin):
+   +    """A test-only model."""
+   +
+   +    __tablename__ = 'foo'
+   +
+   +    id = sa.Column(sa.Integer, primary_key=True)
+   +    uuid = sa.Column(sa.String(36), nullable=True)
+   +    bar = sa.Column(sa.String(255))
+   +
+   +
+    class Service(BASE, models.SoftDeleteMixin):
+        """Represents a running service on a host."""
+
+(you might not be able to apply the diff above cleanly - this is just a demo).
+
+.. rubric:: Auto-generating migration scripts
+
+In order for alembic to compare the migrations with the underlying models, it
+require a database that it can inspect and compare the models against. As such,
+we first need to create a working database. We'll bypass ``cinder-manage`` for
+this and go straight to the :program:`alembic` CLI. The ``alembic.ini`` file
+provided in the ``cinder/db`` directory is helpfully configured to use an
+SQLite database by default (``cinder.db``). Create this database and apply the
+current schema, as dictated by the current migration scripts:
+
+.. code-block:: bash
+
+   $ tox -e venv -- alembic -c cinder/db/alembic.ini \
+       upgrade head
+
+Once done, you should notice the new ``cinder.db`` file in the root of the
+repo. Now, let's generate the new revision:
+
+.. code-block:: bash
+
+   $ tox -e venv -- alembic -c cinder/db/alembic.ini \
+       revision -m "Add foo model" --autogenerate
+
+This will create a new file in ``cinder/db/migrations/versions`` with
+``add_foo_model`` in the name including (hopefully!) the necessary changes to
+add the new ``Foo`` model. You **must** inspect this file once created, since
+there's a chance you'll be missing imports or something else which will need to
+be manually corrected. Once you've inspected this file and made any required
+changes, you can apply the migration and make sure it works:
+
+.. code-block:: bash
+
+   $ tox -e venv -- alembic -c cinder/db/alembic.ini \
+       upgrade head
+
+.. rubric:: Manually generating migration scripts
+
+For trickier migrations or things that alembic doesn't understand, you may need
+to manually create a migration script. This is very similar to the
+auto-generation step, with the exception being that you don't need to have a
+database in place beforehand. As such, you can simply run:
+
+.. code-block:: bash
+
+   $ tox -e venv -- alembic -c cinder/db/alembic.ini \
+       revision -m "Add foo model"
+
+As before, this will create a new file in ``cinder/db/migrations/versions``
+with ``add_foo_model`` in the name. You can simply modify this to make whatever
+changes are necessary. Once done, you can apply the migration and make sure it
+works:
+
+.. code-block:: bash
+
+   $ tox -e venv -- alembic -c cinder/db/alembic.ini \
+       upgrade head
+
+
+Data migrations
+---------------
+
+.. todo: Populate this.

doc/source/contributor/index.rst

@@ -62,6 +62,7 @@ Programming HowTos and Tutorials
     api.apache
     rolling.upgrades
     groups
+    database-migrations
 
 .. _managing-development: