Alembic Migrations

Introduction

The migrations in the alembic/versions contain the changes needed to migrate from older Neutron releases to newer versions. A migration occurs by executing a script that details the changes needed to upgrade the database. The migration scripts are ordered so that multiple scripts can run sequentially to update the database.

The Migration Wrapper

The scripts are executed by Neutron’s migration wrapper neutron-db-manage which uses the Alembic library to manage the migration. Pass the --help option to the wrapper for usage information.

The wrapper takes some options followed by some commands:

neutron-db-manage <options> <commands>

The wrapper needs to be provided with the database connection string, which is usually provided in the neutron.conf configuration file in an installation. The wrapper automatically reads from /etc/neutron/neutron.conf if it is present. If the configuration is in a different location:

neutron-db-manage --config-file /path/to/neutron.conf <commands>

Multiple --config-file options can be passed if needed.

Instead of reading the DB connection from the configuration file(s) the --database-connection option can be used:

neutron-db-manage --database-connection mysql+pymysql://root:secret@127.0.0.1/neutron?charset=utf8 <commands>

For some commands the wrapper needs to know the entrypoint of the core plugin for the installation. This can be read from the configuration file(s) or specified using the --core_plugin option:

neutron-db-manage --core_plugin neutron.plugins.ml2.plugin.Ml2Plugin <commands>

When giving examples below of using the wrapper the options will not be shown. It is assumed you will use the options that you need for your environment.

For new deployments you will start with an empty database. You then upgrade to the latest database version via:

neutron-db-manage upgrade heads

For existing deployments the database will already be at some version. To check the current database version:

neutron-db-manage current

After installing a new version of Neutron server, upgrading the database is the same command:

neutron-db-manage upgrade heads

To create a script to run the migration offline:

neutron-db-manage upgrade heads --sql

To run the offline migration between specific migration versions:

neutron-db-manage upgrade <start version>:<end version> --sql

Upgrade the database incrementally:

neutron-db-manage upgrade --delta <# of revs>

NOTE: Database downgrade is not supported.

Migration Branches

Neutron makes use of alembic branches for two purposes.

1. Indepedent Sub-Project Tables

Various sub-projects can be installed with Neutron. Each sub-project registers its own alembic branch which is responsible for migrating the schemas of the tables owned by the sub-project.

The neutron-db-manage script detects which sub-projects have been installed by enumerating the neutron.db.alembic_migrations entrypoints. For more details see the Entry Points section of Contributing extensions to Neutron.

The neutron-db-manage script runs the given alembic command against all installed sub-projects. (An exception is the revision command, which is discussed in the Developers section below.)

2. Offline/Online Migrations

Since Liberty, Neutron maintains two parallel alembic migration branches.

The first one, called ‘expand’, is used to store expansion-only migration rules. Those rules are strictly additive and can be applied while neutron-server is running. Examples of additive database schema changes are: creating a new table, adding a new table column, adding a new index, etc.

The second branch, called ‘contract’, is used to store those migration rules that are not safe to apply while neutron-server is running. Those include: column or table removal, moving data from one part of the database into another (renaming a column, transforming single table into multiple, etc.), introducing or modifying constraints, etc.

The intent of the split is to allow invoking those safe migrations from ‘expand’ branch while neutron-server is running, reducing downtime needed to upgrade the service.

For more details, see the Expand and Contract Scripts section below.

Developers

A database migration script is required when you submit a change to Neutron or a sub-project that alters the database model definition. The migration script is a special python file that includes code to upgrade the database to match the changes in the model definition. Alembic will execute these scripts in order to provide a linear migration path between revisions. The neutron-db-manage command can be used to generate migration scripts for you to complete. The operations in the template are those supported by the Alembic migration library.

Script Auto-generation

neutron-db-manage revision -m "description of revision" --autogenerate

This generates a prepopulated template with the changes needed to match the database state with the models. You should inspect the autogenerated template to ensure that the proper models have been altered. When running the above command you will probably get the following error message:

Multiple heads are present; please specify the head revision on which the
new revision should be based, or perform a merge.

This is alembic telling you that it does not know which branch (contract or expand) to generate the revision for. You must decide, based on whether you are doing contracting or expanding changes to the schema, and provide either the --contract or --expand option. If you have both types of changes, you must run the command twice, once with each option, and then manually edit the generated revision scripts to separate the migration operations.

In rare circumstances, you may want to start with an empty migration template and manually author the changes necessary for an upgrade. You can create a blank file for a branch via:

neutron-db-manage revision -m "description of revision" --expand
neutron-db-manage revision -m "description of revision" --contract

NOTE: If you use above command you should check that migration is created in a directory that is named as current release. If not, please raise the issue with the development team (IRC, mailing list, launchpad bug).

The timeline on each alembic branch should remain linear and not interleave with other branches, so that there is a clear path when upgrading. To verify that alembic branches maintain linear timelines, you can run this command:

neutron-db-manage check_migration

If this command reports an error, you can troubleshoot by showing the migration timelines using the history command:

neutron-db-manage history

Expand and Contract Scripts

The obsolete “branchless” design of a migration script included that it indicates a specific “version” of the schema, and includes directives that apply all necessary changes to the database at once. If we look for example at the script 2d2a8a565438_hierarchical_binding.py, we will see:

# .../alembic_migrations/versions/2d2a8a565438_hierarchical_binding.py

def upgrade():

    # .. inspection code ...

    op.create_table(
        'ml2_port_binding_levels',
        sa.Column('port_id', sa.String(length=36), nullable=False),
        sa.Column('host', sa.String(length=255), nullable=False),
        # ... more columns ...
    )

    for table in port_binding_tables:
        op.execute((
            "INSERT INTO ml2_port_binding_levels "
            "SELECT port_id, host, 0 AS level, driver, segment AS segment_id "
            "FROM %s "
            "WHERE host <> '' "
            "AND driver <> '';"
        ) % table)

    op.drop_constraint(fk_name_dvr[0], 'ml2_dvr_port_bindings', 'foreignkey')
    op.drop_column('ml2_dvr_port_bindings', 'cap_port_filter')
    op.drop_column('ml2_dvr_port_bindings', 'segment')
    op.drop_column('ml2_dvr_port_bindings', 'driver')

    # ... more DROP instructions ...

The above script contains directives that are both under the “expand” and “contract” categories, as well as some data migrations. the op.create_table directive is an “expand”; it may be run safely while the old version of the application still runs, as the old code simply doesn’t look for this table. The op.drop_constraint and op.drop_column directives are “contract” directives (the drop column moreso than the drop constraint); running at least the op.drop_column directives means that the old version of the application will fail, as it will attempt to access these columns which no longer exist.

The data migrations in this script are adding new rows to the newly added ml2_port_binding_levels table.

Under the new migration script directory structure, the above script would be stated as two scripts; an “expand” and a “contract” script:

# expansion operations
# .../alembic_migrations/versions/liberty/expand/2bde560fc638_hierarchical_binding.py

def upgrade():

    op.create_table(
        'ml2_port_binding_levels',
        sa.Column('port_id', sa.String(length=36), nullable=False),
        sa.Column('host', sa.String(length=255), nullable=False),
        # ... more columns ...
    )


# contraction operations
# .../alembic_migrations/versions/liberty/contract/4405aedc050e_hierarchical_binding.py

def upgrade():

    for table in port_binding_tables:
        op.execute((
            "INSERT INTO ml2_port_binding_levels "
            "SELECT port_id, host, 0 AS level, driver, segment AS segment_id "
            "FROM %s "
            "WHERE host <> '' "
            "AND driver <> '';"
        ) % table)

    op.drop_constraint(fk_name_dvr[0], 'ml2_dvr_port_bindings', 'foreignkey')
    op.drop_column('ml2_dvr_port_bindings', 'cap_port_filter')
    op.drop_column('ml2_dvr_port_bindings', 'segment')
    op.drop_column('ml2_dvr_port_bindings', 'driver')

    # ... more DROP instructions ...

The two scripts would be present in different subdirectories and also part of entirely separate versioning streams. The “expand” operations are in the “expand” script, and the “contract” operations are in the “contract” script.

For the time being, data migration rules also belong to contract branch. There is expectation that eventually live data migrations move into middleware that will be aware about different database schema elements to converge on, but Neutron is still not there.

Scripts that contain only expansion or contraction rules do not require a split into two parts.

If a contraction script depends on a script from expansion stream, the following directive should be added in the contraction script:

depends_on = ('<expansion-revision>',)

Applying database migration rules

To apply just expansion rules, execute:

neutron-db-manage upgrade --expand

After the first step is done, you can stop neutron-server, apply remaining non-expansive migration rules, if any:

neutron-db-manage upgrade --contract

and finally, start your neutron-server again.

If you are not interested in applying safe migration rules while the service is running, you can still upgrade database the old way, by stopping the service, and then applying all available rules:

neutron-db-manage upgrade head[s]

It will apply all the rules from both the expand and the contract branches, in proper order.