12 KiB
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.
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 via:
neutron-db-manage revision -m "description of revision"
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@head
After the first step is done, you can stop neutron-server, apply remaining non-expansive migration rules, if any:
neutron-db-manage upgrade contract@head
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.