2.7 KiB
No More Downward SQL Schema Migrations
SQL Migrations are the accepted method for OpenStack projects to ensure that a given schema is consistent across deployments. Often these migrations include updating of the underlying data as well as modifications to the schema. It is inadvisable to perform a downwards migration in any environment.
Problem description
Best practices state: when reverting post schema migration the correct course is to restore to a known-good state of the database.
Many migrations in OpenStack include data-manipulation to ensure the data conforms to the new schema; often these data-migrations are difficult or impossible to reverse without significant overhead. Performing a downgrade of the schema with such data manipulation can lead to inconsistent or broken state. The possibility of bad-states, relatively minimal testing, and no demand for support renders a downgrade of the schema an unsafe action.
Proposed change
The proposed change is as follows:
- Eliminate the downgrade option(s) from the CLI tools utilized for schema migrations. This is done by rejecting any attempt to migrate that would result in a lower schema version number than current
- Document best practices on restoring to a previous schema version including the steps to take prior to migrating the schema
- Stop support of migrations with downgrade functions across all projects
- Do not add future migrations with downgrade functions
Alternatives
Downgrade paths can continue to be supported.
Implementation
Assignee(s)
Morgan Fainberg (irc: morganfainberg) Matt Fischer (irc: mfisch)
Work Items
- Document best practices for restoring to a previous schema version
- Update oslo.db to return appropriate errors when trying to perform a schema downgrade
- (all core teams) Do not accept new migrations that include downgrade functions
- (all core teams) Projects may drop downgrade functions in all current migration scripts
Dependencies
No external dependencies.
References
- openstack-dev mailing list topic <http://lists.openstack.org/pipermail/ openstack-dev/2015-January/055586.html>
- openstack-operators mailing list topic <http://lists.openstack.org/ pipermail/openstack-operators/2015-January/006082.html>
- Current migration/rollback documentation <http://docs.openstack.org/ openstack-ops/content/ops_upgrades-roll-back.html>
Note
This work is licensed under a Creative Commons Attribution 3.0 Unported License. http://creativecommons.org/licenses/by/3.0/legalcode