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.

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