When removing existing migrations from the GitLab project, you have to take into account the possibility of the migration already been included in past releases or in the current release, and thus already executed on GitLab.com and/or in self-hosted instances.
Because of it, it’s not possible to delete existing migrations, as that could lead to:
- Schema inconsistency, as changes introduced into the database were not rollbacked properly.
- Leaving a record on the
schema_versionstable, that points out to migration that no longer exists on the codebase.
Instead of deleting we can opt for disabling the migration.
Migrations can be disabled if:
- They caused a timeout or general issue on GitLab.com.
- They are obsoleted, e.g. changes are not necessary due to a feature change.
- Migration is a data migration only, i.e. the migration does not change the database schema.
In order to disable a migration, the following steps apply to all types of migrations:
- Turn the migration into a noop by removing the code inside
#performmethods, and adding
- Add a comment explaining why the code is gone.
Disabling migrations requires explicit approval of Database Maintainer.