DB Migrations

Changes to the DB schema should be made in the form of migrations. This ensures that all changes are applied correctly after a DB dump from a previous version is restored.

DB migrations are managed through a system built on top of MyBatis Migrations. The migrations are run automatically when a snapshot is restored, no manual interaction is required by cluster operators.


When adding or altering tables or changing data, in addition to making to change in schema.sql, a new migration class should be created under the org.apache.aurora.scheduler.storage.db.migration package. The class should implement the MigrationScript interface (see V001_TestMigration as an example). The upgrade and downgrade scripts are defined in this class. When restoring a snapshot the list of migrations on the classpath is compared to the list of applied changes in the DB. Any changes that have not yet been applied are executed and their downgrade script is stored alongside the changelog entry in the database to faciliate downgrades in the event of a rollback.


If, while running migrations, a rollback is detected, i.e. a change exists in the DB changelog that does not exist on the classpath, the downgrade script associated with each affected change is applied.


After enough time has passed (at least 1 official release), it should be safe to baseline migrations if desired. This can be accomplished by ensuring the changes from migrations have been applied to schema.sql and then removing the corresponding migration classes and adding a migration to remove the changelog entries.