rails-migrations

Two-phase column removal eliminates runtime errors by deprecating the column first, then removing it in a separate deployment. This ensures older code finishes before the column disappears, maintaining backward compatibility across deployments.

Zero-downtime migrations are schema changes deployed without stopping the application. They prevent errors when old and new code versions run simultaneously, using techniques like concurrent index creation and staged backfills to keep the database accessible during deployment.

Use a two-phase approach: backfill existing rows with a default value before deployment, then add the constraint in a separate migration. This prevents locking tables and ensures all data satisfies the constraint before enforcement.

Pre-deployment cleanup scripts process data in batches outside the migration, avoiding long-running transactions that lock tables. This technique completes backfills before deployment, making the actual migration fast and non-blocking.

Reversible migrations include both `up` and `down` steps, enabling safe rollback if issues arise. Two-phase patterns and strong migrations ensure rollbacks don't corrupt data or break running application code.