database-migration-safety

You prevent database downtime during production schema migrations by using an expand-contract plan that breaks changes into independent phases, ensuring the system never lands in a broken transitional state while minimizing locking risk.

The expand-contract pattern for online schema changes separates risky migrations into expand, migrate, switch, and contract phases, allowing you to add columns and backfill data before switching application traffic and dropping old schema.

To safely backfill large production database tables, execute batched, throttled, and resumable backfills that are idempotent, ensuring the operation can pause under load and resume without data corruption or locking issues.

Database migrations require online schema change tooling when operations involve adding NOT NULL constraints, type changes, unique indexes, or FK validations that risk locking tables or causing outages during production traffic.

Plan rollback criteria for one-way database migrations by enforcing reversible versus explicitly one-way decisions with backup-verified rollback paths, ensuring you have verified data backups before executing irreversible schema changes.

A limitation of using expand-contract migrations for dropping columns is that the contract phase becomes a one-way operation, requiring explicit justification and backup verification because the dropped data cannot be restored without point-in-time recovery.