How does Rails handle large-scale schema evolution with zero downtime in distributed systems?

Rails uses expand-contract migrations, backward-compatible changes, and dual-writing strategies.

In distributed systems, schema changes must not break running services. Rails follows expand-contract patterns: first adding new columns, then deploying code that writes to both old and new schema, migrating data asynchronously, and finally removing old structures. Dual writes ensure compatibility during transition. StrongMigrations gem helps prevent unsafe operations like locking ALTER TABLE in production.