How does Rails handle zero-downtime data migrations at scale?

Rails achieves zero-downtime migrations using expand-contract patterns, backfilling jobs, and backward-compatible schema changes.

At scale, migrations must avoid locking tables or breaking running code. Rails teams use a multi-phase approach: (1) expand schema by adding nullable columns or new tables, (2) deploy application code that writes to both old and new structures, (3) backfill data asynchronously using jobs, and (4) contract by removing legacy fields after full migration. Tools like StrongMigrations help prevent unsafe operations. This ensures continuous availability during deployments.