Migrations

Database migrations are where deploys go to die. Here’s how to make them safe. The Golden Rule Every migration must be backward compatible. Your old code will run alongside the new schema during deployment. If it can’t, you’ll have downtime. Safe Operations These are always safe: Adding a nullable column Adding a table Adding an index (with CONCURRENTLY) Adding a constraint (as NOT VALID, then validate later) Dangerous Operations These need careful handling: ...

The scariest deploy isn’t code—it’s schema changes. One wrong migration can lock tables, corrupt data, or bring down production. Zero-downtime migrations require discipline, but they’re achievable. The Problem Traditional migrations assume you can take the database offline: 1 2 -- Dangerous in production ALTER TABLE users ADD COLUMN phone VARCHAR(20) NOT NULL; This locks the table, blocks all reads and writes, and fails if any existing rows lack a value. In a busy system, that’s an outage. ...