Decoupling database migrations from server startup: why and how

Zero-downtime schema migrations

The general process is:

1. Migrate from schema S to schema S+1, with only additive changes.
2. Over time upgrade some of your processes from application version V to V+1.
3. Eventually everything is on V+1, and you don’t ever expect to rollback to V.
4. Finally, migrate from schema S+1 to S+2, and now you can do destructive schema changes to anything that V+1 no longer uses