Rollback

A rollback is returning to the previous working version after a release goes wrong. It is the single most valuable property a deployment process can have, because it converts a serious incident into a brief one.

Code rolls back easily; data does not. A release that added a column is usually safe to reverse, while one that transformed existing rows or dropped a column may not be reversible at all. This is why schema changes are made in compatible steps — add the new, write to both, migrate, and only then remove the old.

The question worth asking before any significant release is not whether it will work, but how long it takes to undo. If nobody knows, that is the thing to fix first.