Delivery and controlled rollouts

Keep mixed versions compatible while releasing a change

An old server writes email, while a new server expects contact_email. They share a database during rollout. Returning to the old application artifact does not undo rows already changed by the new version.

Plan expansion, compatible readers and writers, backfill, exposure, and retirement. Use the configuration case to study a rollout that is syntactically valid but unusable. You finish with a sequence and recovery boundary, not just a successful deployment command.