Backend & systems·Deployment, release and rollback
you ship in the middle of the afternoon and no request fails, because nothing is ever stopped while it is still serving.
Zero-downtime deployment
Deploying without dropping requests, which requires readiness checks, graceful shutdown, compatible schema changes and a rollout that maintains capacity. It is less a feature than the sum of several other disciplines. Almost all deploy-time error spikes come from missing one of them, usually shutdown handling.