Definition. An operating approach in which servers, nodes and container images are no longer modified after deployment. Changes are made only by building a new image and replacing the old one. No apt upgrade on production systems, no kubectl exec workaround.

Why it matters. Drift is the most common reason why systems at mid-sized companies can no longer be reproduced after a few years. Immutable infrastructure is a technical answer to this that also unifies patching and hardening.

Related. Declarative Infrastructure, GitOps, Talos Linux, Flatcar Linux, Bottlerocket