Immutable Infrastructure

Immutable infrastructure is the principle that once a server, container, or VM image is built and deployed, it is not patched or reconfigured in place. If something needs to change - a security patch, a new application version, a tweak to nginx - you build a new image and replace the running instance. The old one is destroyed.

This is the opposite of the long-standing “pet server” model, where production hosts accumulated months or years of manual fixes that nobody fully remembered.

The benefits:

• No configuration drift. Two servers built from the same image are identical; you can reason about behaviour without ssh-ing in to check.
• Faster recovery. Instead of debugging why a host is sick, you replace it.
• Cleaner audit trails. Every change is a build artefact in a registry, not a series of commands in someone’s terminal history.

Containers and Kubernetes make this approach the default. With VMs it requires more deliberate effort - Packer for image building, an autoscaling group or equivalent for replacement, and IaC for the surrounding plumbing.