immutable infrastructure

Terraform shapes the world; images define what boots inside it. Bake contents into versioned artifacts on a patch cadence, roll fleets by replacement, and keep user_data for identity and wiring only.

SSH plus yum is how outages start. The senior answer replaces nodes from versioned images on a cadence, orchestrates with surge capacity and PDBs, and can explain the kernel-skew problem that makes fleet homogeneity a feature.