Kustomize: Bases, Overlays, and Patches

You've been there: three environments, three directories of YAML, differing by two lines. Dev has two replicas. Staging has a debug flag set. Production has resource limits and a health check. You change the base Deployment and touch all three. You forget one. Production breaks.

Kustomize was built to solve exactly this. Instead of maintaining three copies of your manifests, you define a base once and layer environment-specific changes on top with overlays. No templates, no rendering engine, no {{ }} anywhere. Just plain YAML and a kustomization.yaml file that tells Kustomize what to combine and how.