Using Kustomize Overlays for Per-Environment Config

Put what is shared in a base. Put what differs in overlays, one per environment. Argo CD detects the kustomization.yaml in the overlay path and runs kustomize build to render the manifests. Base holds: Deployment, Service, baseline ConfigMap, common labels. Overlays patch: replicas, resource requests and limits, image tags, ingress hosts, environment-specific ConfigMap values, env vars, and HPA settings. Key rules I follow: 1. Anything that differs between environments lives in overlays, never the base. 2. Image tags belong in overlays so CI can bump dev without touching prod. 3. Patch with strategic merge or JSON patches. Do not copy the whole Deployment YAML into each overlay; that is exactly the duplication Kustomize is supposed to remove. 4. Use configMapGenerator with behavior: merge to add or override values, not replace the whole map. 5. Secrets do not go in Kustomize. Use external-secrets, sealed-secrets, or a secrets manager. On the Argo CD side, the Application points at the overlay path (apps/web/overlays/prod) and Argo CD figures out it is Kustomize from the kustomization.yaml file. No extra config needed. Edge case: if the same overlay applies to two clusters with one tiny difference (a region label, a DNS suffix), do not create a fourth overlay. Use ApplicationSet parameters to pass that one value in. Otherwise overlays multiply fast.