Interview › Infrastructure as Code (Terraform, Ansible)
How do overlays customise a base without copying it?
Infrastructure as Code (Terraform, Ansible) · Advanced level
Answer
Overlays customize a base by referencing it and applying patches or transformations rather than copying the YAML. This keeps common resources in one place and lets dev, staging, and prod differ only where they must.
Technical explanation
Patches are applied at build time, so source control contains both common YAML and environment deltas.
This prevents divergence where dev and prod copies silently drift apart.
Small overlays are easier to audit than full copied manifests.
Keep source manifests or IaC definitions readable enough that reviewers can understand the final desired state.
Use overlays, modules, or roles for reuse, but keep environment-specific differences explicit and reviewable.
Validate generated output in CI before applying it through kubectl, Argo CD, Terraform, or Ansible.
Hands-on example
1. Create a Kustomize base and overlay for: How do overlays customise a base without copying it?
2. Base files:
base/deployment.yaml
base/service.yaml
base/kustomization.yaml
base/kustomization.yaml:
resources:
- deployment.yaml
- service.yaml
commonLabels:
app.kubernetes.io/name: payments
3. prod overlay:
resources:
- ../../base
namePrefix: prod-
namespace: payments-prod
images:
- name: ghcr.io/company/payments
newTag: 1.8.4
configMapGenerator:
- name: app-config
literals:
- LOG_LEVEL=info
patches:
- path: replica-patch.yaml
4. Render and apply:
kubectl kustomize overlays/prod
kubectl diff -k overlays/prod
kubectl apply -k overlays/prod
5. In GitOps, point Argo CD at overlays/prod and let it render, compare, and sync the desired state.
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