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What is the difference between a base and an overlay in Kustomize?

Infrastructure as Code (Terraform, Ansible) · Advanced level

Answer

A base is a reusable set of Kubernetes manifests. An overlay references a base and applies environment-specific changes such as replicas, image tags, labels, patches, namespace, or generated config. Bases define common intent; overlays define differences.

Technical explanation

The base should be environment-neutral.

Overlays should contain only the deltas needed for a target environment.

This model reduces copy-paste and makes production differences reviewable.

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: What is the difference between a base and an overlay in Kustomize?

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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