# Kustomize Overlays — Kubernetes

Source: https://www.skillbyai.com/en/kubernetes/p-kustomize

> One base, many environments.

## Patch without templates

**Kustomize** (built into kubectl) composes plain YAML: a **base** with the common manifests and **overlays** per environment that set a namespace, add labels, change image tags, patch replica counts and generate ConfigMaps and Secrets. There is no templating language; overlays describe differences. Build with `kubectl kustomize dir` to see the result, or apply with `kubectl apply -k dir`.

## The prod overlay's kustomization.yaml

The base contains a plain Deployment (2 replicas, image tag 1.4.0) and Service.

```yaml
resources:
- ../../base
namespace: shop-prod
labels:
- pairs:
    env: prod
images:
- name: ghcr.io/example/web
  newTag: 1.5.2
patches:
- target:
    kind: Deployment
    name: web
  patch: |-
    - op: replace
      path: /spec/replicas
      value: 4
```

## Building the prod overlay, run

I ran this with kubectl 1.37.0 using --dry-run=client (or kubectl kustomize), which generates manifests locally without a cluster; nothing was applied to a live cluster. The overlay sets namespace shop-prod, adds the env: prod label, changes the image tag to 1.5.2 and patches replicas from 2 to 4, without editing the base files.

```bash
kubectl kustomize kz/overlays/prod
```

Output:

```
apiVersion: v1
kind: Service
metadata:
  labels:
    env: prod
  name: web
  namespace: shop-prod
spec:
  ports:
  - port: 80
    targetPort: 8080
  selector:
    app: web
---
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    env: prod
  name: web
  namespace: shop-prod
spec:
  replicas: 4
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - image: ghcr.io/example/web:1.5.2
        name: web
        ports:
        - containerPort: 8080
```

**Quiz:** How does Kustomize customise manifests per environment?

- [ ] It edits files in place
- [x] Overlays patch a shared base without a templating language
- [ ] It uses Go templates
- [ ] It requires a separate server

*Answer:* Overlays patch a shared base without a templating language. Base plus overlays.