How to use Kustomize overlays for multi-environment?

Question

Accepted Answer

**Kustomize overlays** — base manifest + patches for each environment.

**Structure:**
```
k8s/
  base/
    kustomization.yaml
    deployment.yaml
    service.yaml
  overlays/
    dev/
      kustomization.yaml
      patch-replicas.yaml
    staging/
      kustomization.yaml
    production/
      kustomization.yaml
      patch-replicas.yaml
      patch-resources.yaml
```

**base/kustomization.yaml:**
```yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
  - deployment.yaml
  - service.yaml
commonLabels:
  app: myapp
```

**base/deployment.yaml:**
```yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  template:
    spec:
      containers:
        - name: myapp
          image: myapp:latest
          ports:
            - containerPort: 8080
```

**overlays/production/kustomization.yaml:**
```yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
namespace: production
bases:
  - ../../base
patchesStrategicMerge:
  - patch-replicas.yaml
  - patch-resources.yaml
commonLabels:
  environment: production
namePrefix: prod-
```

**overlays/production/patch-replicas.yaml:**
```yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 5
```

**Usage:**
```bash
kubectl apply -k k8s/overlays/dev
kubectl apply -k k8s/overlays/production

# Render (without apply)
kustomize build k8s/overlays/production | kubectl apply -f -
```

**Kustomize vs Helm:** Kustomize is simpler for custom manifests. Helm for charts with templating.

DevOps: How to use Kustomize overlays for multi-environment?