What is Kustomize? Kubernetes Config Management Explained Simply

# Already built into kubectl
kubectl apply -k ./overlays/prod/
 
# Or use the standalone binary
kustomize build ./overlays/prod/ | kubectl apply -f -

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 1
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
        - name: myapp
          image: myapp:latest
          ports:
            - containerPort: 8080

resources:
  - deployment.yaml
  - service.yaml

bases:
  - ../../base
 
images:
  - name: myapp
    newTag: v1.2.3       # override the image tag
 
patches:
  - path: replica-patch.yaml
 
commonLabels:
  environment: production

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 5             # override replicas for prod

kubectl apply -k k8s/overlays/prod/

# kustomization.yaml
images:
  - name: myapp
    newTag: "$(IMAGE_TAG)"    # works with CI/CD variables

# In your CI pipeline:
kustomize edit set image myapp:$CI_COMMIT_SHA
kubectl apply -k .

# kustomization.yaml
configMapGenerator:
  - name: app-config
    files:
      - config/app.properties
    literals:
      - APP_ENV=production
      - LOG_LEVEL=info

# kustomization.yaml
namespace: production    # applies to all resources

# kustomization.yaml
commonLabels:
  team: backend
  app: myapp
 
commonAnnotations:
  managed-by: kustomize

# replica-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 5

# kustomization.yaml
patches:
  - target:
      kind: Deployment
      name: myapp
    patch: |-
      - op: replace
        path: /spec/replicas
        value: 5

# ArgoCD Application
apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: myapp-prod
spec:
  source:
    repoURL: https://github.com/myorg/k8s-configs
    targetRevision: main
    path: overlays/prod      # ArgoCD runs kustomize build here
  destination:
    server: https://kubernetes.default.svc
    namespace: production

# Install kustomize
curl -s "https://raw.githubusercontent.com/kubernetes-sigs/kustomize/master/hack/install_kustomize.sh" | bash
 
# Preview what will be applied (dry run)
kustomize build overlays/prod/
 
# Apply
kubectl apply -k overlays/prod/
 
# Diff against live cluster
kubectl diff -k overlays/prod/