Kustomize Explained

Kustomize lets you customize Kubernetes YAML without templates. Instead of injecting variables into Go templates (like Helm), you write plain, valid YAML and declare how to patch and overlay it for different environments. It's built into kubectl since v1.14.

Canonical Directory Structure

Two Ways to Patch

Kustomize supports two patching strategies. Both modify the base without touching the original files.

Write a partial resource that gets deep-merged into the base. Only specify the fields you want to change.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: api
spec:
  replicas: 2
  template:
    spec:
      containers:
        - name: api
          image: myapp:latest

apiVersion: apps/v1
kind: Deployment
metadata:
  name: api
spec:
  replicas: 5
  template:
    spec:
      containers:
        - name: api
          resources:
            limits:
              memory: 512Mi

apiVersion: apps/v1
kind: Deployment
metadata:
  name: api
spec:
  replicas: 5          # changed
  template:
    spec:
      containers:
        - name: api
          image: myapp:latest # kept
          resources:        # added
            limits:
              memory: 512Mi

An array of operations (add, remove, replace) applied to specific JSON paths. Use when strategic merge can't express your change.

- op: replace
  path: /spec/replicas
  value: 5

- op: add
  path: /metadata/labels/env
  value: prod

- op: remove
  path: /spec/template/spec/containers/0/env/2

patches:
  - path: json-patch.yaml
    target:
      kind: Deployment
      name: api

Generators & Transformers

Beyond patches, kustomization.yaml has built-in generators that create resources and transformers that modify all resources at once.