How to Use Kustomize in Kubernetes: Simple Guide

Use kustomize to customize Kubernetes YAML files by creating a kustomization.yaml file that lists resources and patches. Run kustomize build to generate the final YAML with your customizations applied, then apply it with kubectl apply -k.

Syntax

The main file for Kustomize is kustomization.yaml. It lists resources, patches, and configurations to customize Kubernetes manifests.

Key parts include:

• resources: List of base YAML files or directories.
• patchesStrategicMerge: YAML snippets to modify existing resources.
• configMapGenerator: To create ConfigMaps from literals or files.

Use kustomize build [path] to generate the customized YAML output.

yaml

resources:
- deployment.yaml
- service.yaml

patchesStrategicMerge:
- patch.yaml

configMapGenerator:
- name: example-config
  literals:
  - key1=value1
  - key2=value2

Example

This example shows how to customize a Deployment by changing the image and adding environment variables using Kustomize.

yaml

# deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 2
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: myapp-container
        image: nginx:1.14.2
        ports:
        - containerPort: 80

---
# kustomization.yaml
resources:
- deployment.yaml

patchesStrategicMerge:
- patch.yaml

---
# patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  template:
    spec:
      containers:
      - name: myapp-container
        image: nginx:1.16.0
        env:
        - name: ENV_VAR
          value: "production"

Output

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 2
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: myapp-container
        image: nginx:1.16.0
        ports:
        - containerPort: 80
        env:
        - name: ENV_VAR
          value: "production"

Common Pitfalls

Common mistakes when using Kustomize include:

• Not matching metadata.name exactly in patches, so changes don't apply.
• Using kubectl apply -f instead of kubectl apply -k to apply Kustomize directories.
• Forgetting to include all resources in kustomization.yaml.
• Incorrect indentation or YAML syntax causing build errors.

yaml

Wrong patch name:
apiVersion: apps/v1
kind: Deployment
metadata:
  name: wrongname  # This won't patch the original deployment

Right patch name:
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp  # Must match the original resource name

Quick Reference

Remember these commands and files when using Kustomize:

kubernetes

kustomization.yaml - main config file listing resources and patches

kustomize build [dir] - generates customized YAML

kubectl apply -k [dir] - applies customized resources to cluster

Command/File	Purpose
kustomization.yaml	Defines resources and customizations
kustomize build [dir]	Generates final YAML with customizations
kubectl apply -k [dir]	Applies customized YAML to Kubernetes cluster

Key Takeaways

Create a kustomization.yaml file listing your base resources and patches.

Use kustomize build to generate customized YAML before applying.

Always apply with kubectl apply -k to use Kustomize directories.

Ensure patch metadata names exactly match the resources to modify.

Check YAML syntax carefully to avoid build errors.