This content originally appeared on DEV Community and was authored by Nahla Thabet
What is Kustomize?
Kustomize is a configuration management solution that leverages layering to preserve the base settings of your applications and components by overlaying declarative yaml artifacts (called patches) that selectively override default settings without actually changing the original files.
say that you have 3 env you want to deploy the same application on them but every env has diff number of replica, for example this wordoress deployment, and I have staging and prod environment.
Using Kustomize: A simple guide
first we should create folder for base manifests file and create folders for every env under overlay section.
my-app/
├── base/
│ ├── kustomization.yaml
│ └── deployment.yaml
|
└── overlays/
└── staging/
| ├── kustomization.yaml
| └── replica-patch.yaml
|
|___prod/
├── kustomization.yaml
└── deployment-patch.yaml
Kustomize Patching Strategies
we've set up our directory structure, let's explore the different ways to patch our WordPress deployment for different environments.
- base/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: wordpress
labels:
app: wordpress
spec:
replicas: 2
selector:
matchLabels:
app: wordpress
template:
metadata:
labels:
app: wordpress
spec:
containers:
- name: wordpress
image: wordpress:apache
ports:
- containerPort: 80
env:
- name: WORDPRESS_DB_HOST
value: cluster1-haproxy.mysql-cluster.svc.cluster.local:3306
- name: WORDPRESS_DB_NAME
value: wordpress
- name: WORDPRESS_DB_USER
value: wp
- name: WORDPRESS_DB_PASSWORD
valueFrom:
secretKeyRef:
name: wp-secret
key: wp-password
resources:
requests:
memory: "256Mi"
cpu: "200m"
limits:
memory: "512Mi"
cpu: "400m"
volumeMounts:
- name: wordpress-data
mountPath: /var/www/html
volumes:
- name: wordpress-data
persistentVolumeClaim:
claimName: wordpress-pvc
- base/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- deployment.yaml
Method 1: Strategic Merge Patch
Strategic Merge Patch is the simplest approach. You define only the fields you want to change, and Kustomize intelligently merges them with the base configuration.
- overlays/staging/replica-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: wordpress
spec:
replicas: 3
- overlays/staging/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- ../../base
patches:
- path: replica-patch.yaml
For production, you might want more replicas and different resource limits:
- overlays/prod/deployment-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: wordpress
spec:
replicas: 5
template:
spec:
containers:
- name: wordpress
resources:
requests:
memory: "512Mi"
cpu: "500m"
limits:
memory: "1Gi"
cpu: "1000m"
- overlays/prod/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- ../../base
patches:
- path: deployment-patch.yaml
Method 2: JSON 6902 Patches
JSON 6902 patches provide more precise control over modifications using JSON Patch operations (add, remove, replace, move, copy, test).
- overlays/prod/deployment-patch.yaml
- op: replace
path: /spec/replicas
value: 5
- op: replace
path: /spec/template/spec/containers/0/resources/requests/memory
value: "512Mi"
- op: replace
path: /spec/template/spec/containers/0/resources/limits/memory
value: "1Gi"
- overlays/prod/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- ../../base
patches:
- target:
group: apps
version: v1
kind: Deployment
name: wordpress
path: deployment-patch.yaml
Install Kustomize
- run the following:
curl -s "https://raw.githubusercontent.com/kubernetes-sigs/kustomize/master/hack/install_kustomize.sh" | bash
- Move Kustomize to your path, so that it can be accessed system wide:
sudo mv kustomize /usr/local/bin
Kustomize testing and applying
Testing Kustomize transformations
Always be sure to test Kustomize patches and transformations before applying them. Two useful commands for this are using kubectl apply -k
with the --dry-run=client -o yaml flag as well as and kubectl diffplugin.
Example usage:
kubectl apply -k <path-to-your-kustomization-directory> --dry-run=client -o yaml
This command outputs the YAML rendered by combining the base resources and applying the patches defined in the <path-to-your-kustomization-directory> directory, without actually applying any changes. You can inspect the output to ensure the patches were applied correctly.
Use kubectl diff to compare the changes between your live cluster resources and the new configurations generated by Kustomize.
Example usage:
kubectl diff -k <path-to-your-kustomization-directory>
This will compare the current state of your cluster with the resources that Kustomize would apply and show a diff of the differences.
Apply Patches
Once you have confirmed that your overlays are correct and choose which env you want to apply the changes on it, use the kubectl apply -k overlays/<env-name> command to apply the the settings to your cluster:
kubectl apply -k overlays/staging
kubectl apply -k overlays/prod
This content originally appeared on DEV Community and was authored by Nahla Thabet
Nahla Thabet | Sciencx (2025-10-28T20:56:22+00:00) Kustomization Tutorial. Retrieved from https://www.scien.cx/2025/10/28/kustomization-tutorial/
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