Transformers in Kustomize

What Are Transformers?

In Kustomize, Transformers are modifiers that change the base YAML without editing it directly.

Think of your base YAML as a vanilla ice cream, and transformers are like toppings — chocolate chips, syrup, nuts — all applied without changing the original ice cream.

Common Transformers

Here’s the list of the most used built-in transformers.

Transformer Purpose
labels Add labels to all resources
commonAnnotations Add annotations to all resources
namespace Set namespace for all resources
namePrefix Add a prefix to resource names
nameSuffix Add a suffix to resource names
images Override image name/tag

Usage Pattern

All transformers are defined in kustomization.yaml.

A) Adding commonLabels

Do: Add to kustomization.yaml

# Warning: 'commonLabels' is deprecated. Please use 'labels' instead.
labels:
  app: payment
  team: backend

Where: Inside the same kustomization.yaml that points to your base resources.

See Output: Before:

metadata:
  name: my-deployment

After:

metadata:
  name: my-deployment
  labels:
    app: payment
    team: backend

B) Adding commonAnnotations

commonAnnotations:
  owner: ibtisam
  environment: dev

Adds annotations to all resources.

C) Setting Namespace

namespace: dev

Before:

metadata:
  name: my-deployment

After:

metadata:
  name: my-deployment
  namespace: dev

D) Adding Prefix & Suffix to Names

namePrefix: dev-
nameSuffix: -v2

Before:

metadata:
  name: my-deployment

After:

metadata:
  name: dev-my-deployment-v2

E) Overriding Images

images:
  - name: nginx                              # name of the image, not container
    newName: myregistry.com/custom-nginx
    newTag: "1.19"                          # must contain ""

Before:

image: nginx:latest

After:

image: myregistry.com/custom-nginx:1.19

Why This is Powerful for Exams

  • In CKA, you can quickly change namespace, image tags, or add metadata without touching original YAML.
  • Great for multi-environment configs (dev/prod/staging) from same base.

Pro Tips

  • Transformers apply to all resources unless scoped with selectors (advanced).
  • You can chain multiple transformers in one kustomization.yaml.
  • Always preview with:
kustomize build <dir>
  • Be aware: transformers overwrite existing values if same key exists.

Lab

Assign the following annotation to all nginx and monitoring resources: owner: bob@gmail.com

controlplane ~/code/k8s ➜  cat nginx/kustomization.yaml 
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

resources:
  - nginx-depl.yaml
  - nginx-service.yaml

commonAnnotations:
  owner: bob@gmail.com
controlplane ~/code/k8s ➜  cat monitoring/kustomization.yaml 
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

resources:
  - grafana-depl.yaml
  - grafana-service.yaml

namespace: logging

commonAnnotations:
  owner: bob@gmail.com

Transform all postgres images in the project to mysql.

Since the requirement is to change all postgres images to mysql this means adding an image transformer to the root kustomization.yaml file.

controlplane ~/code/k8s ➜  cat kustomization.yaml 
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

resources:
  - db/
  - monitoring/
  - nginx/

commonLabels:
  sandbox: dev

images:
  - name: postgress
    newName: mysql

Transform all nginx images in the nginx directory to nginx:1.23.

controlplane ~/code/k8s ➜  cat nginx/kustomization.yaml 
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

resources:
  - nginx-depl.yaml
  - nginx-service.yaml

commonAnnotations:
  owner: bob@gmail.com

images:
  - name: nginx
    newTag: "1.23"