Master Kustomize: Simplify Kubernetes Configs with Generators and Transformers

Introduction

Kustomize is built‑in to kubectl (v1.14+) and provides a declarative, template‑free way to customize Kubernetes manifests via a kustomization.yaml file.

Core building blocks

1. Resources – base configuration

The resources field lists YAML files or directories that form the foundation (Deployments, Services, etc.). A common “Base + Overlay” layout places shared files in a base directory and environment‑specific overlays reference them.

# kustomization.yaml
resources:
- ../base   # include all .yaml under base
# or individual files
# - ../base/deployment.yaml
# - ../base/service.yaml

2. Generators – dynamic ConfigMap and Secret creation

Generators automate creation of ConfigMaps and Secrets from files, literals or environment variables.

# configmap-generator.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: ConfigMapGenerator
metadata:
  name: app-config
files:
- application.properties
literals:
- LOG_LEVEL=info

3. Transformers – modify resources

Transformers adjust labels, annotations, names, namespaces, image tags, or apply JSON/strategic‑merge patches.

Add/modify commonLabels or commonAnnotations Prefix or suffix names with namePrefix / nameSuffix Change namespace via namespace field or a patch file

Update container image tags with the images block

Apply complex changes with patchesJson6902 or patchesStrategicMerge Example production overlay:

# kustomization.yaml (production overlay)
namespace: production
commonLabels:
  env: production
images:
- name: my-app
  newTag: v1.2.0
resources:
- ../../base

4. Validators (optional) – enforce policies

Validators run after all resources are generated and can enforce required labels, resource limits, or integrate with policy engines such as OPA Gatekeeper.

Require a label (e.g., owner) on every resource

Check CPU/Memory limits are within allowed ranges

Hook into external policy engines for advanced validation

Processing flow

Load base resources – read files listed in resources Run generators – create ConfigMaps, Secrets, etc.

Apply transformers – modify the resources from the previous steps

(Optional) Run validators – verify the final manifest against policies

Output – merge everything into the final YAML

Preview the result with kubectl kustomize <kustomization_directory> or apply directly using kubectl apply -k <kustomization_directory>.

Key takeaways

Resources define the immutable base.

Generators create mutable configuration data and secrets.

Transformers provide flexible, environment‑specific customisation.

Validators (optional) add a compliance layer before deployment.