What is Kustomize

What is Kustomize

Kustomize is a Kubernetes-native configuration management tool that lets you customize resource YAML files in a reusable, declarative way—without modifying the original (base) files. It was added natively into kubectl (via the -k option) starting in Kubernetes v1.14.

Unlike Helm, which uses templates, Kustomize works solely with plain YAML overlays. It lets teams create a base configuration (shared across environments) and then apply environment-specific patches (for example, dev, staging, or production).

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Core Concepts

1. Base – Common configuration shared by all environments (like Deployments, Services).
2. Overlay – Environment-specific customizations layered over the base using patching and variable substitution.
3. kustomization.yaml – The control file that defines which resources and patches are used together.
4. Patches – YAML fragments that modify specific fields (e.g., replica count, image tag) without editing base files.

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Directory Structure Example

Here’s a simple structure for an app managed with Kustomize:

myapp/
 ├── base/
 │   ├── deployment.yaml
 │   ├── service.yaml
 │   └── kustomization.yaml
 └── overlays/
     ├── dev/
     │   ├── kustomization.yaml
     │   └── patch.yaml
     └── prod/
         ├── kustomization.yaml
         └── patch.yaml

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Example: Basic Usage

1. Base Configuration (base/kustomization.yaml)

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
  - deployment.yaml
  - service.yaml
commonLabels:
  app: myapp

deployment.yaml (in base/):

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 2
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: myapp
        image: nginx:1.20

service.yaml:

apiVersion: v1
kind: Service
metadata:
  name: myapp-service
spec:
  type: ClusterIP
  selector:
    app: myapp
  ports:
  - port: 80
    targetPort: 80

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2. Dev Overlay (overlays/dev/kustomization.yaml)

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
  - ../../base
patchesStrategicMerge:
  - patch.yaml

patch.yaml (only changes dev-specific values):

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 1
  template:
    spec:
      containers:
      - name: myapp
        image: nginx:1.21

To apply the dev configuration to your cluster:

kubectl apply -k overlays/dev

The -k flag tells kubectl to use Kustomize to build and apply manifests.

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Example: Production Overlay

overlays/prod/patch.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 4
  template:
    spec:
      containers:
      - name: myapp
        image: nginx:1.23

Apply with:

kubectl apply -k overlays/prod

This deploys the same base configuration but customizes replicas and image version for production.

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Real‑World Use Cases

1. Multiple Environments Management
   Maintain a single base configuration for all environments (dev/staging/prod) and overlay environment‑specific changes such as image tags, resources, or Ingress settings.

2. Team Collaboration
   Developers can safely adjust configuration previews (like replicas or environment variables) without touching production YAML files.

3. GitOps Pipelines
   Kustomize integrates smoothly with CI/CD pipelines and GitOps tools (Flux, ArgoCD) for environment‑specific deployments.

4. Combining with Helm
   Many teams use Helm for packaging and use Kustomize to patch Helm‑generated files for organization‑specific tweaks without forking the chart.

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Summary

• Tool Type

  • Description: Native YAML overlay configuration manager
  • Command: kubectl apply -k

• Works On

  • Description: Kubernetes manifests (Deployments, Services, ConfigMaps, etc.)

• Strengths

  • Description: Layered configuration, no templates, built into kubectl

• Example Command

  • Description: kustomize build overlays/dev or kubectl apply -k overlays/dev

Kustomize helps you manage different Kubernetes environments cleanly, promoting reusability and safer configuration updates without complex templating.

https://kubernetes.io/docs/tasks/manage-kubernetes-objects/kustomization/