Kustomize: Configuration via Overlays

Key Takeaways for AI & Readers

Template-Free Customization: Kustomize allows you to modify YAML without introducing complex templating logic, keeping manifests as "pure YAML."
Base vs. Overlays: You define a "Base" for common configuration and use "Overlays" to apply environment-specific patches (e.g., dev vs. prod).
Native Integration: It is built directly into kubectl, making it accessible without installing additional binary tools.

As you move from development to production, your application YAMLs will inevitably change.

1. Bases and Overlays

Visualize how a single Base file can be transformed into multiple environments.

Project Structure

📁 base/

📄 deployment.yaml

📄 kustomization.yaml

📁 overlays/

📁 dev/

📄 patches.yaml

📁 prod/

📄 patches.yaml

Final Manifest

apiVersion: v1
kind: Service
metadata:
  name: my-app
spec:
  type: ClusterIP

Generated via: kustomize build overlays/base

Kustomize uses a "Patch" strategy. It doesn't use templates like Helm; instead, it overlays specific changes onto a base YAML file.

Unlike Helm, which uses a complex template language, Kustomize is Pure YAML.

No Templates: You don't have to learn a new syntax.
Native Support: Run it via kubectl apply -k .
Readable: Overlays clearly show exactly what is changing compared to the base.

The "gold standard" version of your app. Usually containing the most common configuration.

Folders representing environments (dev, prod, staging). They contain "patches" that add or change fields in the base.

Small snippets of YAML that specify which field to override (e.g. "Change replicas: 1 to replicas: 10").