What is Kustomize?

Kustomize is a free tool that helps you change Kubernetes configuration files (YAML) without writing templates. You start with a basic set of files and then add small “patches” or “overlays” to adjust them for different environments.

Let's break it down

  • Kustomize - the name of the program; it “customizes” things.
  • Tool - a piece of software you run on your computer.
  • Customize - change something to fit your needs.
  • Raw Kubernetes YAML files - the plain-text files that tell Kubernetes what to run; they use the YAML format.
  • Without using templates - you don’t need special placeholder syntax or a separate templating language.
  • Layering patches and overlays - you add small extra files (patches) on top of the original ones; an overlay is a collection of those patches for a specific situation.
  • Base configuration - the original set of YAML files that work in a default (often “development”) environment.

Why does it matter?

It lets developers and operators keep one clean set of Kubernetes files and reuse them across many environments (dev, test, prod) without copying or manually editing the same text over and over. This reduces errors, saves time, and makes version control easier.

Where is it used?

  • A company deploying the same microservice to development, staging, and production clusters, each with slightly different resource limits.
  • An open-source project that wants to provide a single source of truth for its Helm chart while still allowing users to tweak settings.
  • A CI/CD pipeline that automatically builds and applies environment-specific configurations during automated testing.
  • A cloud-native consulting firm that needs to generate many client-specific Kubernetes manifests from a common template-free base.

Good things about it

  • No templating language to learn; you work directly with standard YAML.
  • Clear separation of base files and environment-specific changes, which improves readability.
  • Works natively with kubectl (you can run kubectl apply -k).
  • Supports generators (e.g., ConfigMap, Secret) that create resources automatically.
  • Fully open source and integrated into many GitOps tools.

Not-so-good things

  • Complex overlays can become hard to visualize when many layers are stacked.
  • Lacks some of the advanced logic (loops, conditionals) that full templating engines like Helm provide.
  • Learning the patch syntax (strategic merge, JSON patches) can be confusing for beginners.
  • Not ideal for completely dynamic configurations that need runtime calculations.