Home » Blog » Kustomize 101 for Kubernetes: An Introduction

Information

Kustomize 101 for Kubernetes: An Introduction

Benhcmark Administrator

21/08/2025

0 Views 0

SaveSavedRemoved 0

What is Kustomize? The Ultimate Guide to Template-Free Kubernetes Management

Managing configurations in Kubernetes can quickly become a complex task. As your applications grow, you’ll find yourself needing to maintain separate YAML files for different environments like development, staging, and production. This often leads to duplicating files and manually editing values, a process that is both tedious and prone to error. If you’ve ever felt buried in a mountain of nearly identical YAML files, you’re not alone.

Fortunately, there’s a powerful tool built directly into kubectl that solves this exact problem: Kustomize. It offers a clean, declarative, and template-free way to customize your Kubernetes configurations.

This guide will walk you through what Kustomize is, how it works, and why it might be the perfect solution for streamlining your Kubernetes workflow.

The Core Problem with Standard Kubernetes YAML

The challenge with native Kubernetes manifests is their static nature. A Deployment.yaml file written for a development environment might require a different number of replicas, a different container image tag, or different resource limits for a production environment.

Without a management tool, teams often resort to one of these methods:

• Copy and Paste: Creating separate copies of YAML files for each environment and manually changing the values. This is not scalable and makes updates a nightmare.
• Templating Tools: Using external tools with complex templating languages (like Jinja2 or Go templates) to inject variables into YAML files. While powerful, this can add a layer of complexity and make the manifests harder to read and debug.

Kustomize introduces a third, more elegant option.

What is Kustomize? A Simpler Approach to Configuration

Kustomize is a standalone tool—and a native feature of kubectl—that lets you customize raw, template-free YAML files for multiple purposes, leaving the original YAML untouched and usable as is.

Its core philosophy revolves around a few key principles:

• It’s Template-Free: You work with standard, valid Kubernetes YAML. There are no special template syntaxes to learn. This makes your configurations easier to read and validate.
• It’s Declarative: You specify what you want to change, not how to change it. You provide a base configuration and then declare patches or modifications for each environment.
• It’s Natively Integrated: Since Kubernetes v1.14, Kustomize has been built directly into kubectl. This means you don’t need to install any extra software to use it. You can apply a configuration simply by running kubectl apply -k <directory>.

How Kustomize Works: Understanding Bases and Overlays

The magic of Kustomize lies in its structured approach using bases and overlays. This structure allows you to define a common foundation and then layer environment-specific changes on top of it.

A typical Kustomize project structure looks like this:

my-application/
├── base/
│   ├── deployment.yaml
│   ├── service.yaml
│   └── kustomization.yaml
└── overlays/
    ├── staging/
    │   ├── deployment-patch.yaml
    │   ├── replica-count.yaml
    │   └── kustomization.yaml
    └── production/
        ├── deployment-patch.yaml
        └── kustomization.yaml

Let’s break down these components:

• The base Directory: This contains the core, shared Kubernetes resource definitions (like Deployments, Services, and ConfigMaps). The YAML in the base is standard and can be applied to a cluster on its own.
• The overlays Directory: This directory contains subdirectories for each of your environments (e.g., staging, production). Each overlay specifies the differences from the base. It doesn’t contain full copies of the YAML files, only the specific modifications needed for that environment.
• The kustomization.yaml File: This is the heart of Kustomize. It’s a special YAML file that tells Kustomize how to assemble the final configuration. You’ll have one in your base directory and one in each overlay directory.

The kustomization.yaml in an overlay will reference the base and then list the patches or changes to apply, such as updating an image tag, changing the number of replicas, or adding an annotation.

Key Benefits of Using Kustomize

Adopting Kustomize can bring significant advantages to your configuration management process:

1. Drastically Reduced Duplication: By defining a common base and only specifying the differences in overlays, you eliminate redundant YAML. This adheres to the Don’t Repeat Yourself (DRY) principle.
2. Improved Readability and Maintainability: Since you are working with standard YAML, your configurations are easy for anyone on the team to read and understand. There’s no need to decipher complex template logic.
3. Seamless kubectl Integration: The ability to run kubectl apply -k makes the workflow incredibly smooth. There are no extra steps or plugins required.
4. Enhanced Auditability: It’s much easier to see exactly what changes between environments. You can simply inspect the overlay’s patch files to understand the specific configurations for production versus staging.
5. Reusable and Composable: Kustomize components (bases and overlays) can be versioned in Git and reused across different projects, promoting consistency and best practices.

Kustomize vs. Helm: Which One Should You Use?

Helm is another popular tool in the Kubernetes ecosystem, and it’s often compared to Kustomize. However, they serve slightly different purposes.

• Helm is a package manager for Kubernetes. It excels at bundling, sharing, and deploying pre-packaged applications (called charts). It uses a powerful Go-based templating system. Think of it like apt or yum for Kubernetes.
• Kustomize is a configuration customizer. It is designed specifically for modifying existing YAML files for your own applications across different environments.

You don’t always have to choose between them. Many teams use Helm to deploy third-party applications (like Prometheus or Redis) and use Kustomize to manage their in-house application configurations.

Actionable Best Practices for Using Kustomize

To get the most out of Kustomize, follow these security and operational best practices:

• Keep Overlays Lean: An overlay should only contain the specific changes for that environment. Avoid putting anything in an overlay that could be in the base.
• Version Control Everything: Store your entire Kustomize configuration directory (bases and overlays) in a Git repository. This is a foundational practice for GitOps and provides a clear audit trail.
• Manage Secrets Separately: Never commit plain-text secrets to your Git repository. Use a tool like Sealed Secrets, SOPS, or a secrets provider like HashiCorp Vault to manage sensitive information. Kustomize can then integrate with these tools.
• Use commonLabels and commonAnnotations: The kustomization.yaml file allows you to apply labels and annotations to every resource it manages. This is perfect for ensuring consistent metadata for tracking and policy enforcement.

By embracing a template-free, declarative approach, Kustomize empowers teams to manage Kubernetes configurations with clarity and confidence. It reduces complexity, minimizes errors, and provides a scalable framework for handling diverse environments—all from within the kubectl command you already use every day.

Source: https://kifarunix.com/kubernetes-kustomize-101-introduction-and-basics/