Introduction

Kubernetes is a powerful platform that simplifies the deployment, scaling, and management of containerized applications. However, as your application grows and requires different configurations across multiple environments, managing Kubernetes resources can quickly become a challenge. This is where Kustomization comes in. Kustomization is a feature in Kubernetes that allows you to customize and manage your resources in a more reusable and efficient manner, especially when dealing with various environments like development, staging, and production.

What is Kustomization?

Kustomization refers to the process of managing Kubernetes resources using the Kustomize tool, which allows users to customize, patch, and modify YAML configuration files declaratively. It provides a way to avoid redundancy when configuring resources in different environments by enabling you to reuse a base set of configurations and apply environment-specific modifications without directly modifying the original files.

In simple terms, Kustomization helps you to define Kubernetes resources in a more flexible and reusable manner. It also eliminates the need to maintain multiple separate YAML files for each environment.

Kustomize: The Tool Behind Kustomization

Kustomize is a Kubernetes-native tool that enables users to create and manage these customizations. It integrates seamlessly with kubectl, the Kubernetes command-line interface, making it easy to deploy customized resources directly to a Kubernetes cluster.

Unlike Helm or other templating tools, Kustomize doesn’t use templating. Instead, it focuses on a declarative approach to customize existing Kubernetes resources by layering patches on top of base configurations. This makes it an ideal choice for managing configurations that need to differ slightly across environments but share a common core structure.

How Kustomization Works

To better understand how Kustomization works, let's break it down into its core components:

1. Base Resources

Base resources are the foundational Kubernetes configuration files that are common across environments. These typically include YAML files for core Kubernetes objects like:

• Deployments

• Services

• ConfigMaps

• Secrets

• PersistentVolumeClaims (PVCs)

These resources define the default settings for your application, and they can be reused in multiple environments without modification. For example, you might have a base deployment file that defines the containers and volumes for your app, but the replica count or resource limits might change depending on whether you are in a development or production environment.

2. Overlay Resources

Overlay resources are customizations or environment-specific changes that are applied on top of the base resources. Overlays allow you to modify or extend base resources without changing the original configurations.

For example, in a development environment, you might want to set a lower replica count and fewer resource limits, while in production, you would increase the replica count and allocate more resources. Rather than duplicating entire YAML files for each environment, you can create overlays that apply the necessary changes.

3. Kustomization File (kustomization.yaml)

The kustomization.yaml file is the heart of the Kustomization process. It defines the structure and logic for how base resources and overlays come together. This file specifies the base configurations to use, the patches to apply, and any additional resources that should be included or excluded.

Here’s an example of a simple kustomization.yaml file:

# kustomization.yaml
resources:
  - deployment.yaml
  - service.yaml
  - configmap.yaml

patchesStrategicMerge:
  - replica-patch.yaml
  - resource-limits-patch.yaml

In this example:

• The resources section lists the base resources that will be included (like deployment.yaml, service.yaml, etc.).

• The patchesStrategicMerge section specifies the overlays (e.g., replica-patch.yaml and resource-limits-patch.yaml) that will modify the base resources.

4. Patches

Patches are files that contain the modifications or customizations to the base resources. They are typically used to update specific fields in the YAML files, such as increasing replica counts, changing resource limits, or modifying environment variables. Patches can be applied either through strategic merge or JSON patch.

A strategic merge patch looks like this:

# replica-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 5  # Override replica count for production environment

This patch changes the replica count for the my-app deployment without altering the original deployment.yaml file.

Advantages of Kustomization

Kustomization provides several benefits, especially when managing complex Kubernetes configurations:

1. Avoid Redundancy

Instead of having separate YAML files for each environment (e.g., dev-deployment.yaml, prod-deployment.yaml), Kustomization allows you to use a single base configuration and apply different customizations through overlays. This reduces duplication and simplifies maintenance.

2. Declarative Configuration

Kustomization is built on a declarative model, where you define the desired state of your Kubernetes resources. You specify what needs to be changed, and Kustomize takes care of applying the changes. This makes it easier to maintain and track changes in version control.

3. Environment-Specific Customizations

Kustomization makes it easy to manage resources for different environments. By creating different overlays (e.g., dev, staging, prod), you can tailor your configurations for each environment without duplicating the base configurations.

4. Simplified Resource Management

Kustomize integrates directly with kubectl, meaning you can deploy customized configurations to your Kubernetes cluster without needing additional tools. The workflow becomes straightforward: you define your resources in the kustomization.yaml file, and then apply them using kubectl apply -k.

Example Use Case: Managing Multiple Environments

Let’s say you are managing a web application that needs to be deployed in both a development and production environment. The application uses a Kubernetes Deployment with a replica count, resource limits, and environment variables.

Step 1: Create Base Resources

First, create a base deployment.yaml that defines the default configuration:

# deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-web-app
spec:
  replicas: 3  # Default replica count
  selector:
    matchLabels:
      app: my-web-app
  template:
    metadata:
      labels:
        app: my-web-app
    spec:
      containers:
        - name: web
          image: my-web-app:latest
          resources:
            limits:
              cpu: 500m
              memory: 1Gi

Step 2: Create Overlays

Next, create overlays for the development and production environments:

• Dev Overlay (dev/replica-patch.yaml): In dev, reduce the replica count and resources.

# dev/replica-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-web-app
spec:
  replicas: 1  # Lower replica count for development
  template:
    spec:
      containers:
        - name: web
          resources:
            limits:
              cpu: 100m
              memory: 512Mi

• Prod Overlay (prod/replica-patch.yaml): In production, increase the replica count and resources.

# prod/replica-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-web-app
spec:
  replicas: 5  # Higher replica count for production
  template:
    spec:
      containers:
        - name: web
          resources:
            limits:
              cpu: 1000m
              memory: 2Gi

Step 3: Create kustomization.yaml

In both the dev and prod directories, create a kustomization.yaml file to define the resources and overlays.

# dev/kustomization.yaml
resources:
  - ../deployment.yaml
  - ../service.yaml
  - ../configmap.yaml
patchesStrategicMerge:
  - replica-patch.yaml

# prod/kustomization.yaml
resources:
  - ../deployment.yaml
  - ../service.yaml
  - ../configmap.yaml
patchesStrategicMerge:
  - replica-patch.yaml

Step 4: Deploy with Kustomize

To deploy to the dev environment, run:

kubectl apply -k dev/

To deploy to the prod environment, run:

kubectl apply -k prod/

This approach keeps your configurations clean, reusable, and easy to manage.

Directory Structure

The directory structure for managing Kubernetes configurations with Kustomization will look like this:

my-k8s-configs/
├── base/
│   └── deployment.yaml
│   └── service.yaml
│   └── configmap.yaml
│   └── kustomization.yaml
├── dev/
│   └── replica-patch.yaml
│   └── kustomization.yaml
├── prod/
│   └── replica-patch.yaml
│   └── kustomization.yaml
└── README.md

Breakdown of the Directory:

1. base/: Contains the common Kubernetes configuration files shared across environments (e.g., deployment.yaml, service.yaml, configmap.yaml). The kustomization.yaml file here defines the base resources.

2. dev/: Contains the replica-patch.yaml for the development environment and the kustomization.yaml file that specifies the base resources and dev-specific customizations.

3. prod/: Contains the replica-patch.yaml for the production environment and the kustomization.yaml file that specifies the base resources and prod-specific customizations.

4. README.md (optional): A documentation file that explains the directory structure, how to use Kustomize, and any specific details for managing different environments.

Conclusion

Kustomization is a powerful tool for managing Kubernetes configurations in a flexible and declarative manner. By using base resources and applying environment-specific overlays, you can streamline the process of deploying applications across multiple environments. It simplifies configuration management, reduces duplication, and ensures that changes are easy to track and apply.

By incorporating Kustomization into your Kubernetes workflows, you can significantly improve the maintainability and scalability of your infrastructure, all while keeping things organized and efficient.