Day 37: Using Kustomize to Manage Kubernetes Configurations

In Kubernetes, deploying applications across multiple environments—such as development, staging, and production—often requires variations in configuration. These variations can lead to repetitive and hard-to-maintain YAML files. Enter Kustomize, a tool designed to simplify this process by enabling customizations without altering the original configurations.

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What is Kustomize?

Kustomize is a native Kubernetes configuration management tool that uses a declarative approach to apply environment-specific customizations. Instead of duplicating your YAML files for every environment, Kustomize allows you to define a base configuration and apply targeted overlays for specific needs.

This ensures that your configurations remain DRY (Don't Repeat Yourself) and maintainable.

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Why Kustomize?

Managing environment-specific variations often requires changes like:

• Adjusting replica counts for resource optimization.
• Modifying environment variables for debugging or production.
• Switching resource limits or service types.

Kustomize makes these changes easy and scalable while reducing errors caused by manual updates.

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Key Concepts in Kustomize

1. Base Configuration:
   
   The core, reusable Kubernetes resources, applicable across environments (e.g., Deployment, Service).

2. Overlay:
   
   Environment-specific customizations or patches (e.g., changing replicas, service types).

3. Kustomization File (kustomization.yaml):
   
   The heart of Kustomize that declares resources and patches.

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Hands-On with Kustomize

Step 1: Set Up a Base Configuration

Start by defining the shared resources for all environments.

Folder Structure:

kustomize/
└── base/
    ├── deployment.yaml
    ├── service.yaml
    └── kustomization.yaml

• deployment.yaml: Defines an Nginx Deployment with 2 replicas.

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

• service.yaml: Creates a Service to expose the Nginx app.

apiVersion: v1
kind: Service
metadata:
  name: nginx-service
spec:
  selector:
    app: nginx
  ports:
  - protocol: TCP
    port: 80
    targetPort: 80

• kustomization.yaml: Lists resources in the base configuration.

resources:
- deployment.yaml
- service.yaml

Deploy the base configuration:

kubectl apply -k base/

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Step 2: Add Environment-Specific Overlays

Next, create environment-specific overlays (e.g., dev and prod). Each overlay modifies the base configuration using patches.

Folder Structure:

kustomize/
├── base/
└── overlays/
    ├── dev/
    │   ├── kustomization.yaml
    │   └── replica-count.yaml
    └── prod/
        ├── kustomization.yaml
        └── replica-count.yaml

• replica-count.yaml (dev overlay): Reduces replicas for development.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx
spec:
  replicas: 1

• kustomization.yaml (dev overlay): Refers to the base and applies the patch.

resources:
- ../../base
patches:
- replica-count.yaml

Deploy the dev overlay:

kubectl apply -k overlays/dev/

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Step 3: Testing the Overlays

To confirm the changes:

1. Check the Deployment:

   kubectl get deployment nginx -o yaml

Observe that the replicas have been updated to the overlay's configuration.

1. Scale further or apply additional patches (e.g., resource limits, environment variables).

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Going Beyond Basics

While this guide introduces the core capabilities of Kustomize, you can extend its use in more complex scenarios:

• Combine Kustomize with Helm for advanced templating and package management.
• Use ConfigMapGenerator or SecretGenerator for dynamic ConfigMaps and Secrets.
• Integrate with CI/CD pipelines for seamless deployments.