Kustomize and overlay in Kubernetes Open SHift - unix1998/technical_notes GitHub Wiki

Kustomize and overlays are related concepts in Kubernetes that help manage and customize Kubernetes resources declaratively. Here's an overview of each:

Kustomize

Kustomize is a configuration management tool built into kubectl that allows you to customize Kubernetes resources without modifying the original YAML files. It provides a way to manage configuration variants using a kustomization.yaml file.

Key Features of Kustomize:

1. No Template Language:

   • Unlike Helm, Kustomize does not use a template language. Instead, it uses plain YAML and layering to customize configurations.

2. Declarative Management:

   • Kustomize allows you to manage Kubernetes configurations declaratively, meaning you specify the desired state rather than how to achieve it.

3. Layering:

   • You can define a base configuration and overlay customizations on top of it for different environments (e.g., development, staging, production).

4. Built-in Support in kubectl:

   • Kustomize is integrated into kubectl, so you can use it directly with commands like kubectl apply -k.

Basic Kustomize Example:

Suppose you have a base configuration for a Deployment and a Service.

• base/deployment.yaml:

  apiVersion: apps/v1
  kind: Deployment
  metadata:
    name: myapp
  spec:
    replicas: 1
    selector:
      matchLabels:
        app: myapp
    template:
      metadata:
        labels:
          app: myapp
      spec:
        containers:
        - name: myapp
          image: myapp:latest
          ports:
          - containerPort: 80
  

• base/service.yaml:

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

• base/kustomization.yaml:

  resources:
  - deployment.yaml
  - service.yaml
  

Overlays

Overlays in Kustomize are used to customize or extend the base configurations for different environments or purposes without duplicating the entire configuration.

Overlays Example:

You might have different configurations for development and production environments. You can create overlays to modify the base configuration accordingly.

• overlays/dev/kustomization.yaml:

  resources:
  - ../../base
  
  patchesStrategicMerge:
  - deployment-patch.yaml
  

• overlays/dev/deployment-patch.yaml:

  apiVersion: apps/v1
  kind: Deployment
  metadata:
    name: myapp
  spec:
    replicas: 2
    template:
      spec:
        containers:
        - name: myapp
          image: myapp:dev
  

• overlays/prod/kustomization.yaml:

  resources:
  - ../../base
  
  patchesStrategicMerge:
  - deployment-patch.yaml
  

• overlays/prod/deployment-patch.yaml:

  apiVersion: apps/v1
  kind: Deployment
  metadata:
    name: myapp
  spec:
    replicas: 5
    template:
      spec:
        containers:
        - name: myapp
          image: myapp:prod
  

Applying Kustomize Configurations

To apply the configuration for a specific environment, you use the -k flag with kubectl:

• Apply the development configuration:

  kubectl apply -k overlays/dev
  

• Apply the production configuration:

  kubectl apply -k overlays/prod
  

Summary

• Kustomize is a tool for customizing Kubernetes configurations using plain YAML files and a kustomization.yaml file to manage configuration variants.
• Overlays are used in Kustomize to customize base configurations for different environments or purposes without duplicating the entire configuration.
• Kustomize is integrated into kubectl, making it easy to use for declarative configuration management and environment-specific customization.