Kubernetes Kustomize 101: Introduction and Basics

This guide provides a foundational understanding of Kubernetes Kustomize. It will introduce you to Kustomize’s core concepts and the basics. With more organizations increasingly adopting the use of Kubernetes as the de facto standard for container orchestration, managing Kubernetes configurations across different environments such development, staging, production often requiring repetitive adjustments and configurations, can become cumbersome to manage. Kubernetes Kustomize addresses this challenge by offering a declarative approach to managing Kubernetes manifests. It simplifies the process of customizing configurations without the need for extensive templating or duplication of YAML files.

Kubernetes Kustomize 101: Introduction and Basics

What is Kubernetes Kustomize?

Kubernetes Kustomize is a tool used to customize Kubernetes YAML configurations without the need to use templating engines like Helm. It provides a declarative way to manage Kubernetes manifests by overlaying patches on top of a base configuration, i.e without duplicating or modifying the base YAML files directly. It provides a way to manage configuration drifts hence maintaining consistency across deployments.

Key Concepts in Kustomize

Some of the key concepts that allows Kubernetes customize to simplify configuration management across different environments include:

• Base:
  • This is the foundational directory in a Kustomize project where the original or default Kubernetes application manifests reside. It represents the base configuration of your application.
• Overlay:
  • An overlay is a directory that contains additional or modified Kubernetes manifests that overlay (or apply on top of) the base configuration.
  • They allow customization of the base configuration for different environments, regions, or specific use cases without modifying the original base files directly.
  • You can create separate overlay directories (e.g., overlays/dev, overlays/prod) where you can add or override resources, apply environment-specific configurations (like different resource limits, environment variables, etc.), and manage variations in configurations.
• Patch:
  • Patches are configurations that specifies the changes to apply to a resource. They provide a mechanism to modify or extend existing Kubernetes resources without directly modifying the original YAML files.
• Kustomization file:
  • kustomization.yaml file is the recipe for building a customized deployment configuration in Kustomize. It tells Kustomize what resources to include (base manifests, overlays, additional YAMLs), how to modify them (patches), and where to deploy them (namespace) within your Kubernetes cluster.

Why use Kubernetes Kustomize?

Kubernetes Kustomize offers different benefits including:

• Makes it easy to manage configurations for different environments. With Kubernetes Kustomize, you can create a base manifest with common configurations and then use overlays to apply environment-specific updates, keeping your base manifests clean and reusable.
• Since Kubernetes Kustomize allows you to maintain a base manifest that can be customized whenever a need arises, it reduces the hussle of having to duplicate entire manifests for different environments.
• The flexibility of Kubernetes Kustomize provides easy configurations version control thus ensuring clear traceability and reproducibility of your deployments.

Common Use cases of Kubernetes Kustomize

Some of the common use cases of Kubernetes Kustomize include:

• Composing and Customizing resources.
• Generating ConfigMaps and Secrets
• Applying Cross-Cutting Fields for example to set common labels, annotations, or namespaces for all resources in a project.

Read more on Kustomize overview.

Getting Started with Kustomize

Installing Kubernetes Kustomize

From Kubernetes v1.14, Kustomize is integrated directly into kubectl, so you can use it without needing to install additional tools.

To check if Kustomize is installed, just check the version of kubectl;

kubectl version

Client Version: v1.30.2
Kustomize Version: v5.0.4-0.20230601165947-6ce0bf390ce3
Server Version: v1.30.2

Therefore, you can view resources available in a directory using the command;

kubectl kustomize <kustomization_file_directory>

Or apply resources in a directory using the command;

kubectl apply [-k|--kustomize] <kustomization_directory>

Running StandAlone Kustomize Tool

If you want, you can install a standalone Kustomize tool on your system.

The installation steps have been extensively described in the documentation, link given below.

Install Kustomize

Using Kustomize to Generate Resources from Other Sources

Kustomize can be used to dynamically generate Kubernetes resources from files or literals, simplifying resource management and avoiding hard-coded values.

As an example, Kustomize can be used to generate ConfigMaps and Secrets using Kustomize configMapGenerator and secretGenerator features respectively. Similarly, it can be used to generate other Kubernetes resources such as deployments, services, etc

We will be creating our resources on our custom namespace, kube-kustomize-base.

kubectl create namespace kube-kustomize-base

Then, let’s create our base configuration file.

mkdir base

Within the directory, we will create the Kustomization file and config directory for storing configMaps and Secrets.

mkdir base/config

In our example, we will be using a simple Nginx app serving the following content;

cat base/config/index.html

<html>
<head>
    <style>
        body {
            text-align: center; /* Centers text and inline elements */
            font-family: Arial, sans-serif; /* Optional: sets a nicer font */
        }
    </style>
</head>
<body>
    <h1>Kube Kustomize Base Environment!</h1>
    <p>This is the base version of our web app.</p>
    <p>Feel free to explore and innovate!</p>
</body>
</html>

And Nginx configuration with basic authentication.

server {
    listen 80;

    location / {
        auth_basic "Restricted Access";
        auth_basic_user_file /etc/nginx/.htpasswd;
        root /usr/share/nginx/html;
        index index.html;
    }
}

So, we will store the basic authentication password in a file (base/config/.htpasswd);

htpasswd -c base/config/.htpasswd admin

Next, let’s create Kubernetes kustomization file in the base directory. The Kustomization file defines how to use the configMapGenerator and secretGenerator to generate the configMap and Secrets.

cat base/kustomization.yaml

namespace: kube-kustomize-base
configMapGenerator:
  - name: nginx-config
    files:
      - config/nginx.conf
      - config/index.html

secretGenerator:
  - name: nginx-auth
    files:
      - htpasswd=config/.htpasswd

generatorOptions:
  disableNameSuffixHash: true

The:

configMapGenerator Section:

• name: nginx-config: Creates a ConfigMap named nginx-config.
• files:: Includes nginx.conf and index.html in the ConfigMap.

secretGenerator Section:

• name: nginx-auth: Creates a Secret named nginx-auth.
• files:: Reads config/.htpasswd and adds it to the Secret under the key htpasswd.

generatorOptions:

• The option disableNameSuffixHash is used to avoid adding a hash suffix to the names of generated ConfigMaps and Secrets. This is useful if you want to keep the names consistent and predictable.

Note that you can generate your configMaps/Secrets from the literals or even the environment variables.

Thus, this is our basic base directory structure;

tree -a base

base
├── config
│   ├── .htpasswd
│   ├── index.html
│   └── nginx.conf
└── kustomization.yaml

2 directories, 4 files

You can examine the configMap and Secret to be generated using the command;

kubectl kustomize base

Sample output;

apiVersion: v1
data:
  index.html: |
    <html>
    <head>
        <style>
            body {
                text-align: center; /* Centers text and inline elements */
                font-family: Arial, sans-serif; /* Optional: sets a nicer font */
            }
        </style>
    </head>
    <body>
        <h1>Kube Kustomize Base Environment!</h1>
        <p>This is the base version of our web app.</p>
        <p>Feel free to explore and innovate!</p>
    </body>
    </html>
  nginx.conf: |
    server {
        listen 80;

        location / {
            auth_basic "Restricted Access";
            auth_basic_user_file /etc/nginx/.htpasswd;
            root /usr/share/nginx/html;
            index index.html;
        }
    }
kind: ConfigMap
metadata:
  name: nginx-config
  namespace: kube-kustomize-base
---
apiVersion: v1
data:
  .htpasswd: |
    YWRtaW46JDJ5JDEwJG5uc25kTFozSTYzWENNOVRxbFF2bi45LzRWVC8vbGxWajRKSWY1UW
    M4RlFGYmEwS1JOSlptCg==
kind: Secret
metadata:
  name: nginx-auth
  namespace: kube-kustomize-base
type: Opaque

To create the configMap and Secrets, simply apply using kustomize;

kubectl apply [-k|--kustomize] <path to directory containing kustomization yaml>

E.g

kubectl apply -k base

Sample output;

configmap/nginx-config created
secret/nginx-auth created

You can customize how the resources are generated with generator options.

View the created configMap and Secret;

kubectl get configmap,secrets -n kube-kustomize-base

NAME                                DATA   AGE
configmap/kube-root-ca.crt          1      7m26s
configmap/nginx-config              1      104s

NAME                           TYPE     DATA   AGE
secret/nginx-auth              Opaque   1      104s

Now, let’s add a deployment that will utilize the configMap and Secrets above, as well a service that exposes the Deployment.

cat base/deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:latest
        ports:
        - containerPort: 80
        volumeMounts:
        - name: nginx-auth
          mountPath: /etc/nginx/.htpasswd
          subPath: .htpasswd
          readOnly: true
        - name: nginx-config
          mountPath: /etc/nginx/conf.d/default.conf
          subPath: nginx.conf
        - name: nginx-config
          mountPath: /usr/share/nginx/html/index.html
          subPath: index.html
      volumes:
      - name: nginx-auth
        secret:
          secretName: nginx-auth
      - name: nginx-config
        configMap:
          name: nginx-config

And the service to expose our deployment via the NodePort.

cat base/service.yaml

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

At this point, these are all the files in my base directory;

tree -a base

base
├── config
│   ├── .htpasswd
│   ├── index.html
│   └── nginx.conf
├── deployment.yaml
├── kustomization.yaml
└── service.yaml

2 directories, 6 files

Next, include the Deployment and Service resources in the kustomization file.

cat base/kustomization.yaml

namespace: kube-kustomize-base
configMapGenerator:
  - name: nginx-config
    files:
      - config/nginx.conf
      - config/index.html

secretGenerator:
  - name: nginx-auth
    files:
      - config/.htpasswd

generatorOptions:
  disableNameSuffixHash: true

resources:
- deployment.yaml
- service.yaml

To see what the final YAML output would look like after all customizations are applied.

kubectl kustomize base/

To apply the customizations into Kubernetes cluster;

kubectl apply -k base/

You can check the Deployment and Service created.

kubectl get deployment,svc -n kube-kustomize-base

NAME                               READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/nginx-deployment   1/1     1            1           31s

NAME                    TYPE       CLUSTER-IP       EXTERNAL-IP   PORT(S)        AGE
service/nginx-service   NodePort   10.110.182.223   <none>        80:32169/TCP   31s

Your basic Nginx app is now running.

Customizing Kubernetes Resources with Kustomize Overlays

To update the above Nginx app to reflect different environments (dev, staging, production) using Kustomize overlays, you can create a configuration that includes patches to modify the content served by Nginx or any specific configuration that reflects specific environment of the app.

Base Configuration

In the base configuration above, we have the app configuration directory which contains the Secrets, ConfigMap, the deployment and service manifest and the kustomization file. We have used Kubernetes Kustomize to generate Secrets, ConfigMaps, and our Nginx deployment and service.

Base Directory Structure:

tree -a base

base
├── config
│   ├── .htpasswd
│   ├── index.html
│   └── nginx.conf
├── deployment.yaml
├── kustomization.yaml
└── service.yaml

2 directories, 6 files

Overlay Configuration

To demonstrate how you can use Kustomize to customize/patch the app for each environment (dev, staging, production), we will update our ConfigMap using patches to customize the base configuration without altering the original files.

We already have a base directory with base configurations. Next, let’s create overlays directory for the three environments:

mkdir -p overlays/{dev,staging,production}

Each overlay will provide a patch to modify the index.html content for that environment.

Similarly, I will put each environment under its own namespace. So, let’s create three namespaces.

kubectl create ns kube-kustomize-dev

kubectl create ns kube-kustomize-staging

kubectl create ns kube-kustomize-production

In summary, this is what we want to try out:

• Customize the nginx default page for each environment.
• Set different replica counts for development, staging, and production.
• Deploy to different namespaces for each environment.

Overlay directory structure:

tree -a overlays

overlays/
├── dev
│   ├── configmap-patch.yaml
│   ├── deployment-patch.yaml
│   └── kustomization.yaml
├── production
│   ├── configmap-patch.yaml
│   ├── deployment-patch.yaml
│   └── kustomization.yaml
└── staging
    ├── configmap-patch.yaml
    ├── deployment-patch.yaml
    └── kustomization.yaml

4 directories, 9 files

In general, this is how our custom app directory structure is like;

tree -a kustomize-app/

kustomize-app/
├── base
│   ├── config
│   │   ├── .htpasswd
│   │   ├── index.html
│   │   └── nginx.conf
│   ├── deployment.yaml
│   ├── kustomization.yaml
│   └── service.yaml
└── overlays
    ├── dev
    │   ├── configmap-patch.yaml
    │   ├── deployment-patch.yaml
    │   └── kustomization.yaml
    ├── production
    │   ├── configmap-patch.yaml
    │   ├── deployment-patch.yaml
    │   └── kustomization.yaml
    └── staging
        ├── configmap-patch.yaml
        ├── deployment-patch.yaml
        └── kustomization.yaml

7 directories, 15 files

Development Overlays:

cat overlays/dev/kustomization.yaml

namespace: kube-kustomize-dev

resources:
  - ../../base

patches:
  - path: deployment-patch.yaml
  - path: configmap-patch.yaml

cat overlays/dev/deployment-patch.yaml

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

cat overlays/dev/configmap-patch.yaml

apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-config
data:
  index.html: |
    <html>
    <head>
        <style>
            body {
                text-align: center;
                font-family: Arial, sans-serif;
            }
        </style>
    </head>
    <body>
        <h1>Kube Kustomize Dev Environment!</h1>
        <p>This is the Development version of our web app.</p>
        <p>Feel free to explore and innovate!</p>
    </body>
    </html>

Now, you can review your patches;

kubectl kustomize overlays/dev

apiVersion: v1
data:
  index.html: |
    <html>
    <head>
        <style>
            body {
                text-align: center;
                font-family: Arial, sans-serif;
            }
        </style>
    </head>
    <body>
        <h1>Kube Kustomize Dev Environment!</h1>
        <p>This is the Development version of our web app.</p>
        <p>Feel free to explore and innovate!</p>
    </body>
    </html>
  nginx.conf: |
    server {
        listen 80;

        location / {
            auth_basic "Restricted Access";
            auth_basic_user_file /etc/nginx/.htpasswd;
            root /usr/share/nginx/html;
            index index.html;
        }
    }
kind: ConfigMap
metadata:
  name: nginx-config
  namespace: kube-kustomize-dev
---
apiVersion: v1
data:
  .htpasswd: |
    YWRtaW46JDJ5JDEwJG5uc25kTFozSTYzWENNOVRxbFF2bi45LzRWVC8vbGxWajRKSWY1UW
    M4RlFGYmEwS1JOSlptCg==
kind: Secret
metadata:
  name: nginx-auth
  namespace: kube-kustomize-dev
type: Opaque
---
apiVersion: v1
kind: Service
metadata:
  name: nginx-service
  namespace: kube-kustomize-dev
spec:
  ports:
  - port: 80
    protocol: TCP
    targetPort: 80
  selector:
    app: nginx
  type: NodePort
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  namespace: kube-kustomize-dev
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:latest
        name: nginx
        ports:
        - containerPort: 80
        volumeMounts:
        - mountPath: /etc/nginx/.htpasswd
          name: nginx-auth
          readOnly: true
          subPath: .htpasswd
        - mountPath: /etc/nginx/conf.d/default.conf
          name: nginx-config
          subPath: nginx.conf
        - mountPath: /usr/share/nginx/html/index.html
          name: nginx-config
          subPath: index.html
      volumes:
      - name: nginx-auth
        secret:
          secretName: nginx-auth
      - configMap:
          name: nginx-config
        name: nginx-config

The apply if all is good.

kubectl apply -k overlays/dev

List all resources in the Development namespace.

kubectl get all -n kube-kustomize-dev

NAME                                   READY   STATUS    RESTARTS   AGE
pod/nginx-deployment-85c575f57-kjn24   1/1     Running   0          53s

NAME                    TYPE       CLUSTER-IP      EXTERNAL-IP   PORT(S)        AGE
service/nginx-service   NodePort   10.105.39.168   <none>        80:32056/TCP   53s

NAME                               READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/nginx-deployment   1/1     1            1           53s

NAME                                         DESIRED   CURRENT   READY   AGE
replicaset.apps/nginx-deployment-85c575f57   1         1         1       53s

You can access your app via port 32056. Remember we didnt change the basic auth password. So use the one set in the base configuration.

If you want, you can update other settings as you see fit.

For our production, this is how we customized the app: 4 replicas and updated web page.

cat overlays/production/kustomization.yaml

namespace: kube-kustomize-production

resources:
  - ../../base

patches:
  - path: deployment-patch.yaml
  - path: configmap-patch.yaml

cat overlays/production/deployment-patch.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 4

overlays/production/configmap-patch.yaml

apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-config
data:
  index.html: |
    <html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <style>
            body {
                font-family: Arial, sans-serif;
                text-align: center;
                padding: 50px;
            }
            h1 {
                color: #009688;
            }
            p {
                color: #607d8b;
            }
            .container {
                margin-top: 30px;
                display: flex;
                justify-content: center;
                align-items: center;
            }
            .kube-logo {
                width: 150px;
                height: 150px;
                margin-right: 20px;
            }
        </style>
    </head>
    <body>
        <h1>Welcome to Kubernetes Kustomize Basics!</h1>
        <div class="container">
            <img class="kube-logo" src="https://raw.githubusercontent.com/kubernetes/kubernetes/master/logo/logo.png" alt="Kubernetes Logo">
            <p>This is the Production Kustomize Overlay with an updated web page and with four replicas!</p>
        </div>
        <p>Feel free to explore and innovate!</p>
    </body>
    </html>

You can review the changes;

kubectl kustomize overlays/production

Then apply;

kubectl apply --kustomize overlays/production

View resources;

kubectl get all -n kube-kustomize-production

NAME                                   READY   STATUS    RESTARTS   AGE
pod/nginx-deployment-85c575f57-2rl9k   1/1     Running   0          13s
pod/nginx-deployment-85c575f57-2vt49   1/1     Running   0          13s
pod/nginx-deployment-85c575f57-4x656   1/1     Running   0          13s
pod/nginx-deployment-85c575f57-t9ggv   1/1     Running   0          13s

NAME                    TYPE       CLUSTER-IP      EXTERNAL-IP   PORT(S)        AGE
service/nginx-service   NodePort   10.107.20.199           80:31487/TCP   13s

NAME                               READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/nginx-deployment   4/4     4            4           13s

NAME                                         DESIRED   CURRENT   READY   AGE
replicaset.apps/nginx-deployment-85c575f57   4         4         4       13s

And there you go!

And that closes our basic guide on Kubernetes Kustomize 101.

Conclusion

Kustomize provides robust features for managing Kubernetes configurations, including:

• Generating Resources from Other Sources: Combine and reuse configurations to create complex deployments.
• Setting Cross-Cutting Fields for Resources: Apply consistent settings such as labels and annotations across multiple resources.
• Composing and Customizing Collections of Resources: Build and manage complex configurations in a modular and flexible manner.

Read more on Kustomize page.