Using a Kubernetes Configmap in a Pod

A ConfigMap is provided as a way to inject configuration data into a pod/pods. It can be included as files or be used as environment variables. Below are a couple of examples of mounting a configmap using both methods.

Use as File

In this first example I’ll show how to create a configmap and mount it using the key(s) as filenames and the data as the file content. It will then be used in an Nginx container as the index page.

First create the configmap if the pod is created first it won’t start properly as it wouldn’t be able to find the configmap.

# file: nginx-cm.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-cm
data:
  index.html: Hello, World!

apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-cm
binaryData:
  index.html: PCFET0NUWVBFIGh0bWw+CjxodG1sPgo8aGVhZD4KCTxtZXRhIGNoYXJzZXQ9InV0Zi04Ij4KCTxtZXRhIG5hbWU9InZpZXdwb3J0IiBjb250ZW50PSJ3aWR0aD1kZXZpY2Utd2lkdGgsIGluaXRpYWwtc2NhbGU9MSI+Cgk8dGl0bGU+SGVsbG8sIFdvcmxkITwvdGl0bGU+Cgk8c3R5bGUgdHlwZT0idGV4dC9jc3MiPgoJCWh0bWwgewoJCQljb2xvci1zY2hlbWU6IGxpZ2h0IGRhcms7CgkJfQoJPC9zdHlsZT4KPC9oZWFkPgo8Ym9keT4KCTxoMT5IZWxsbyBXb3JsZCE8L2gxPgoJPHA+SWYgeW91IHNlZSB0aGlzIHdlYiBwYWdlIHRoZSBzZXJ2ZXIgaXMgd29ya2luZy48L3A+CjwvYm9keT4KPC9odG1sPg==

You can check if the configmap is available using kubectl get configmaps. The output should show nginx-cm.

Next create the pod, once it is up and running create the service and view the page in the browser. It should appear plain white with the only words being “Hello, World!”.

# file: nginx-pod.yaml
apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - image: nginx:alpine
    imagePullPolicy: IfNotPresent
    name: nginx
    volumeMounts:
    - name: nginx-files
      mountPath: /usr/share/nginx/html
      readOnly: true
  volumes:
  - name: nginx-files
    configMap:
      name: nginx-cm

apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: dep-nginx
  name: dep-nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: dep-nginx
  strategy: {}
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: dep-nginx
    spec:
      containers:
      - image: nginx:alpine
        name: nginx-pod
        imagePullPolicy: IfNotPresent
        volumeMounts:
        - name: nginx-files
          mountPath: /usr/share/nginx/html
          readOnly: true
      volumes:
      - name: nginx-files
        configMap:
          name: nginx-cm

# file: nginx-service.yaml
apiVersion: v1
kind: Service
metadata:
  creationTimestamp: null
  labels:
    run: nginx
  name: nginx
spec:
  ports:
  - port: 80
    protocol: TCP
    targetPort: 80
    nodePort: 30080
  selector:
    run: nginx
  type: NodePort
status:
  loadBalancer: {}

apiVersion: v1
kind: Service
metadata:
  name: dep-nginx-service
spec:
  selector:
    app: dep-nginx
  ports:
    - port: 80
      targetPort: 80
  type: LoadBalancer

Use as Environment Variable

Now let’s take a look at using the configmap as environment variables. In this example I’ll use the Kode Kloud webapp color so the value and any changes will be easily visible.

Like last time the first thing to do is create the configmap so it will be available to the pod. To start we’ll use the color green.

# file: webapp-color-cm.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: webapp-color-cm
data:
  app-color: green

Check if the configmap is available using kubectl get configmaps. The output should show webapp-color-cm.

Next create the pod, I added some comments in the yaml to help point out the configmap mapping and how to choose the environment variable name.

# file: webapp-color-pod.yaml
apiVersion: v1
kind: Pod
metadata:
  labels:
    run: webapp-color
  name: webapp-color
spec:
  containers:
    - name: test-container
      image: kodekloud/webapp-color:latest
      imagePullPolicy: IfNotPresent
      env:
        # the environment variable that will be set
        - name: APP_COLOR
          valueFrom:
            configMapKeyRef:
              # the name of the configmap created previously
              name: webapp-color-cm
              # the key to use the data from 
              key: app-color

apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: dep-web-color
  name: dep-web-color
spec:
  replicas: 3
  selector:
    matchLabels:
      app: dep-web-color
  strategy: {}
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: dep-web-color
    spec:
      containers:
      - image: kodekloud/webapp-color:latest
        name: webapp-color
        imagePullPolicy: IfNotPresent
        env:
          - name: APP_COLOR
            valueFrom:
              configMapKeyRef:
                name: webapp-color-cm
                key: app-color

Next create the service and view the resulting page. Hopefully it appears green!

# file: webapp-color-service.yaml
apiVersion: v1
kind: Service
metadata:
  creationTimestamp: null
  labels:
    run: webapp-color
  name: webapp-color
spec:
  ports:
  - port: 8080
    protocol: TCP
    targetPort: 8080
    nodePort: 30080
  selector:
    run: webapp-color
  type: NodePort
status:
  loadBalancer: {}

apiVersion: v1
kind: Service
metadata:
  name: webapp-color-service
spec:
  selector:
    app: dep-web-color
  ports:
    - port: 80
      targetPort: 8080
  type: LoadBalancer

If you would like to change the background color of the page; it’s possible to edit the configmap. You’ll have to recreate the pod(s) for the changes to be picked up though.

kubectl edit configmaps webapp-color-cm

Change ‘green’ to ‘red’ then save and exit the editor.

Finally recreate the pod, you can either delete and then create again or run the following kubectl replace -f webapp-color-pod.yaml --force. Once this finishes and the pod is ready again, refresh the page and the background will now be red.

Originally posted Feb 12, 2024 last updated Jun 5, 2024 by bmcculley Tagged: