Using ConfigMaps via Volume Mounts

Environment variables work well for short strings. They break down for multi-line config files. You cannot inject an Nginx configuration block or a JSON settings file as a single env var. Volume mounts solve this: each ConfigMap key becomes a file in a directory inside the container.

Start with a ConfigMap that holds a config file:

nano nginx-config.yaml

apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-config
data:
  nginx.conf: |
    server {
      listen 80;
      server_name _;
      location /health {
        return 200 'ok';
      }
    }
  log-format: combined

kubectl apply -f nginx-config.yaml

Mounting all keys as files

A configMap volume type mounts every key as a separate file in the specified directory. The key name becomes the filename. The value becomes the file content.

nano nginx-pod.yaml

apiVersion: v1
kind: Pod
metadata:
  name: nginx-pod
spec:
  containers:
    - name: nginx
      image: busybox:1.36
      command: ['sh', '-c', 'ls /etc/nginx-config/ && cat /etc/nginx-config/nginx.conf']
      volumeMounts:
        - name: config-vol
          mountPath: /etc/nginx-config
  volumes:
    - name: config-vol
      configMap:
        name: nginx-config
  restartPolicy: Never

kubectl apply -f nginx-pod.yaml
kubectl logs nginx-pod

The output lists both files: nginx.conf and log-format. Then it prints the full contents of nginx.conf. Each ConfigMap key became a separate file.

Mounting a specific key

Sometimes you want only one file from the ConfigMap, not all of them. Use the items field to select specific keys and map them to specific filenames:

nano nginx-selective.yaml

apiVersion: v1
kind: Pod
metadata:
  name: nginx-selective
spec:
  containers:
    - name: nginx
      image: busybox:1.36
      command: ['sh', '-c', 'ls /etc/nginx/ && cat /etc/nginx/server.conf']
      volumeMounts:
        - name: config-vol
          mountPath: /etc/nginx
  volumes:
    - name: config-vol
      configMap:
        name: nginx-config
        items:
          - key: nginx.conf
            path: server.conf
  restartPolicy: Never

kubectl apply -f nginx-selective.yaml
kubectl logs nginx-selective

Only one file appears: server.conf. The log-format key was not listed in items, so it was not mounted. The file is named server.conf instead of nginx.conf because the path field controls the destination filename.

Live updates without restart

When you update a ConfigMap, volume-mounted files are updated automatically. The kubelet periodically refreshes ConfigMap volumes (typically within a minute). The container sees the new file content without being restarted.

Edit the ConfigMap:

kubectl edit configmap nginx-config

Change log-format from combined to short. Save and exit. Wait a moment, then exec into the Pod to see the change:

kubectl exec nginx-pod -- cat /etc/nginx-config/log-format

The file content reflects the updated value. The container did not restart.

kubectl delete pod nginx-pod nginx-selective
kubectl delete configmap nginx-config

Volume mounts handle multi-line config files cleanly and support live updates. The next lesson covers immutable ConfigMaps, a performance optimization for high-scale clusters.