ConfigMap and env

Hub › Kubernetes › 02

Goal

Externalise configuration from the container image into a Kubernetes ConfigMap and inject it as an environment variable into the web Deployment.

Why

Hard-coding config in an image means a rebuild every time a value changes. A ConfigMap lets you update values without touching the image — you just re-apply the manifest.

Step 1 — Create the ConfigMap

Create configmap.yaml:

apiVersion: v1
kind: ConfigMap
metadata:
  name: web-config
data:
  GREETING: "hello from configmap"

Apply it:

kubectl apply -f configmap.yaml

Verify it exists:

kubectl get configmap web-config

Expected output:

NAME         DATA   AGE
web-config   1      5s

Step 2 — Reference the ConfigMap from the Deployment

Open deployment.yaml and add an env block to the container spec so it reads GREETING from web-config:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
spec:
  replicas: 1
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
        - name: nginx
          image: nginx:1.27
          ports:
            - containerPort: 80
          env:
            - name: GREETING
              valueFrom:
                configMapKeyRef:
                  name: web-config
                  key: GREETING

Apply the updated Deployment:

kubectl apply -f deployment.yaml

Step 3 — Verify the env var is injected

Once the pod restarts, exec into it and print the variable:

kubectl exec deploy/web -- printenv GREETING

Expected output:

hello from configmap

Checkpoint

You now have a running web Deployment with GREETING sourced from a ConfigMap. No image rebuild was required — changing the value in configmap.yaml and re-applying is all that's needed.

Next: 03 Probes