Updating ConfigMaps and propagation in Kubernetes - Mini Project: Build & Apply

Updating ConfigMaps and Propagation in Kubernetes

📖 Scenario: You are managing a Kubernetes cluster for a small web application. The application uses a ConfigMap to store configuration settings like the welcome message. You need to update the ConfigMap and see how the changes propagate to the running pods.

🎯 Goal: Learn how to create a ConfigMap, update it, and verify that the changes propagate to pods using volume mounts.

📋 What You'll Learn

Create a ConfigMap with a key-value pair

Create a pod that uses the ConfigMap as a volume

Update the ConfigMap with a new value

Verify the updated value inside the pod

💡 Why This Matters

🌍 Real World

ConfigMaps are used in Kubernetes to separate configuration from container images. Updating ConfigMaps and understanding how changes propagate helps keep applications flexible and easy to manage.

💼 Career

DevOps engineers and Kubernetes administrators frequently update ConfigMaps to change application behavior without rebuilding images. Knowing how to update and verify ConfigMaps is essential for managing live applications.

1

Create a ConfigMap with initial data

Create a ConfigMap named app-config with a key welcome_message and value Hello, Kubernetes! using the kubectl create configmap command.

Kubernetes

# Your code here

Hint

Use kubectl create configmap app-config --from-literal=welcome_message='Hello, Kubernetes!' to create the ConfigMap.

2

Create a pod that uses the ConfigMap as a volume

Create a pod named configmap-pod using the busybox image. Mount the app-config ConfigMap as a volume at /etc/config. Use the command sleep 3600 to keep the pod running.

Kubernetes

kubectl create configmap app-config --from-literal=welcome_message='Hello, Kubernetes!'
# Your code here

Hint

Use kubectl run to create the pod and patch it to add the ConfigMap volume and mount.

3

Update the ConfigMap with a new welcome message

Update the app-config ConfigMap to change the welcome_message value to Welcome to Kubernetes! using the kubectl create configmap --from-literal --dry-run=client -o yaml | kubectl apply -f - pattern.

Kubernetes

kubectl create configmap app-config --from-literal=welcome_message='Hello, Kubernetes!'
kubectl run configmap-pod --image=busybox --restart=Never -- sleep 3600
kubectl patch pod configmap-pod -p '{"spec":{"volumes":[{"name":"config-volume","configMap":{"name":"app-config"}}],"containers":[{"name":"configmap-pod","image":"busybox","command":["sleep","3600"],"volumeMounts":[{"name":"config-volume","mountPath":"/etc/config"}]}]}}' --type=merge
# Your code here

Hint

Use kubectl create configmap app-config --from-literal=welcome_message='Welcome to Kubernetes!' --dry-run=client -o yaml | kubectl apply -f - to update the ConfigMap.

4

Verify the updated ConfigMap value inside the pod

Use kubectl exec to open a shell inside the configmap-pod pod and use cat /etc/config/welcome_message to display the updated welcome message.

Kubernetes

kubectl create configmap app-config --from-literal=welcome_message='Hello, Kubernetes!'
kubectl run configmap-pod --image=busybox --restart=Never -- sleep 3600
kubectl patch pod configmap-pod -p '{"spec":{"volumes":[{"name":"config-volume","configMap":{"name":"app-config"}}],"containers":[{"name":"configmap-pod","image":"busybox","command":["sleep","3600"],"volumeMounts":[{"name":"config-volume","mountPath":"/etc/config"}]}]}}' --type=merge
kubectl create configmap app-config --from-literal=welcome_message='Welcome to Kubernetes!' --dry-run=client -o yaml | kubectl apply -f -
# Your code here

Hint

Use kubectl exec configmap-pod -- cat /etc/config/welcome_message to see the updated message inside the pod.

Practice

1. What happens when you update a ConfigMap in Kubernetes without restarting the pods using it?

easy

A. Pods continue using the old ConfigMap values until restarted

B. Pods automatically reload the new ConfigMap values immediately

C. Kubernetes deletes the pods using the old ConfigMap

D. ConfigMap update fails if pods are running

Solution

1. Step 1: Understand ConfigMap behavior on update

   Updating a ConfigMap does not notify or refresh pods automatically.

2. Step 2: Effect on running pods

   Pods keep using the old ConfigMap data until they are restarted or redeployed.

3. Final Answer:

   Pods continue using the old ConfigMap values until restarted -> Option A

4. Quick Check:

   Pods do not auto-refresh ConfigMaps = B [OK]

Hint: Pods need restart to use updated ConfigMaps [OK]

Common Mistakes:

• Assuming pods auto-refresh ConfigMap changes
• Thinking ConfigMaps update triggers pod restart automatically
• Believing ConfigMap updates delete pods

2. Which command correctly updates a ConfigMap named app-config from a file config.yaml?

easy

A. kubectl update configmap app-config --from-file=config.yaml

B. kubectl edit configmap app-config --from-file=config.yaml

C. kubectl replace configmap app-config --from-file=config.yaml

D. kubectl create configmap app-config --from-file=config.yaml --dry-run=client -o yaml | kubectl apply -f -

Solution

1. Step 1: Understand updating ConfigMap from file

   Direct update requires creating new YAML and applying it.

2. Step 2: Use dry-run and apply for update

   Command C creates new ConfigMap YAML from file and applies it, updating existing ConfigMap.

3. Final Answer:

   kubectl create configmap app-config --from-file=config.yaml --dry-run=client -o yaml | kubectl apply -f - -> Option D

4. Quick Check:

   Use create with dry-run + apply to update ConfigMap = C [OK]

Hint: Use create with dry-run and apply to update ConfigMap [OK]

Common Mistakes:

• Using 'kubectl update' which is not a valid command
• Trying to edit ConfigMap with --from-file option
• Using replace without proper YAML input

3. Given a pod using a ConfigMap mounted as a volume, what is the output of cat /etc/config/app.properties inside the pod immediately after updating the ConfigMap but before restarting the pod?

medium

A. Shows the updated content from the ConfigMap

B. Shows the old content from the ConfigMap before update

C. File is empty because ConfigMap is not mounted

D. Command returns an error because ConfigMap changed

Solution

1. Step 1: Understand ConfigMap volume mount behavior

   ConfigMap mounted as volume is projected as files inside pod at start time.

2. Step 2: Effect of ConfigMap update on mounted files

   Files inside pod do not update automatically; they keep old content until pod restart.

3. Final Answer:

   Shows the old content from the ConfigMap before update -> Option B

4. Quick Check:

   Mounted ConfigMap files do not auto-update = A [OK]

Hint: Mounted ConfigMap files update only after pod restart [OK]

Common Mistakes:

• Assuming mounted files auto-refresh on ConfigMap update
• Expecting empty files if ConfigMap changed
• Thinking command errors due to ConfigMap update

4. You updated a ConfigMap but your pods still use old values. Which command fixes this without deleting pods manually?

medium

A. kubectl rollout restart deployment/myapp

B. kubectl delete pod -l app=myapp

C. kubectl apply -f configmap.yaml

D. kubectl configmap refresh myapp

Solution

1. Step 1: Identify how to refresh pods after ConfigMap update

   Pods must restart to load new ConfigMap values.

2. Step 2: Use rollout restart to restart pods gracefully

   Command 'kubectl rollout restart' triggers pod restart without manual deletion.

3. Final Answer:

   kubectl rollout restart deployment/myapp -> Option A

4. Quick Check:

   Use rollout restart to refresh pods = D [OK]

Hint: Use rollout restart to refresh pods after ConfigMap update [OK]

Common Mistakes:

• Deleting pods manually instead of rollout restart
• Expecting apply to restart pods automatically
• Using non-existent commands like configmap refresh

5. You have a deployment using a ConfigMap for environment variables. After updating the ConfigMap, you want all pods to use the new values without downtime. What is the best approach?

hard

A. Update the ConfigMap and wait; pods auto-refresh environment variables

B. Delete all pods manually to force recreation with new ConfigMap

C. Update the ConfigMap and run kubectl rollout restart deployment/myapp

D. Edit each pod to mount the new ConfigMap version

Solution

1. Step 1: Understand environment variable update behavior

   Pods do not auto-refresh environment variables from ConfigMap updates.

2. Step 2: Use rollout restart for zero downtime update

   Rolling restart updates pods one by one, applying new ConfigMap values without downtime.

3. Final Answer:

   Update the ConfigMap and run kubectl rollout restart deployment/myapp -> Option C

4. Quick Check:

   Rollout restart updates pods gracefully = A [OK]

Hint: Use rollout restart for zero downtime ConfigMap update [OK]

Common Mistakes:

• Expecting pods to auto-refresh env vars
• Deleting pods causing downtime
• Editing pods directly which is not recommended