Lab Solution ReplicaSets optional

This lesson walks through the ReplicaSet lab solution step-by-step, preserving the lab question order and showing the exact commands and sample outputs used to inspect and fix the resources. Use this as a reference to understand ReplicaSet behavior, debugging steps, and common fixes. Quick reference - common commands:

Action	Command
List pods	`kubectl get pods`
List ReplicaSets	`kubectl get replicaset` or `kubectl get rs`
Describe ReplicaSet	`kubectl describe replicaset <name>`
Describe Pod	`kubectl describe pod <name>`
Create from file	`kubectl create -f <file>`
Edit resource	`kubectl edit rs <name>`
Scale ReplicaSet	`kubectl scale rs <name> --replicas=<n>`
Watch pods	`kubectl get pods --watch`

1) How many pods exist on the system?

Run:

kubectl get pods

Initial output:

No resources found in default namespace.

Conclusion: there are 0 pods initially.

2) How many ReplicaSets exist on the system?

Run:

kubectl get replicaset

Initial output:

No resources found in default namespace.

Conclusion: there are 0 ReplicaSets initially. Later, after a ReplicaSet is created, it can be observed:

kubectl get replicaset

Example output:

NAME               DESIRED   CURRENT   READY   AGE
new-replica-set    4         4         0       9s

3) What is the desired number of pods in the new ReplicaSet?

From the kubectl get replicaset output:

Desired = 4

Answer: 4

4) What image is used to create the pods in the new ReplicaSet?

To inspect the ReplicaSet template:

kubectl describe replicaset new-replica-set

Relevant excerpt:

Name:         new-replica-set
Namespace:    default
Selector:     name=busybox-pod
Replicas:     4 current / 4 desired
Pods Status:  0 Running / 4 Waiting / 0 Succeeded / 0 Failed
Pod Template:
  Labels:  name=busybox-pod
  Containers:
    busybox-container:
      Image:        busybox777
      Command:
        sh
        -c
        echo Hello Kubernetes! && sleep 3600
Events:
  Normal   SuccessfulCreate   60s    replicaset-controller   Created pod: new-replica-set-7r2qw
  Normal   SuccessfulCreate   60s    replicaset-controller   Created pod: new-replica-set-wkzjh
  Normal   SuccessfulCreate   60s    replicaset-controller   Created pod: new-replica-set-tn2mp
  Normal   SuccessfulCreate   60s    replicaset-controller   Created pod: new-replica-set-vpkh8

From the pod template, the image is busybox777. Answer: busybox777

5) How many pods are Ready in the new ReplicaSet?

From the kubectl get replicaset / kubectl describe output:

Ready = 0

So none of the pods are Ready.

6) Why are the pods not Ready?

Inspect a failing pod to find the root cause:

kubectl describe pod new-replica-set-7r2qw

Relevant excerpt:

Containers:
  busybox-container:
    Command:
      sh
      -c
      echo Hello Kubernetes! && sleep 3600
    State:          Waiting
      Reason:       ImagePullBackOff
      Ready:        False
    Restart Count: 0
Events:
  Normal   Scheduled  2m23s            default-scheduler  Successfully assigned default/new-replica-set-7r2qw to controlplane
  Normal   Pulling    46s (x4 over 2m23s) kubelet         Pulling image "busybox777"
  Warning  Failed     46s (x4 over 2m22s) kubelet         Failed to pull image "busybox777": rpc error: code = Unknown desc = failed to pull and unpack image "docker.io/library/busybox777:latest": failed to resolve reference "docker.io/library/busybox777:latest": pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed
  Warning  Failed     34s (x6 over 2m21s) kubelet         Error: ImagePullBackOff
  Normal   BackOff    23s (x7 over 2m21s) kubelet         Back-off pulling image "busybox777"

Reason: the kubelet cannot pull image busybox777 — the repository does not exist (invalid image name), causing ImagePullBackOff. Therefore pods are not Ready.

7) Delete a pod from the ReplicaSet and observe behavior

Delete one pod:

kubectl delete pod new-replica-set-wkzjh

Then list pods:

kubectl get pods

Example output after deletion:

NAME                      READY   STATUS         RESTARTS   AGE
new-replica-set-hcmbw     0/1     ErrImagePull   0          17s
new-replica-set-vpkh8     0/1     ErrImagePull   0          3m26s
new-replica-set-7r2qw     0/1     ErrImagePull   0          3m26s
new-replica-set-tn2mp     0/1     ErrImagePull   0          3m26s

Explanation: Deleting a pod managed by a ReplicaSet causes the ReplicaSet controller to immediately create a replacement to maintain the desired count. That’s why you still see four pods even after deleting one.

8) Create a ReplicaSet from a definition file and fix apiVersion issue

List files in /root:

ls /root
# replicaset-definition-1.yaml  replicaset-definition-2.yaml  sample.yaml

Attempt to create the first definition:

kubectl create -f /root/replicaset-definition-1.yaml

Error:

error: unable to recognize "/root/replicaset-definition-1.yaml": no matches for kind "ReplicaSet" in version "v1"

Inspect the file:

# /root/replicaset-definition-1.yaml
apiVersion: v1
kind: ReplicaSet
metadata:
  name: replicaset-1
spec:
  replicas: 2
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx

Problem: apiVersion is set to v1 but ReplicaSet is in the apps API group (apps/v1). Discover the correct API with:

kubectl explain replicaset

Fix the file by updating apiVersion to apps/v1, then create it:

# Update apiVersion to apps/v1 in replicaset-definition-1.yaml
kubectl create -f /root/replicaset-definition-1.yaml

Success:

replicaset.apps/replicaset-1 created

ReplicaSet belongs to the apps API group. Always use apiVersion: apps/v1. Use kubectl explain <kind> to discover the correct API version and available fields.

9) Fix the second definition file: selector vs template labels

Attempt to create the second file:

kubectl create -f replicaset-definition-2.yaml

Error:

The ReplicaSet "replicaset-2" is invalid: spec.template.metadata.labels: Invalid value: map[string]string{"tier":"nginx"}: `selector` does not match template `labels`

Inspect replicaset-definition-2.yaml (excerpt):

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: replicaset-2
spec:
  replicas: 2
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: nginx
    spec:
      containers:
      - name: nginx
        image: nginx

Problem: spec.selector.matchLabels is tier: frontend while the Pod template labels are tier: nginx. A ReplicaSet’s selector must match the Pod template labels so it can manage the created pods. Fix: make selector.matchLabels and template.metadata.labels match (for example, set both to tier: frontend), then create:

kubectl create -f replicaset-definition-2.yaml

Success:

replicaset.apps/replicaset-2 created

Check ReplicaSets:

kubectl get rs

Example output:

NAME                DESIRED   CURRENT   READY   AGE
new-replica-set     4         4         0       10m
replicaset-1        2         2         2       3m4s
replicaset-2        2         2         2       22s

10) Delete replicaset-1 and replicaset-2

Delete both ReplicaSets:

kubectl delete rs replicaset-1 replicaset-2

Output:

replicaset.apps "replicaset-1" deleted
replicaset.apps "replicaset-2" deleted

11) Fix the original ReplicaSet to use the correct busybox image

The new-replica-set still references the invalid image busybox777. Edit the ReplicaSet to correct the container image:

kubectl edit rs new-replica-set

In the editor, change the container image from busybox777 to busybox. Example edited snippet:

spec:
  replicas: 4
  selector:
    matchLabels:
      name: busybox-pod
  template:
    metadata:
      labels:
        name: busybox-pod
    spec:
      containers:
      - command:
        - sh
        - -c
        - echo Hello Kubernetes! && sleep 3600
        image: busybox
        imagePullPolicy: Always
        name: busybox-container

Save the change. The ReplicaSet object is updated, but existing pods do not automatically get replaced just because the ReplicaSet pod template changed.

Editing a ReplicaSet’s pod template updates the ReplicaSet object, but it does not replace existing pods. To apply the new template to running pods, either delete the existing pods (so the ReplicaSet recreates them) or recreate the ReplicaSet.

12) Recreate pods to pick up the new image

Delete the ReplicaSet’s pods so the controller will create new pods using the corrected image:

kubectl delete pod new-replica-set-vpkh8 new-replica-set-tn2mp new-replica-set-7r2qw new-replica-set-hcmbw

Watch pods being recreated:

kubectl get pods --watch

Sample outputs as pods are created and become Running:

NAME                             READY   STATUS              RESTARTS   AGE
new-replica-set-8z7z5            0/1     ContainerCreating   0          12s
new-replica-set-f5gth            0/1     ContainerCreating   0          12s
new-replica-set-nsbgx            0/1     ContainerCreating   0          12s
new-replica-set-whhll            0/1     ContainerCreating   0          12s

# A short while later
NAME                             READY   STATUS    RESTARTS   AGE
new-replica-set-f5gth            1/1     Running   0          18s
new-replica-set-nsbgx            1/1     Running   0          18s
new-replica-set-8z7z5            1/1     Running   0          18s
new-replica-set-whhll            1/1     Running   0          18s

Verify ReplicaSet ready pods:

kubectl get rs

Example output:

NAME              DESIRED   CURRENT   READY   AGE
new-replica-set   4         4         4       15m

Now the pods are running with the corrected busybox image. Note: When you change a ReplicaSet’s pod template, existing pods are not automatically replaced. Deleting the pods causes the ReplicaSet to recreate them with the new template.

13) Scale the ReplicaSet to 5 pods

Scale with kubectl scale:

kubectl scale rs new-replica-set --replicas=5

Confirm:

kubectl get pods

Example output:

NAME                                READY   STATUS    RESTARTS   AGE
new-replica-set-f5gth               1/1     Running   0          55s
new-replica-set-nsbgx               1/1     Running   0          55s
new-replica-set-8z7z5               1/1     Running   0          55s
new-replica-set-whhll               1/1     Running   0          55s
new-replica-set-mwpnz               1/1     Running   0          4s

You can also edit the ReplicaSet and modify spec.replicas to scale.

14) Scale the ReplicaSet down to 2 pods (edit approach)

Edit the ReplicaSet:

kubectl edit rs new-replica-set

Change spec.replicas to 2:

spec:
  replicas: 2
  selector:
    matchLabels:
      name: busybox-pod
  template:
    metadata:
      labels:
        name: busybox-pod
    ...

Save the change. The ReplicaSet controller will terminate excess pods to match the desired replica count of 2.

That’s it for this lab lesson. Links and References

Introduction

Core Concepts

Scheduling

Logging & Monitoring

Application Lifecycle Management

Cluster Maintenance

Security

Storage

Networking

Design and Install a Kubernetes Cluster

Install "Kubernetes the kubeadm way"

Troubleshooting

Helm Basics (2025 Updates)

Kustomize Basics (2025 Updates)

Other Topics

Mock Exams

Lab Solution ReplicaSets optional

1) How many pods exist on the system?

2) How many ReplicaSets exist on the system?

3) What is the desired number of pods in the new ReplicaSet?

4) What image is used to create the pods in the new ReplicaSet?

5) How many pods are Ready in the new ReplicaSet?

6) Why are the pods not Ready?

7) Delete a pod from the ReplicaSet and observe behavior

8) Create a ReplicaSet from a definition file and fix apiVersion issue

9) Fix the second definition file: selector vs template labels

10) Delete replicaset-1 and replicaset-2

11) Fix the original ReplicaSet to use the correct busybox image

12) Recreate pods to pick up the new image

13) Scale the ReplicaSet to 5 pods

14) Scale the ReplicaSet down to 2 pods (edit approach)

Watch Video

​1) How many pods exist on the system?

​2) How many ReplicaSets exist on the system?

​3) What is the desired number of pods in the new ReplicaSet?

​4) What image is used to create the pods in the new ReplicaSet?

​5) How many pods are Ready in the new ReplicaSet?

​6) Why are the pods not Ready?

​7) Delete a pod from the ReplicaSet and observe behavior

​8) Create a ReplicaSet from a definition file and fix apiVersion issue

​9) Fix the second definition file: selector vs template labels

​10) Delete replicaset-1 and replicaset-2

​11) Fix the original ReplicaSet to use the correct busybox image

​12) Recreate pods to pick up the new image

​13) Scale the ReplicaSet to 5 pods

​14) Scale the ReplicaSet down to 2 pods (edit approach)

Watch Video

1) How many pods exist on the system?

2) How many ReplicaSets exist on the system?

3) What is the desired number of pods in the new ReplicaSet?

4) What image is used to create the pods in the new ReplicaSet?

5) How many pods are Ready in the new ReplicaSet?

6) Why are the pods not Ready?

7) Delete a pod from the ReplicaSet and observe behavior

8) Create a ReplicaSet from a definition file and fix apiVersion issue

9) Fix the second definition file: selector vs template labels

10) Delete replicaset-1 and replicaset-2

11) Fix the original ReplicaSet to use the correct busybox image

12) Recreate pods to pick up the new image

13) Scale the ReplicaSet to 5 pods

14) Scale the ReplicaSet down to 2 pods (edit approach)