ReplicaSet definition in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of a ReplicaSet in Kubernetes?

Choose the best description of what a ReplicaSet does in a Kubernetes cluster.

A It ensures a specified number of pod replicas are running at any time.

B It manages network policies between pods.

C It stores persistent data for pods.

D It schedules pods to specific nodes based on resource availability.

💡 Hint

Think about what keeps pods running in the right quantity.

A ReplicaSet's main job is to keep the desired number of pod replicas running. It does not handle storage, networking, or scheduling directly.

❓ Configuration

intermediate

Identify the correct ReplicaSet YAML snippet for 3 replicas of a pod named 'nginx'.

Which YAML snippet correctly defines a ReplicaSet with 3 replicas running the 'nginx' container?

A

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-rs
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx

B

apiVersion: v1
kind: ReplicaSet
metadata:
  name: nginx-rs
spec:
  replicas: 3
  selector:
    app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx

C

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-rs
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:latest
        ports:
        - containerPort: 80

D

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

💡 Hint

Check apiVersion, replicas count, and selector format carefully.

Option A uses the correct apiVersion 'apps/v1', sets replicas to 3, and uses 'matchLabels' under selector which is required. Option A has wrong apiVersion and selector format. Option A adds ports and image tag but is valid YAML; however, the question asks for a basic correct snippet, so the extra port is not necessary but not wrong. Option A has replicas set to 2, which is incorrect.

💻 Command Output

advanced

What is the output of 'kubectl get rs' after applying this ReplicaSet YAML?

Given this ReplicaSet YAML applied to the cluster, what will be the output of kubectl get rs?

Kubernetes

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: test-rs
spec:
  replicas: 4
  selector:
    matchLabels:
      app: test
  template:
    metadata:
      labels:
        app: test
    spec:
      containers:
      - name: test-container
        image: busybox
        command: ['sleep', '3600']

A

NAME      DESIRED   CURRENT   READY   AGE
test-rs   1         1         1       <age>

B

NAME      DESIRED   CURRENT   READY   AGE
test-rs   4         0         0       <age>

C

NAME      DESIRED   CURRENT   READY   AGE
test-rs   4         4         4       <age>

D Error from server (NotFound): replicasets.apps "test-rs" not found

💡 Hint

Think about what happens when you apply a ReplicaSet with 4 replicas.

When a ReplicaSet with 4 replicas is applied, Kubernetes creates 4 pods matching the selector. So DESIRED, CURRENT, and READY will all be 4 if pods are running correctly.

❓ Troubleshoot

advanced

Why does this ReplicaSet fail to create pods?

Given this ReplicaSet YAML, pods are not being created. What is the most likely cause?

Kubernetes

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: fail-rs
spec:
  replicas: 2
  selector:
    matchLabels:
      app: fail
  template:
    metadata:
      labels:
        app: fail-app
    spec:
      containers:
      - name: fail-container
        image: nginx

A The apiVersion 'apps/v1' is incorrect for ReplicaSet.

B The selector labels do not match the pod template labels.

C The container image 'nginx' is invalid and causes pod creation failure.

D The replicas count must be at least 3 for pods to be created.

💡 Hint

Check if the selector and pod labels match exactly.

The selector uses label 'app: fail' but the pod template labels use 'app: fail-app'. They must match exactly for pods to be created.

✅ Best Practice

expert

Which practice ensures safe updates to ReplicaSets in production?

When updating a ReplicaSet in a production environment, which practice is best to avoid downtime?

A Scale the ReplicaSet to zero, update the pods manually, then scale back.

B Delete the existing ReplicaSet and create a new one with updated specs.

C Directly edit the ReplicaSet to change the container image version.

D Use a Deployment to manage ReplicaSets and perform rolling updates.

💡 Hint

Think about how Kubernetes manages updates without downtime.

ReplicaSets do not support rolling updates directly. Deployments manage ReplicaSets and provide rolling update features to avoid downtime.