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.

Practice

1. What is the main purpose of a ReplicaSet in Kubernetes?

easy

A. To manage network policies between pods

B. To store persistent data for pods

C. To ensure a specified number of pod replicas are running at all times

D. To schedule pods on specific nodes

Solution

1. Step 1: Understand ReplicaSet role

   A ReplicaSet's job is to keep a stable set of pod replicas running.

2. Step 2: Compare options with ReplicaSet function

   Only To ensure a specified number of pod replicas are running at all times describes maintaining pod replicas, which is the ReplicaSet's purpose.

3. Final Answer:

   To ensure a specified number of pod replicas are running at all times -> Option C

4. Quick Check:

   ReplicaSet manages pod count = A [OK]

Hint: ReplicaSet = keeps pods running in set number [OK]

Common Mistakes:

• Confusing ReplicaSet with PersistentVolume
• Thinking ReplicaSet manages network policies
• Mixing ReplicaSet with pod scheduling

2. Which of the following is the correct minimal YAML snippet to define a ReplicaSet with 3 replicas?

easy

A. apiVersion: apps/v1\nkind: ReplicaSet\nmetadata:\n name: my-rs\nspec:\n replicas: 3\n selector:\n matchLabels:\n app: myapp\n template:\n metadata:\n labels:\n app: myapp\n spec:\n containers:\n - name: nginx\n image: nginx

B. apiVersion: v1\nkind: ReplicaSet\nmetadata:\n name: my-rs\nspec:\n replicas: 3\n selector:\n app: myapp\n template:\n metadata:\n labels:\n app: myapp\n spec:\n containers:\n - name: nginx\n image: nginx

C. apiVersion: apps/v1\nkind: ReplicaSet\nmetadata:\n name: my-rs\nspec:\n replicas: three\n selector:\n matchLabels:\n app: myapp\n template:\n metadata:\n labels:\n app: myapp\n spec:\n containers:\n - name: nginx\n image: nginx

D. apiVersion: apps/v1\nkind: ReplicaSet\nmetadata:\n name: my-rs\nspec:\n replicas: 3\n selector:\n matchLabels:\n app: myapp\n template:\n metadata:\n labels:\n app: otherapp\n spec:\n containers:\n - name: nginx\n image: nginx

Solution

1. Step 1: Check apiVersion and kind

   ReplicaSet requires apiVersion: apps/v1 and kind: ReplicaSet. The option with apiVersion: v1 is invalid.

2. Step 2: Validate replicas and selector-template match

   Replicas must be an integer like 3, not string 'three'. Selector labels must match template labels exactly (one option has template app: otherapp mismatch). Only one option has all correct: apps/v1, replicas: 3, matching labels with proper structure.

3. Final Answer:

   Correct YAML with apiVersion apps/v1, replicas: 3, matching selector and template labels -> Option A

4. Quick Check:

   YAML syntax + matching labels = D [OK]

Hint: Match selector labels exactly with template labels [OK]

Common Mistakes:

• Using wrong apiVersion
• Setting replicas as string instead of number
• Mismatching selector and template labels

3. Given this ReplicaSet YAML snippet, what will be the number of pods running after applying it?

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: test-rs
spec:
  replicas: 2
  selector:
    matchLabels:
      app: test
  template:
    metadata:
      labels:
        app: test
    spec:
      containers:
      - name: busybox
        image: busybox
        command: ['sleep', '3600']

medium

A. 3 pods

B. 1 pod

C. 0 pods

D. 2 pods

Solution

1. Step 1: Check replicas field

   The spec.replicas is set to 2, so Kubernetes will try to run 2 pods matching the selector.

2. Step 2: Confirm selector and template labels match

   Selector matches label app: test, and template labels also have app: test, so pods will be created.

3. Final Answer:

   2 pods -> Option D

4. Quick Check:

   replicas = 2 pods running [OK]

Hint: replicas number = pods running if labels match [OK]

Common Mistakes:

• Ignoring label selector and template label matching
• Assuming pods run without matching labels
• Confusing replicas with container count

4. You applied a ReplicaSet YAML but no pods are created. Which of these is the most likely cause?

medium

A. The replicas field is set to 0

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

C. The container image name is misspelled

D. The ReplicaSet name is missing

Solution

1. Step 1: Understand pod creation depends on selector matching template labels

   If selector labels don't match pod template labels, ReplicaSet won't manage any pods.

2. Step 2: Evaluate other options

   Replicas 0 means no pods, but question says pods expected. Misspelled image causes pod crash, not zero pods. Name missing causes YAML error, not silent no pods.

3. Final Answer:

   The selector labels do not match the pod template labels -> Option B

4. Quick Check:

   Selector-template label mismatch = no pods [OK]

Hint: Selector and template labels must match exactly [OK]

Common Mistakes:

• Assuming image typo prevents pod creation
• Ignoring selector-template label mismatch
• Thinking ReplicaSet name is optional

5. You want to update a ReplicaSet to increase replicas from 2 to 5 but keep the same pods running without downtime. What is the best approach?

hard

A. Scale the ReplicaSet using kubectl scale command to 5 replicas

B. Delete the ReplicaSet and create a new one with replicas: 5

C. Edit the ReplicaSet YAML to set replicas: 5 and apply it

D. Change the pod template labels to new values and apply

Solution

1. Step 1: Understand scaling ReplicaSet

   Scaling with kubectl scale updates replicas live without downtime.

2. Step 2: Evaluate other options

   Editing YAML and applying works but less safe than scale command. Deleting ReplicaSet causes downtime. Changing labels breaks pod matching.

3. Final Answer:

   Scale the ReplicaSet using kubectl scale command to 5 replicas -> Option A

4. Quick Check:

   Use kubectl scale for live replica changes [OK]

Hint: Use kubectl scale to change replicas live [OK]

Common Mistakes:

• Deleting ReplicaSet causes downtime
• Changing labels breaks pod matching
• Editing YAML directly risks errors