Deployment as higher-level abstraction in Kubernetes - Mini Project: Build & Apply

Deployment as higher-level abstraction in Kubernetes

📖 Scenario: You are managing a simple web application that needs to run reliably on Kubernetes. Instead of creating Pods directly, you will use a Deployment to manage the Pods. This helps Kubernetes keep your app running smoothly even if some Pods fail.

🎯 Goal: Create a Kubernetes Deployment YAML file that runs an NGINX web server with 3 replicas. Then, update the Deployment to change the number of replicas. Finally, display the Deployment status using kubectl commands.

📋 What You'll Learn

Create a Deployment YAML file named nginx-deployment.yaml with 3 replicas

Use the nginx:1.21 image for the container

Add a label app: nginx to the Deployment and Pod template

Update the Deployment to scale replicas from 3 to 5

Use kubectl commands to apply the Deployment and check its status

💡 Why This Matters

🌍 Real World

Deployments are used in real Kubernetes clusters to manage application Pods. They ensure your app stays running and can be updated without downtime.

💼 Career

Understanding Deployments is essential for DevOps engineers and site reliability engineers who manage containerized applications in production.

1

Create the initial Deployment YAML

Create a file named nginx-deployment.yaml with a Deployment that has replicas: 3, uses the container image nginx:1.21, and has the label app: nginx in both metadata and pod template.

Kubernetes

# Write the Deployment YAML with 3 replicas, nginx:1.21 image, and label app: nginx

Hint

Remember to set replicas: 3 under spec and add app: nginx labels in both metadata and template.metadata.

2

Add a variable for replicas count

Add a comment at the top of nginx-deployment.yaml defining a variable replicaCount set to 3 to represent the number of replicas.

Kubernetes

## replicaCount: 3
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.21
        ports:
        - containerPort: 80
# Add a comment defining replicaCount variable above

Hint

Add a YAML comment at the very top like ## replicaCount: 3 to represent the replicas count.

3

Update replicas count in Deployment

Change the replicas field in nginx-deployment.yaml from 3 to 5 to scale the Deployment.

Kubernetes

## replicaCount: 3
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 5
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.21
        ports:
        - containerPort: 80
# Change replicas from 3 to 5

Hint

Find the replicas field under spec and change its value to 5.

4

Apply Deployment and check status

Run the command kubectl apply -f nginx-deployment.yaml to create or update the Deployment. Then run kubectl get deployment nginx-deployment to display the Deployment status.

Kubernetes

# Run kubectl commands below

Hint

Use kubectl apply -f nginx-deployment.yaml to apply the Deployment and kubectl get deployment nginx-deployment to see the status. The output should show 5 replicas ready.

Practice

1. What is the main purpose of a Kubernetes Deployment?

easy

A. To provide network access to pods

B. To store data persistently across pods

C. To schedule pods on specific nodes manually

D. To manage multiple copies of an application and keep them running

Solution

1. Step 1: Understand Deployment role

   A Deployment manages the desired number of pod replicas and ensures they stay running.

2. Step 2: Compare other options

   Options A, B, and D describe Services, Persistent Volumes, and manual scheduling, which are not Deployment's main purpose.

3. Final Answer:

   To manage multiple copies of an application and keep them running -> Option D

4. Quick Check:

   Deployment manages pods = C [OK]

Hint: Deployment controls app copies and uptime [OK]

Common Mistakes:

• Confusing Deployment with Service or Volume
• Thinking Deployment schedules pods manually
• Assuming Deployment stores data

2. Which of the following is the correct minimal YAML snippet to define a Deployment selector matching pods with label app: myapp?

easy

A. selector:
matchLabels:
app: myapp

B. selector:
labels:
app: myapp

C. selector:
matchLabels:
name: myapp

D. selector:
matchExpressions:
- key: app
operator: In
values: [myapp]

Solution

1. Step 1: Recall Deployment selector syntax

   The selector must use matchLabels with key-value pairs matching pod labels.

2. Step 2: Check options

   selector: matchLabels: app: myapp uses correct matchLabels with app: myapp. selector: labels: app: myapp uses wrong key labels. selector: matchLabels: name: myapp uses wrong label key name. selector: matchExpressions: - key: app operator: In values: [myapp] uses matchExpressions which is valid but more complex and not minimal.

3. Final Answer:

   selector: matchLabels: app: myapp -> Option A

4. Quick Check:

   Use matchLabels with correct key = A [OK]

Hint: Use matchLabels with exact pod label keys [OK]

Common Mistakes:

• Using 'labels' instead of 'matchLabels'
• Mismatching label keys in selector
• Confusing matchLabels with matchExpressions

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

apiVersion: apps/v1
kind: Deployment
metadata:
  name: test-deploy
spec:
  replicas: 3
  selector:
    matchLabels:
      app: test
  template:
    metadata:
      labels:
        app: test
    spec:
      containers:
      - name: nginx
        image: nginx:latest

medium

A. 0 pods running due to label mismatch

B. 1 pod running

C. 3 pods running

D. Deployment will fail to create pods

Solution

1. Step 1: Check replicas and labels

   The Deployment requests 3 replicas and the selector matches pods with label app: test. The pod template labels match this exactly.

2. Step 2: Confirm pod creation

   Since labels match, Kubernetes will create 3 pods as requested.

3. Final Answer:

   3 pods running -> Option C

4. Quick Check:

   Matching labels + replicas 3 = 3 pods [OK]

Hint: Match selector and template labels to get pods [OK]

Common Mistakes:

• Assuming label mismatch causes zero pods
• Thinking Deployment fails without errors
• Ignoring replicas count

4. You applied a Deployment YAML but no pods are created. Which issue is most likely causing this?

medium

A. The Deployment metadata name is missing

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

C. The replicas field is set to zero

D. The container image name is missing

Solution

1. Step 1: Understand Deployment selector importance

   The Deployment uses the selector to find pods it manages. If selector labels don't match pod template labels, no pods are created.

2. Step 2: Evaluate other options

   Missing image name causes pod creation failure but usually with errors. Replicas zero means no pods but is explicit. Missing metadata name causes YAML validation error.

3. Final Answer:

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

4. Quick Check:

   Selector-template label mismatch = no pods [OK]

Hint: Check selector and template labels match exactly [OK]

Common Mistakes:

• Ignoring label mismatch
• Assuming missing image silently creates no pods
• Confusing replicas zero with label mismatch

5. You want to update your app smoothly without downtime using a Deployment. Which feature of Deployment helps achieve this?

hard

A. Rolling update strategy that replaces pods gradually

B. Manual pod deletion to force restart

C. Setting replicas to zero before update

D. Using a StatefulSet instead of Deployment

Solution

1. Step 1: Identify update strategies in Deployment

   Deployment supports rolling updates that replace pods one by one, keeping the app available.

2. Step 2: Compare other options

   Manual deletion causes downtime. Setting replicas to zero stops all pods. StatefulSet is for stateful apps, not smooth rolling updates.

3. Final Answer:

   Rolling update strategy that replaces pods gradually -> Option A

4. Quick Check:

   Rolling update = smooth app update [OK]

Hint: Rolling update replaces pods one by one [OK]

Common Mistakes:

• Deleting pods manually causes downtime
• Scaling replicas to zero stops app
• Confusing StatefulSet with Deployment update