Service selectors and labels in Kubernetes - Mini Project: Build & Apply

Service selectors and labels

📖 Scenario: You are managing a Kubernetes cluster for a small online store. You want to organize your pods and services so that the services can find the right pods using labels and selectors.

🎯 Goal: Learn how to create pods with labels and a service that selects those pods using matching labels.

📋 What You'll Learn

Create pods with specific labels

Create a service with a selector matching the pod labels

Verify the service selects the correct pods

💡 Why This Matters

🌍 Real World

Labels and selectors help organize and manage groups of pods in Kubernetes. Services use selectors to route traffic to the right pods automatically.

💼 Career

Understanding labels and selectors is essential for Kubernetes administrators and DevOps engineers to deploy and maintain scalable applications.

1

Create pods with labels

Create two pod YAML definitions named pod1.yaml and pod2.yaml. Each pod should have the label app: webstore. Use the image nginx for both pods.

Kubernetes

# Create pod1.yaml with label app: webstore
# Create pod2.yaml with label app: webstore

Hint

Use metadata.labels to add labels to pods. Both pods must have the label app: webstore.

2

Create a service with a selector

Create a service YAML definition named service.yaml that selects pods with the label app: webstore. Use the service type ClusterIP and expose port 80.

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: pod1
  labels:
    app: webstore
spec:
  containers:
  - name: nginx
    image: nginx
---
apiVersion: v1
kind: Pod
metadata:
  name: pod2
  labels:
    app: webstore
spec:
  containers:
  - name: nginx
    image: nginx

# Create service.yaml with selector app: webstore, type ClusterIP, port 80

Hint

Use spec.selector to match the label app: webstore. Set spec.type to ClusterIP and expose port 80.

3

Verify the service selects the pods

Write the kubectl command to list pods selected by the service webstore-service using the label selector app=webstore.

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: pod1
  labels:
    app: webstore
spec:
  containers:
  - name: nginx
    image: nginx
---
apiVersion: v1
kind: Pod
metadata:
  name: pod2
  labels:
    app: webstore
spec:
  containers:
  - name: nginx
    image: nginx
---
apiVersion: v1
kind: Service
metadata:
  name: webstore-service
spec:
  selector:
    app: webstore
  ports:
  - protocol: TCP
    port: 80
    targetPort: 80
  type: ClusterIP

# Write the kubectl command to list pods with label app=webstore

Hint

Use kubectl get pods -l app=webstore to list pods with the label app=webstore.

4

Display the pods selected by the service

Run the command kubectl get pods -l app=webstore and print the output exactly as it appears.

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: pod1
  labels:
    app: webstore
spec:
  containers:
  - name: nginx
    image: nginx
---
apiVersion: v1
kind: Pod
metadata:
  name: pod2
  labels:
    app: webstore
spec:
  containers:
  - name: nginx
    image: nginx
---
apiVersion: v1
kind: Service
metadata:
  name: webstore-service
spec:
  selector:
    app: webstore
  ports:
  - protocol: TCP
    port: 80
    targetPort: 80
  type: ClusterIP

# Print the output of kubectl get pods -l app=webstore

Hint

The output shows the pods with label app=webstore selected by the service.

Practice

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

easy

A. To match labels on pods so the service knows which pods to send traffic to

B. To define the IP address of the service

C. To specify the container image used by pods

D. To set resource limits for pods

Solution

1. Step 1: Understand what selectors do

   A selector in a Kubernetes Service is used to find pods with matching labels.

2. Step 2: Connect selectors to pods

   The service uses the selector to route traffic only to pods that have the matching labels.

3. Final Answer:

   To match labels on pods so the service knows which pods to send traffic to -> Option A

4. Quick Check:

   Selector matches pod labels [OK]

Hint: Selector matches pod labels to connect service and pods [OK]

Common Mistakes:

• Thinking selector sets IP or image
• Confusing selector with resource limits
• Assuming selector is for service IP

2. Which of the following is the correct way to specify a selector in a Service YAML to match pods with label app: frontend?

easy

A. matchLabels:
app: frontend

B. labels:
app: frontend

C. podSelector:
app: frontend

D. selector:
app: frontend

Solution

1. Step 1: Identify correct selector syntax in Service YAML

   The Service spec uses selector: followed by key-value pairs to match pod labels.

2. Step 2: Compare options

   selector: app: frontend uses selector: with app: frontend, which is correct. Others use incorrect keys like labels, matchLabels, or podSelector which are not valid in Service spec.

3. Final Answer:

   selector: app: frontend -> Option D

4. Quick Check:

   Service selector key is 'selector' [OK]

Hint: Service selector key is always 'selector' not 'labels' or others [OK]

Common Mistakes:

• Using 'labels' instead of 'selector'
• Confusing with pod label syntax
• Using 'matchLabels' which is for label selectors, not Service

3. Given this Service selector:

selector:
  tier: backend
  env: prod

Which pods will this Service select?

medium

A. Pods with label tier=backend only

B. Pods with labels tier=backend and env=prod

C. Pods with label env=prod only

D. Pods with either tier=backend or env=prod

Solution

1. Step 1: Understand selector matching logic

   A Service selector matches pods that have all the specified labels with exact values.

2. Step 2: Apply to given selector

   The selector requires pods to have both tier=backend and env=prod labels to be selected.

3. Final Answer:

   Pods with labels tier=backend and env=prod -> Option B

4. Quick Check:

   Selector matches all labels, not any [OK]

Hint: Selector matches pods with all labels, not just one [OK]

Common Mistakes:

• Thinking selector matches pods with any one label
• Ignoring one of the labels in selector
• Confusing OR with AND logic in selectors

4. You have a Service with this selector:

selector:
  app: myapp
  version: v1

But your pods have labels:

app: myapp
version: v2

What is the problem?

medium

A. The Service selector syntax is invalid

B. The Service will select pods with version v2 anyway

C. The Service selector does not match any pods because version labels differ

D. Pods need a label 'service: myapp' to be selected

Solution

1. Step 1: Compare Service selector and pod labels

   The Service selector requires version: v1, but pods have version: v2.

2. Step 2: Understand label matching rules

   Labels must match exactly for the pod to be selected. Different version values mean no match.

3. Final Answer:

   The Service selector does not match any pods because version labels differ -> Option C

4. Quick Check:

   Exact label match required for selection [OK]

Hint: Labels must match exactly; different values mean no selection [OK]

Common Mistakes:

• Assuming partial label match works
• Thinking selector syntax is wrong
• Believing extra pod labels are needed

5. You want a Service to select pods with label app=web but exclude pods with label env=dev. How can you configure the selector?

hard

A. Use a selector with app: web and add a matchExpressions to exclude env=dev

B. Use selector:
app: web
env: !dev

C. Set selector:
app: web
env: dev

D. Label pods with app=web and remove env label from dev pods

Solution

1. Step 1: Understand label selector limitations

   Simple selectors only match labels present; to exclude labels, use matchExpressions with operators.

2. Step 2: Use matchExpressions to exclude

   You can specify matchExpressions with operator NotIn to exclude pods with env=dev.

3. Step 3: Review options

   Use a selector with app: web and add a matchExpressions to exclude env=dev correctly describes using matchExpressions to exclude labels. Use selector: app: web env: !dev syntax is invalid. Set selector: app: web env: dev includes the label to exclude. Label pods with app=web and remove env label from dev pods is about pod labeling, not selector configuration.

4. Final Answer:

   Use a selector with app: web and add a matchExpressions to exclude env=dev -> Option A

5. Quick Check:

   Exclude labels using matchExpressions NotIn [OK]

Hint: Use matchExpressions with NotIn to exclude labels in selector [OK]

Common Mistakes:

• Trying to exclude labels with simple selector syntax
• Using invalid selector syntax like env: !dev
• Confusing pod labeling with selector configuration