Service selectors and labels in Kubernetes - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select pods with label app=web.

Kubernetes

selector:
  matchLabels:
    app: [1]

Drag options to blanks, or click blank then click option'

Aweb

Bdatabase

Ccache

Dfrontend

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong label value like 'database' or 'cache'.

Confusing the label key with the value.

The selector must match the label value 'web' to select the correct pods.

2fill in blank

medium

Complete the service selector to match pods with label tier=backend.

Kubernetes

selector:
  matchLabels:
    tier: [1]

Drag options to blanks, or click blank then click option'

Afrontend

Bdatabase

Ccache

Dbackend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'frontend' instead of 'backend'.

Mixing up label keys and values.

The selector must match the label value 'backend' to select the correct pods.

3fill in blank

hard

Fix the error in the selector to correctly match pods with label env=prod.

Kubernetes

selector:
  matchLabels:
    env: [1]

Drag options to blanks, or click blank then click option'

Aproduction

Bprod

Cdevelopment

Dtest

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'production' instead of 'prod'.

Using unrelated label values like 'test' or 'development'.

The pods are labeled with env=prod, so the selector must use 'prod'.

4fill in blank

hard

Fill both blanks to select pods with labels app=api and version=v1.

Kubernetes

selector:
  matchLabels:
    app: [1]
    version: [2]

Drag options to blanks, or click blank then click option'

Aapi

Bv2

Cv1

Dweb

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing version values like using 'v2' instead of 'v1'.

Using wrong app label like 'web' instead of 'api'.

The selector must match app=api and version=v1 to select the correct pods.

5fill in blank

hard

Fill all three blanks to create a selector for pods with labels role=frontend, env=staging, and track=canary.

Kubernetes

selector:
  matchLabels:
    role: [1]
    env: [2]
    track: [3]

Drag options to blanks, or click blank then click option'

Afrontend

Bstaging

Ccanary

Dproduction

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'production' instead of 'staging' for env.

Mixing up the label values for role or track.

The selector must match role=frontend, env=staging, and track=canary to select the correct pods.

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