Service selectors and labels in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Service Selectors in Kubernetes

What is the primary purpose of a selector in a Kubernetes Service?

A To define the network port the Service listens on

B To configure the storage volume attached to the Pods

C To assign a unique IP address to the Service

D To specify which Pods the Service should route traffic to based on matching labels

💡 Hint

Think about how a Service knows which Pods to send traffic to.

A selector in a Service matches labels on Pods to determine which Pods receive traffic. It does not configure ports, IPs, or storage.

💻 Command Output

intermediate

Output of kubectl get pods with label selector

Given the following Pods with labels:

pod1: app=web, env=prod
pod2: app=web, env=dev
pod3: app=db, env=prod

What is the output of the command kubectl get pods -l app=web,env=prod --no-headers -o custom-columns=:metadata.name?

A pod1

B pod2

C pod3

D

pod1
pod2

💡 Hint

The command filters Pods matching both labels app=web and env=prod.

Only pod1 has both labels app=web and env=prod. pod2 has env=dev, pod3 has app=db.

❓ Configuration

advanced

Correct Service selector for multiple labels

You want to create a Service that routes traffic only to Pods labeled app=frontend and tier=production. Which selector configuration is correct?

A

selector:
  app: frontend
  tier: production

B

selector:
  app: frontend,tier: production

C

selector:
  app: frontend
  tier-production: true

D

selector:
  - app=frontend
  - tier=production

💡 Hint

Selectors use key-value pairs in YAML mapping format.

The selector must be a map with keys and values. Option A correctly defines two keys with their values. Option A is invalid syntax, A uses a list which is incorrect, and D uses a wrong key name.

❓ Troubleshoot

advanced

Why does a Service not route traffic to Pods?

You created a Service with selector app: backend. However, no traffic reaches the Pods. The Pods have labels app: backend and version: v1. What is the most likely cause?

A The Service type is ClusterIP, which cannot route traffic

B The Pods are not running on the same node as the Service

C The Service selector matches the Pods, but the Pods are not ready or have no endpoints

D The Service selector does not match the Pod labels exactly due to a typo

💡 Hint

Check Pod readiness and endpoints associated with the Service.

If Pods match the selector but are not ready or have no endpoints, the Service will not route traffic. Node location does not affect routing, and ClusterIP Services route traffic inside the cluster.

🔀 Workflow

expert

Order the steps to expose a Deployment using a Service with selectors

Arrange the steps in the correct order to expose a Deployment named myapp using a Service that routes traffic to its Pods via selectors.

A 3,1,4,2

B 1,3,2,4

C 1,2,3,4

D 2,1,3,4

💡 Hint

Think about creating resources and applying them in order.

First define the Deployment with labels, then apply it. Next define the Service with matching selectors, then apply it. This ensures Pods exist before Service tries to select them.