Using labels for service routing in Kubernetes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a label in Kubernetes?

A label is a key-value pair attached to Kubernetes objects like pods. It helps identify and organize resources for selection and routing.

beginner

How do labels help with service routing in Kubernetes?

Labels allow services to select and route traffic only to pods with matching labels, enabling targeted communication between components.

beginner

What is a selector in a Kubernetes Service?

A selector is a query that matches pods with specific labels. The service routes traffic to all pods matching this selector.

beginner

Example: How to label a pod with app=frontend?

In the pod YAML, under metadata, add:
labels:
app: frontend

beginner

How does a Kubernetes Service use labels to route traffic?

The service uses a selector to find pods with matching labels and sends network requests only to those pods.

What does a Kubernetes Service selector do?

A Creates new pods automatically

B Deletes pods without labels

C Matches pods with specific labels to route traffic

D Changes pod labels dynamically

A selector matches pods with labels so the service knows where to send traffic.

Which of these is a valid label format?

A app/frontend

B app frontend

C app=frontend

D app:frontend

Labels use key:value format in YAML, like app: frontend.

If a service selector is 'app=backend', which pods receive traffic?

A All pods in the cluster

B Pods labeled with app=backend

C Pods without labels

D Pods labeled with app=frontend

Only pods with the label app=backend get traffic from that service.

Can a pod have multiple labels?

A Yes, pods can have many labels

B No, only one label is allowed

C Only if the pod is running

D Only for system pods

Pods can have many labels to describe different attributes.

What happens if no pods match a service selector?

A The service has no endpoints and traffic fails

B The service routes traffic to all pods

C The service creates new pods automatically

D The service ignores the selector

If no pods match, the service has no endpoints and cannot route traffic.

Explain how labels and selectors work together to route traffic in Kubernetes services.

Think about how a service finds the right pods to send requests.

Describe how you would label a pod and create a service to route traffic to it.

Consider the YAML structure for both pod and service.

Practice

1. What is the main purpose of using labels in Kubernetes service routing?

easy

A. To connect services to the correct pods

B. To store pod logs

C. To define pod resource limits

D. To schedule pods on specific nodes

Solution

1. Step 1: Understand labels role

   Labels are key-value pairs attached to pods to identify them.

2. Step 2: Connect services using selectors

   Services use selectors to find pods with matching labels for routing traffic.

3. Final Answer:

   To connect services to the correct pods -> Option A

4. Quick Check:

   Labels link services and pods = D [OK]

Hint: Labels identify pods for services to route traffic [OK]

Common Mistakes:

• Confusing labels with resource limits
• Thinking labels store logs
• Assuming labels control pod scheduling

2. Which of the following is the correct syntax to select pods with label app=frontend in a Service YAML?

easy

A. selector:
app: frontend

B. selector:
app = frontend

C. selector:
app: 'frontend'
version: 'v1'

D. selector:
- app: frontend

Solution

1. Step 1: Check YAML selector syntax

   Selectors use key: value pairs without '=' sign or list format.

2. Step 2: Validate options

   selector: app: frontend uses correct key: value syntax. selector: app = frontend uses '=' which is invalid. selector: app: 'frontend' version: 'v1' adds extra label, which is valid but not asked. selector: - app: frontend uses list format which is invalid for selector.

3. Final Answer:

   selector: app: frontend -> Option A

4. Quick Check:

   Selector uses key: value, no '=' or list [OK]

Hint: Use key: value pairs for selectors, no '=' or lists [OK]

Common Mistakes:

• Using '=' instead of ':' in YAML
• Writing selectors as lists
• Adding extra labels not requested

3. Given this Service YAML snippet:

selector:
  app: backend
  tier: api

Which pods will this Service route traffic to?

medium

A. Pods labeled only with app=backend

B. Pods labeled with either app=backend or tier=api

C. Pods labeled with both app=backend and tier=api

D. Pods labeled only with tier=api

Solution

1. Step 1: Understand selector matching

   Selectors match pods that have all specified labels (AND logic).

2. Step 2: Apply to given labels

   Pods must have both app=backend and tier=api labels to be selected.

3. Final Answer:

   Pods labeled with both app=backend and tier=api -> Option C

4. Quick Check:

   Selector matches all labels (AND) = A [OK]

Hint: Selectors match pods with all labels, not just one [OK]

Common Mistakes:

• Assuming selector matches any label (OR logic)
• Ignoring one of the labels in selector
• Confusing selector with pod annotations

4. You created a Service with selector:

selector:
  app: web
  env: prod

But traffic is not reaching pods. What is the most likely cause?

medium

A. Pods are not running on the same node

B. Service type is not LoadBalancer

C. Service port is missing

D. Pods do not have both labels app=web and env=prod

Solution

1. Step 1: Check label matching

   Service routes only to pods matching all selector labels.

2. Step 2: Identify common mismatch

   If pods lack either label, service finds no endpoints, so no traffic flows.

3. Final Answer:

   Pods do not have both labels app=web and env=prod -> Option D

4. Quick Check:

   Missing labels cause no pod selection = C [OK]

Hint: Verify pods have all selector labels to receive traffic [OK]

Common Mistakes:

• Blaming service type instead of labels
• Assuming node location affects routing
• Ignoring missing service port errors

5. You want to update your app without downtime. You have pods labeled app=frontend, version=v1. You deploy new pods with app=frontend, version=v2. How should you configure the Service selector to route traffic to both versions during rollout?

hard

A. Use selector: version=v2 only

B. Use selector: app=frontend only

C. Use selector: app=frontend and version=v2

D. Use selector: app=frontend and version=v1

Solution

1. Step 1: Understand rolling update strategy

   To route traffic to both old and new pods, selector must match both versions.

2. Step 2: Choose selector that matches both versions

   Selector with only app=frontend matches all pods regardless of version label.

3. Step 3: Avoid version-specific selectors

   Selectors with version=v1 or version=v2 exclude the other version, causing downtime.

4. Final Answer:

   Use selector: app=frontend only -> Option B

5. Quick Check:

   Selector matches all versions for smooth rollout = B [OK]

Hint: Use broad selector to include all versions during rollout [OK]

Common Mistakes:

• Selecting only new version pods causing downtime
• Selecting only old version pods ignoring new pods
• Using multiple selectors that exclude pods