Pod affinity and anti-affinity in Kubernetes - Mini Project: Build & Apply

Pod Affinity and Anti-Affinity in Kubernetes

📖 Scenario: You are managing a Kubernetes cluster for a small online store. You want to control how pods are placed on nodes to improve reliability and performance.Specifically, you want some pods to be placed together (affinity) and some pods to be placed apart (anti-affinity).

🎯 Goal: Learn how to write pod specifications with affinity rules using podAffinity and podAntiAffinity in Kubernetes pod manifests.You will create a pod manifest with labels, then add affinity and anti-affinity rules, and finally check the pod spec output.

📋 What You'll Learn

Create a pod manifest with specific labels

Add pod affinity rules to schedule pods together

Add pod anti-affinity rules to avoid scheduling pods together

Print the final pod manifest YAML

💡 Why This Matters

🌍 Real World

Pod affinity and anti-affinity help control pod placement in Kubernetes clusters to improve availability and performance by grouping or separating pods.

💼 Career

Understanding pod affinity rules is important for Kubernetes administrators and DevOps engineers to optimize workload distribution and fault tolerance.

1

Create a basic pod manifest with labels

Create a YAML manifest for a pod named web-server in namespace default. The pod should have a container named nginx using image nginx:latest. Add labels app: web and tier: frontend under metadata.labels.

Kubernetes

# Write the pod manifest YAML with name, namespace, container, and labels

Hint

Use metadata.labels to add labels. Containers go under spec.containers.

2

Add pod affinity to schedule pods together

Add a podAffinity rule under spec.affinity to require scheduling this pod on nodes where pods with label app: web are already running. Use requiredDuringSchedulingIgnoredDuringExecution with topologyKey: kubernetes.io/hostname.

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: web-server
  namespace: default
  labels:
    app: web
    tier: frontend
spec:
  containers:
  - name: nginx
    image: nginx:latest
  # Add podAffinity rule below

Hint

Put podAffinity under spec.affinity. Use requiredDuringSchedulingIgnoredDuringExecution with a list of rules.

3

Add pod anti-affinity to avoid scheduling pods together

Add a podAntiAffinity rule under spec.affinity to avoid scheduling this pod on nodes where pods with label tier: frontend are running. Use preferredDuringSchedulingIgnoredDuringExecution with weight 100 and topologyKey: kubernetes.io/hostname.

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: web-server
  namespace: default
  labels:
    app: web
    tier: frontend
spec:
  affinity:
    podAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
          matchLabels:
            app: web
        topologyKey: kubernetes.io/hostname
  # Add podAntiAffinity rule below
  containers:
  - name: nginx
    image: nginx:latest

Hint

Put podAntiAffinity under spec.affinity. Use preferredDuringSchedulingIgnoredDuringExecution with weight and podAffinityTerm.

4

Print the final pod manifest YAML

Print the complete pod manifest YAML with the affinity and anti-affinity rules included.

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: web-server
  namespace: default
  labels:
    app: web
    tier: frontend
spec:
  affinity:
    podAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
          matchLabels:
            app: web
        topologyKey: kubernetes.io/hostname
    podAntiAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 100
        podAffinityTerm:
          labelSelector:
            matchLabels:
              tier: frontend
          topologyKey: kubernetes.io/hostname
  containers:
  - name: nginx
    image: nginx:latest

# Print the pod manifest YAML below

Hint

Use a print statement to confirm the manifest is ready.

Practice

1. What is the main purpose of podAffinity in Kubernetes?

easy

A. To automatically restart pods on failure

B. To prevent pods from running on the same node

C. To limit the number of pods per node

D. To schedule pods close to other pods with matching labels

Solution

1. Step 1: Understand podAffinity concept

   Pod affinity is used to place pods close to other pods that have specific labels, improving communication or resource sharing.

2. Step 2: Compare options with podAffinity purpose

   Only To schedule pods close to other pods with matching labels describes placing pods close to others with matching labels, which matches podAffinity's goal.

3. Final Answer:

   To schedule pods close to other pods with matching labels -> Option D

4. Quick Check:

   podAffinity = place pods close [OK]

Hint: Affinity means placing pods near each other [OK]

Common Mistakes:

• Confusing affinity with anti-affinity
• Thinking affinity limits pods per node
• Mixing affinity with pod restart policies

2. Which of the following is the correct syntax to specify a required pod anti-affinity rule in a pod spec?

easy

A. "podAntiAffinity": {"requiredDuringSchedulingIgnoredDuringExecution": [{"labelSelector": {"matchLabels": {"app": "web"}}, "topologyKey": "kubernetes.io/hostname"}]}

B. "podAntiAffinity": {"preferredDuringSchedulingIgnoredDuringExecution": [{"weight": 100, "podAffinityTerm": {"labelSelector": {"matchLabels": {"app": "web"}}, "topologyKey": "kubernetes.io/hostname"}}]}

C. "podAffinity": {"requiredDuringSchedulingIgnoredDuringExecution": [{"labelSelector": {"matchLabels": {"app": "web"}}, "topologyKey": "kubernetes.io/hostname"}]}

D. "podAffinity": {"preferredDuringSchedulingIgnoredDuringExecution": [{"weight": 50, "podAffinityTerm": {"labelSelector": {"matchLabels": {"app": "web"}}, "topologyKey": "kubernetes.io/zone"}}]}

Solution

1. Step 1: Identify required pod anti-affinity syntax

   Required pod anti-affinity uses podAntiAffinity with requiredDuringSchedulingIgnoredDuringExecution and a label selector plus topology key.

2. Step 2: Match syntax with options

   "podAntiAffinity": {"requiredDuringSchedulingIgnoredDuringExecution": [{"labelSelector": {"matchLabels": {"app": "web"}}, "topologyKey": "kubernetes.io/hostname"}]} correctly uses podAntiAffinity with requiredDuringSchedulingIgnoredDuringExecution and proper labelSelector and topologyKey.

3. Final Answer:

   Correct required pod anti-affinity syntax -> Option A

4. Quick Check:

   Required anti-affinity uses podAntiAffinity + requiredDuringScheduling [OK]

Hint: Required anti-affinity uses podAntiAffinity + requiredDuringScheduling [OK]

Common Mistakes:

• Using podAffinity instead of podAntiAffinity
• Confusing required and preferred rules
• Wrong topologyKey values

3. Given this pod spec snippet, what will happen if a pod with label app: frontend is scheduled?

affinity:
  podAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
    - labelSelector:
        matchLabels:
          app: backend
      topologyKey: kubernetes.io/hostname

medium

A. The pod will be scheduled only if no pod with label app=backend exists

B. The pod will be scheduled on any node regardless of other pods

C. The pod will be scheduled on a node that already runs a pod with label app=backend

D. The pod scheduling will fail due to syntax error

Solution

1. Step 1: Understand required podAffinity behavior

   Required podAffinity means the pod must be placed on a node where pods with matching labels already run, here app=backend.

2. Step 2: Apply to given pod spec

   The pod with label app=frontend requires a node with pods labeled app=backend, so it will schedule only on such nodes.

3. Final Answer:

   The pod will be scheduled on a node that already runs a pod with label app=backend -> Option C

4. Quick Check:

   Required podAffinity = schedule near matching pods [OK]

Hint: Required podAffinity means schedule near matching pods [OK]

Common Mistakes:

• Thinking podAffinity prevents scheduling near pods
• Confusing required and preferred affinity
• Assuming scheduling fails without matching pods

4. You wrote this anti-affinity rule but pods are still scheduled on the same node:

podAntiAffinity:
  requiredDuringSchedulingIgnoredDuringExecution:
  - labelSelector:
      matchLabels:
        app: database
    topologyKey: ""

What is the likely cause?

medium

A. Empty topologyKey causes the rule to be ignored

B. Label selector is incorrect syntax

C. Required rules are always ignored by scheduler

D. podAntiAffinity must be under preferredDuringSchedulingIgnoredDuringExecution

Solution

1. Step 1: Check topologyKey usage

   The topologyKey must be a valid node label key like kubernetes.io/hostname. Empty string disables the rule.

2. Step 2: Understand scheduler behavior

   With empty topologyKey, the scheduler ignores the anti-affinity rule, allowing pods on same node.

3. Final Answer:

   Empty topologyKey causes the rule to be ignored -> Option A

4. Quick Check:

   Valid topologyKey needed for anti-affinity [OK]

Hint: topologyKey cannot be empty for anti-affinity rules [OK]

Common Mistakes:

• Leaving topologyKey empty
• Misplacing anti-affinity under preferred instead of required
• Assuming required rules are ignored

5. You want to deploy multiple replicas of a pod with label app: cache so that no two pods run on the same node, but you want the scheduler to try to keep pods in the same zone if possible. Which affinity configuration achieves this?

hard

A. Use podAffinity with requiredDuringSchedulingIgnoredDuringExecution on app: cache and topologyKey: kubernetes.io/hostname only

B. Use podAntiAffinity with requiredDuringSchedulingIgnoredDuringExecution on app: cache and topologyKey: kubernetes.io/hostname, plus podAffinity with preferredDuringSchedulingIgnoredDuringExecution on topologyKey: failure-domain.beta.kubernetes.io/zone

C. Use podAntiAffinity with preferredDuringSchedulingIgnoredDuringExecution on app: cache and topologyKey: kubernetes.io/hostname only

D. Use podAffinity with preferredDuringSchedulingIgnoredDuringExecution on app: cache and topologyKey: failure-domain.beta.kubernetes.io/zone only

Solution

1. Step 1: Prevent pods on same node strictly

   Use podAntiAffinity with requiredDuringSchedulingIgnoredDuringExecution and topologyKey: kubernetes.io/hostname to ensure no two pods run on the same node.

2. Step 2: Prefer pods in same zone

   Add podAffinity with preferredDuringSchedulingIgnoredDuringExecution and topologyKey: failure-domain.beta.kubernetes.io/zone to encourage scheduling pods in the same zone but not strictly.

3. Step 3: Combine both rules

   Use podAntiAffinity with requiredDuringSchedulingIgnoredDuringExecution on app: cache and topologyKey: kubernetes.io/hostname, plus podAffinity with preferredDuringSchedulingIgnoredDuringExecution on topologyKey: failure-domain.beta.kubernetes.io/zone combines strict anti-affinity on node and preferred affinity on zone, matching the requirement.

4. Final Answer:

   Use required podAntiAffinity on hostname and preferred podAffinity on zone -> Option B

5. Quick Check:

   Strict anti-affinity + preferred affinity = Use podAntiAffinity with requiredDuringSchedulingIgnoredDuringExecution on app: cache and topologyKey: kubernetes.io/hostname, plus podAffinity with preferredDuringSchedulingIgnoredDuringExecution on topologyKey: failure-domain.beta.kubernetes.io/zone [OK]

Hint: Use required anti-affinity on node + preferred affinity on zone [OK]

Common Mistakes:

• Using affinity instead of anti-affinity to separate pods
• Using preferred instead of required for strict separation
• Ignoring zone preference