Node affinity and anti-affinity in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Node Affinity Types

Which of the following best describes the difference between requiredDuringSchedulingIgnoredDuringExecution and preferredDuringSchedulingIgnoredDuringExecution in Kubernetes node affinity?

A requiredDuringSchedulingIgnoredDuringExecution means pods prefer matching nodes but can schedule elsewhere; preferredDuringSchedulingIgnoredDuringExecution means pods must be scheduled on matching nodes.

B requiredDuringSchedulingIgnoredDuringExecution means pods must be scheduled on matching nodes; preferredDuringSchedulingIgnoredDuringExecution means pods prefer matching nodes but can schedule elsewhere if none match.

C Both mean pods must be scheduled on matching nodes, but preferredDuringSchedulingIgnoredDuringExecution also enforces node labels at runtime.

D Both mean pods prefer matching nodes, but requiredDuringSchedulingIgnoredDuringExecution ignores node labels during scheduling.

💡 Hint

Think about what happens if no nodes match the affinity rules.

requiredDuringSchedulingIgnoredDuringExecution enforces a hard rule: pods will only schedule on nodes that match the criteria. preferredDuringSchedulingIgnoredDuringExecution is a soft preference: the scheduler tries to place pods on matching nodes but will schedule elsewhere if needed.

💻 Command Output

intermediate

Output of Pod Scheduling with Node Affinity

Given a Kubernetes cluster with nodes labeled zone=us-east-1a and zone=us-east-1b, and a pod spec with this node affinity:

{
  "requiredDuringSchedulingIgnoredDuringExecution": {
    "nodeSelectorTerms": [
      {"matchExpressions": [{"key": "zone", "operator": "In", "values": ["us-east-1a"]}]}
    ]
  }
}

What will be the output of kubectl get pods -o wide regarding the node the pod is scheduled on?

A The pod will be scheduled only on a node labeled zone=us-east-1a.

B The pod will be scheduled on any node regardless of zone label.

C The pod will fail to schedule because no nodes match the affinity.

D The pod will be scheduled on a node labeled zone=us-east-1b.

💡 Hint

Check the meaning of requiredDuringSchedulingIgnoredDuringExecution.

The requiredDuringSchedulingIgnoredDuringExecution affinity restricts scheduling to nodes with the specified label. So the pod will only run on nodes labeled zone=us-east-1a.

❓ Configuration

advanced

Correct Node Anti-Affinity YAML

Which of the following YAML snippets correctly configures a pod to avoid scheduling on nodes that already run pods with label app=frontend using requiredDuringSchedulingIgnoredDuringExecution node anti-affinity?

A

affinity:
  nodeAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
    - labelSelector:
        matchExpressions:
        - key: app
          operator: In
          values:
          - frontend
      topologyKey: kubernetes.io/hostname

B

affinity:
  podAntiAffinity:
    preferredDuringSchedulingIgnoredDuringExecution:
    - labelSelector:
        matchExpressions:
        - key: app
          operator: In
          values:
          - frontend
      topologyKey: kubernetes.io/hostname

C

affinity:
  podAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
    - labelSelector:
        matchExpressions:
        - key: app
          operator: In
          values:
          - frontend
      topologyKey: kubernetes.io/hostname

D

affinity:
  podAntiAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
    - labelSelector:
        matchExpressions:
        - key: app
          operator: In
          values:
          - frontend
      topologyKey: kubernetes.io/hostname

💡 Hint

Remember that node anti-affinity uses podAntiAffinity, not nodeAffinity or podAffinity.

Node anti-affinity to avoid nodes running pods with certain labels uses podAntiAffinity with requiredDuringSchedulingIgnoredDuringExecution. Option D correctly uses this syntax.

❓ Troubleshoot

advanced

Pod Fails to Schedule Due to Node Affinity

A pod with this node affinity fails to schedule:

affinity:
  nodeAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
      nodeSelectorTerms:
      - matchExpressions:
        - key: disk
          operator: In
          values:
          - ssd

What is the most likely reason?

A No nodes in the cluster have the label <code>disk=ssd</code>.

B The pod spec is missing a container image.

C The pod is trying to schedule on nodes labeled <code>disk=hdd</code> which conflicts with affinity.

D The nodeAffinity field is misspelled and ignored by the scheduler.

💡 Hint

Check if any nodes have the required label.

If no nodes have the label disk=ssd, the requiredDuringSchedulingIgnoredDuringExecution affinity cannot be satisfied, so the pod will not schedule.

✅ Best Practice

expert

Best Practice for High Availability Using Node Anti-Affinity

You want to deploy multiple replicas of a critical service to ensure high availability. Which node anti-affinity configuration best helps spread pods across different nodes to avoid single points of failure?

A Use <code>preferredDuringSchedulingIgnoredDuringExecution</code> pod affinity with <code>topologyKey: kubernetes.io/hostname</code> to prefer pods on the same node.

B Use <code>preferredDuringSchedulingIgnoredDuringExecution</code> pod anti-affinity with <code>topologyKey: kubernetes.io/hostname</code> to softly prefer pods on different nodes.

C Use <code>requiredDuringSchedulingIgnoredDuringExecution</code> pod anti-affinity with <code>topologyKey: kubernetes.io/hostname</code> to force pods on different nodes.

D Use <code>requiredDuringSchedulingIgnoredDuringExecution</code> node affinity with <code>topologyKey: failure-domain.beta.kubernetes.io/zone</code> to schedule all pods in the same zone.

💡 Hint

Think about how to guarantee pods do not land on the same node.

Using requiredDuringSchedulingIgnoredDuringExecution pod anti-affinity with topologyKey: kubernetes.io/hostname ensures pods are scheduled on different nodes, improving availability by avoiding single-node failure.