Pod priority and preemption in Kubernetes - Mini Project: Build & Apply

Pod Priority and Preemption in Kubernetes

📖 Scenario: You are managing a Kubernetes cluster where some applications are more important than others. You want to make sure that when resources are tight, the most important pods get scheduled first by using pod priority and preemption.

🎯 Goal: Learn how to create pod priority classes and assign priorities to pods so that Kubernetes can preempt lower priority pods when scheduling higher priority pods.

📋 What You'll Learn

Create a pod priority class with a specific priority value

Create a pod that uses the priority class

Create a lower priority pod

Observe pod scheduling and preemption behavior

💡 Why This Matters

🌍 Real World

Pod priority and preemption help ensure that critical applications get the resources they need in a busy Kubernetes cluster, improving reliability and user experience.

💼 Career

Understanding pod priority and preemption is essential for Kubernetes administrators and DevOps engineers to manage cluster resources effectively and maintain application uptime.

1

Create a Pod Priority Class

Create a pod priority class named high-priority with a value of 1000 and a globalDefault set to false. Use the kubectl apply -f command with a YAML manifest.

Kubernetes

# Write the YAML manifest for the high-priority pod priority class here
# Your code here

Hint

Use kind: PriorityClass and set value to 1000.

2

Create a Pod with Lower Priority

Create a pod named low-priority-pod without specifying a priority class. The pod should run the nginx container. Use the kubectl apply -f command with a YAML manifest.

Kubernetes

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000
globalDefault: false
description: "High priority class for important pods"

# Write the YAML manifest for the low-priority-pod here
# Your code here

Hint

Do not set priorityClassName for the low priority pod.

3

Create a Pod with High Priority

Create a pod named important-pod that uses the high-priority priority class. Use the kubectl apply -f command with a YAML manifest. The pod should run the nginx container.

Kubernetes

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000
globalDefault: false
description: "High priority class for important pods"
---
apiVersion: v1
kind: Pod
metadata:
  name: low-priority-pod
spec:
  containers:
  - name: nginx
    image: nginx

# Write the YAML manifest for the important-pod here
# Your code here

Hint

Set priorityClassName to high-priority in the pod spec.

4

Observe Pod Scheduling and Preemption

Run kubectl get pods to see the status of the pods. Then run kubectl describe pod low-priority-pod to check if it was preempted when important-pod was scheduled.

Kubernetes

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000
globalDefault: false
description: "High priority class for important pods"
---
apiVersion: v1
kind: Pod
metadata:
  name: low-priority-pod
spec:
  containers:
  - name: nginx
    image: nginx
---
apiVersion: v1
kind: Pod
metadata:
  name: important-pod
spec:
  priorityClassName: high-priority
  containers:
  - name: nginx
    image: nginx

# Run the commands below to check pod status
# Your code here

Hint

Use kubectl get pods and kubectl describe pod low-priority-pod to observe scheduling and preemption.

Practice

1. What is the main purpose of pod priority in Kubernetes?

easy

A. To decide which pods get scheduled first when resources are limited

B. To assign network policies to pods

C. To control pod security settings

D. To manage pod storage volumes

Solution

1. Step 1: Understand pod priority concept

   Pod priority is used to rank pods by importance for scheduling decisions.

2. Step 2: Identify scheduling impact

   When resources are scarce, higher priority pods are scheduled before lower priority ones.

3. Final Answer:

   To decide which pods get scheduled first when resources are limited -> Option A

4. Quick Check:

   Pod priority controls scheduling order = A [OK]

Hint: Pod priority ranks pods for scheduling under resource limits [OK]

Common Mistakes:

• Confusing priority with security or network policies
• Thinking priority controls storage or volumes
• Assuming priority affects pod runtime behavior

2. Which YAML field correctly assigns a priority class to a pod?

easy

A. spec.priority: high-priority

B. metadata.priorityClass: high-priority

C. metadata.priorityClassName: high-priority

D. spec.priorityClassName: high-priority

Solution

1. Step 1: Recall pod spec fields

   The priority class is set under the pod's spec using priorityClassName.

2. Step 2: Verify correct YAML syntax

   Correct field is spec.priorityClassName, not metadata or spec.priority.

3. Final Answer:

   spec.priorityClassName: high-priority -> Option D

4. Quick Check:

   Priority class set in spec.priorityClassName = C [OK]

Hint: Use spec.priorityClassName to assign priority class in pod YAML [OK]

Common Mistakes:

• Placing priorityClassName under metadata instead of spec
• Using spec.priority instead of priorityClassName
• Misspelling the field name

3. Given these pods with priorities: Pod A (priority 1000), Pod B (priority 500), Pod C (priority 1000). If resources are limited and only two pods can run, which pods will Kubernetes schedule?

medium

A. Pod B and Pod C

B. Pod A and Pod C

C. Pod A and Pod B

D. Pod B only

Solution

1. Step 1: Compare pod priorities

   Pod A and Pod C have priority 1000, Pod B has 500.

2. Step 2: Select highest priority pods

   Kubernetes schedules pods with highest priority first, so Pod A and Pod C run.

3. Final Answer:

   Pod A and Pod C -> Option B

4. Quick Check:

   Highest priority pods scheduled = A [OK]

Hint: Schedule pods with highest priority first when resources are limited [OK]

Common Mistakes:

• Choosing pods with lower priority over higher
• Assuming all pods run regardless of resources
• Ignoring equal priority pods

4. You created a pod with priorityClassName: "high-priority" but it is not preempting lower priority pods when scheduled. What could be the reason?

medium

A. The priority class "high-priority" is not defined in the cluster

B. Pod preemption is disabled by default in Kubernetes

C. The pod spec is missing resource requests

D. The pod is running on a node with no other pods

Solution

1. Step 1: Check priority class existence

   If the priority class "high-priority" is not defined, the pod cannot use it for preemption.

2. Step 2: Understand preemption behavior

   Preemption requires a valid priority class; missing it disables preemption.

3. Final Answer:

   The priority class "high-priority" is not defined in the cluster -> Option A

4. Quick Check:

   Undefined priority class blocks preemption = D [OK]

Hint: Ensure priority class exists before expecting preemption [OK]

Common Mistakes:

• Assuming preemption is disabled by default
• Ignoring missing resource requests impact
• Not checking node pod status

5. You have a cluster with pods of different priorities. A new pod with priority 2000 is pending due to lack of resources. How does Kubernetes handle this situation?

hard

A. It waits until resources are manually freed without preemption

B. It schedules the new pod alongside others, causing resource overcommit

C. It preempts lower priority pods to free resources for the new pod

D. It deletes the new pod automatically due to insufficient resources

Solution

1. Step 1: Understand preemption role

   When a high priority pod is pending, Kubernetes can preempt lower priority pods to free resources.

2. Step 2: Apply preemption behavior

   Kubernetes stops lower priority pods to schedule the higher priority pod if possible.

3. Final Answer:

   It preempts lower priority pods to free resources for the new pod -> Option C

4. Quick Check:

   High priority pod triggers preemption = B [OK]

Hint: High priority pods preempt lower ones to get scheduled [OK]

Common Mistakes:

• Thinking Kubernetes waits without preemption
• Assuming resource overcommit happens automatically
• Believing new pods get deleted if pending