Priority classes for critical workloads in Kubernetes - Commands & Configuration

Introduction

Sometimes, many applications run on the same Kubernetes cluster. Priority classes help decide which apps get resources first when the cluster is busy. This way, important apps keep running smoothly even if the cluster is crowded.

When you have a critical app that must keep running even if the cluster is full.

When you want to make sure system services get resources before less important apps.

When you want to avoid important workloads being stopped during resource shortages.

When you want to organize apps by importance to manage cluster resources better.

When you want to control which pods get deleted first if the cluster runs out of memory or CPU.

Config File - priorityclass.yaml

priorityclass.yaml

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: critical-priority
value: 1000000
globalDefault: false
description: "Priority class for critical workloads that must not be evicted"

This file creates a PriorityClass named critical-priority with a very high value (1000000). The value sets how important pods with this class are. globalDefault: false means this priority is not the default for all pods. The description explains its purpose.

Commands

This command creates the priority class in the Kubernetes cluster so pods can use it to mark their importance.

Terminal

kubectl apply -f priorityclass.yaml

Expected Output

priorityclass.scheduling.k8s.io/critical-priority created

This command lists all priority classes in the cluster to verify that the critical priority class was created.

Terminal

kubectl get priorityclass

Expected Output

NAME                     VALUE        GLOBAL-DEFAULT   DESCRIPTION

critical-priority         1000000      false            Priority class for critical workloads that must not be evicted

system-cluster-critical   2000000000   true             system critical priority class

system-node-critical      2000001000   true             system node critical priority class

This command runs a pod named critical-app using the nginx image and assigns it the critical priority class to ensure it is treated as very important.

Terminal

kubectl run critical-app --image=nginx --restart=Never --priority-class-name=critical-priority

Expected Output

pod/critical-app created

→

--priority-class-name - Assigns the pod to the specified priority class

This command checks the priority class assigned to the critical-app pod to confirm it is set correctly.

Terminal

kubectl get pods critical-app -o jsonpath='{.spec.priorityClassName}'

Expected Output

critical-priority

Key Concept

If you remember nothing else from this pattern, remember: priority classes let Kubernetes know which pods are most important during resource shortages.

Common Mistakes

Not creating the PriorityClass resource before assigning it to pods.

Pods will fail to start or ignore the priority class if it does not exist.

Always create the PriorityClass resource first using kubectl apply.

Using a low value for critical workloads.

Low values mean the pod is treated as less important and can be evicted first.

Use a high numeric value for critical workloads to ensure they get priority.

Setting globalDefault to true for a critical priority class.

This makes all pods use this priority by default, which defeats the purpose of prioritizing only critical pods.

Set globalDefault to false and assign the priority class only to important pods.

Summary

Create a PriorityClass resource with a high value to mark critical workloads.

Assign the priority class to pods using the --priority-class-name flag.

Verify the priority class exists and pods have the correct priority assigned.

Practice

1. What does a higher value in a Kubernetes PriorityClass mean?

easy

A. The pod will be scheduled on nodes with more memory.

B. The pod will use less CPU resources.

C. The pod has a higher priority and is more important.

D. The pod will restart automatically on failure.

Solution

1. Step 1: Understand PriorityClass value meaning

   In Kubernetes, the value field in a PriorityClass defines the importance of the pod. Higher values mean higher priority.

2. Step 2: Relate priority to pod importance

   Pods with higher priority are considered more critical and get scheduled before lower priority pods.

3. Final Answer:

   The pod has a higher priority and is more important. -> Option C

4. Quick Check:

   Higher value = higher priority [OK]

Hint: Higher PriorityClass value means more important pod [OK]

Common Mistakes:

• Confusing priority with resource limits
• Thinking priority controls pod restart behavior
• Assuming priority affects node selection by memory

2. Which of the following is the correct YAML snippet to define a PriorityClass named high-priority with value 1000 and globalDefault: false?

easy

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

B. apiVersion: v1
kind: PriorityClass
metadata:
name: high-priority
priority: 1000
default: false

C. apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: high-priority
value: 1000
globalDefault: true
description: "High priority class"

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

Solution

1. Step 1: Check correct apiVersion and kind

   The correct apiVersion for PriorityClass is scheduling.k8s.io/v1 and kind is PriorityClass.

2. Step 2: Verify fields and types

   The field for priority is value (integer), not priority. globalDefault is a boolean. The value must be an integer, not a string.

3. Final Answer:

   YAML with apiVersion scheduling.k8s.io/v1, kind PriorityClass, value 1000 as integer, globalDefault false -> Option A

4. Quick Check:

   Correct apiVersion and value field [OK]

Hint: Use 'value' as integer and correct apiVersion [OK]

Common Mistakes:

• Using wrong apiVersion or kind
• Using 'priority' instead of 'value'
• Setting value as string instead of integer

3. Given this PriorityClass YAML and pod spec, what priority value will the pod have?

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: critical
value: 2000
globalDefault: false
description: "Critical priority"

---

apiVersion: v1
kind: Pod
metadata:
  name: test-pod
spec:
  priorityClassName: critical
  containers:
  - name: app
    image: nginx

medium

A. 1000

B. 2000

C. 0

D. Pod will fail to schedule

Solution

1. Step 1: Identify PriorityClass used by pod

   The pod specifies priorityClassName: critical, so it uses the PriorityClass named 'critical'.

2. Step 2: Find priority value of 'critical'

   The PriorityClass 'critical' has value: 2000, so the pod's priority is 2000.

3. Final Answer:

   2000 -> Option B

4. Quick Check:

   Pod priority matches PriorityClass value [OK]

Hint: Pod priority equals PriorityClass value it references [OK]

Common Mistakes:

• Assuming default priority 0 without PriorityClass
• Confusing priorityClassName with container image
• Thinking pod fails without globalDefault

4. You created a PriorityClass with globalDefault: true but pods without priorityClassName still have priority 0. What is the likely cause?

medium

A. The PriorityClass value is set to 0.

B. globalDefault only works for DaemonSets, not pods.

C. Pods must specify priorityClassName to get any priority.

D. The PriorityClass resource was not applied correctly.

Solution

1. Step 1: Understand globalDefault behavior

   A PriorityClass with globalDefault: true sets the default priority for pods without a specified class.

2. Step 2: Check why pods have priority 0

   If pods still have priority 0, likely the PriorityClass was not created or applied properly, so Kubernetes does not see it as default.

3. Final Answer:

   The PriorityClass resource was not applied correctly. -> Option D

4. Quick Check:

   globalDefault requires correct PriorityClass creation [OK]

Hint: Check if PriorityClass resource is applied when globalDefault fails [OK]

Common Mistakes:

• Assuming pods need priorityClassName despite globalDefault
• Setting globalDefault on PriorityClass with value 0
• Believing globalDefault only applies to DaemonSets

5. You want to ensure that all pods without a specified PriorityClass get a default priority of 500, but also have a critical class with priority 2000. Which YAML snippet correctly sets this up?

hard

A. apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: default-priority
value: 500
globalDefault: true
description: "Default priority"
---
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: critical
value: 2000
globalDefault: false
description: "Critical priority"

B. apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: critical
value: 2000
globalDefault: true
description: "Critical priority"
---
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: default-priority
value: 500
globalDefault: false
description: "Default priority"

C. apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: default-priority
value: 500
globalDefault: false
description: "Default priority"
---
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: critical
value: 2000
globalDefault: true
description: "Critical priority"

D. apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: default-priority
value: 500
globalDefault: true
description: "Default priority"
---
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
name: critical
value: 2000
globalDefault: true
description: "Critical priority"

Solution

1. Step 1: Identify globalDefault usage

   Only one PriorityClass can have globalDefault: true. This sets the default priority for pods without a class.

2. Step 2: Assign correct priorities

   Set the default-priority class with value 500 and globalDefault true. Set critical class with value 2000 and globalDefault false.

3. Final Answer:

   Default priority 500 with globalDefault true, critical 2000 without globalDefault -> Option A

4. Quick Check:

   Only one globalDefault PriorityClass allowed [OK]

Hint: Only one PriorityClass can have globalDefault true [OK]

Common Mistakes:

• Setting globalDefault true on multiple PriorityClasses
• Confusing which class should be default
• Using same priority value for default and critical