Why Priority classes for critical workloads in Kubernetes? - Purpose & Use Cases

The Big Idea

What if your most important app could never be slowed down, no matter what?

The Scenario

Imagine you run many apps on a shared computer, but some apps are super important, like a hospital system. You try to start all apps by hand, hoping the important ones get enough power and time.

The Problem

Doing this manually is slow and risky. Important apps might not get resources when needed, causing delays or crashes. You can't easily tell the system which apps matter most, so everything competes equally and chaos happens.

The Solution

Priority classes let you label apps by importance. The system then knows which apps to run first and protect during busy times. This way, critical apps keep working smoothly, even if less important ones slow down.

Before vs After

✗ Before

kubectl run app1
kubectl run app2
# No priority set, all treated equal

✓ After

kubectl create priorityclass critical --value=1000 --global-default=false --description="Critical priority class"
kubectl run app1 --priority-class-name=critical

What It Enables

It enables your system to smartly manage resources, always keeping critical apps running without interruption.

Real Life Example

In a hospital, patient monitoring apps use high priority classes so they never lose connection, even if many other apps are running.

Key Takeaways

Manual resource sharing can cause important apps to fail.

Priority classes tell the system which apps matter most.

This keeps critical workloads running smoothly under pressure.

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