Priority classes for critical workloads in Kubernetes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a PriorityClass in Kubernetes?

A PriorityClass is a Kubernetes resource that defines the priority level for pods. It helps the system decide which pods to schedule or evict first when resources are limited.

intermediate

How does Kubernetes use PriorityClasses during resource contention?

Kubernetes schedules pods with higher priority classes before lower priority ones. If resources run out, it evicts lower priority pods first to make room for higher priority pods.

beginner

What is the significance of the 'value' field in a PriorityClass?

The 'value' field is a number that defines the priority level. Higher numbers mean higher priority. For example, a PriorityClass with value 1000 is more important than one with 500.

intermediate

Can you create a PriorityClass for a critical workload? What key fields must it include?

Yes. A PriorityClass must include: metadata.name (unique name), value (priority number), and optionally description and globalDefault (if it should be default).

beginner

What happens if a pod does not specify a PriorityClass?

If a pod does not specify a PriorityClass, it gets the default priority, which is usually zero or the globalDefault PriorityClass if set. It will be scheduled and evicted with lower priority compared to pods with higher PriorityClasses.

What does a higher 'value' in a PriorityClass mean?

A Higher priority for scheduling and eviction decisions

B Lower priority for scheduling

C No effect on pod scheduling

D Only affects pod labels

Higher 'value' means the pod is more important and gets scheduled first or evicted last.

Which pods are evicted first when the node runs out of resources?

A Pods with the lowest PriorityClass value

B Pods with the highest PriorityClass value

C Pods with the most CPU usage

D Pods created last

Pods with lower priority are evicted first to free resources for higher priority pods.

What is the purpose of the 'globalDefault' field in a PriorityClass?

A Makes the PriorityClass apply only to global pods

B Marks this PriorityClass as the default for pods without a specified priority

C Disables the PriorityClass

D Sets the priority to zero

'globalDefault: true' means pods without a PriorityClass get this priority by default.

How do you assign a PriorityClass to a pod?

A Use an annotation 'priority-class' on the pod

B Set a label 'priorityClass' on the pod

C Specify 'priorityClassName' in the pod spec

D PriorityClass is assigned automatically

Pods specify the PriorityClass by setting 'priorityClassName' in their spec.

What happens if two pods have the same PriorityClass value?

A Both pods are scheduled simultaneously always

B Pods with the same value are evicted immediately

C Pods with the same value cannot exist

D Kubernetes uses other factors like creation time to decide scheduling

If priorities are equal, Kubernetes uses other criteria like creation time or resource requests.

Explain how PriorityClasses help Kubernetes manage critical workloads during resource shortages.

Think about how Kubernetes decides which pods to keep or remove when resources are tight.

Describe the steps to create and use a PriorityClass for a critical workload pod.

Focus on resource creation and pod specification.

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