Why Quality of Service classes (Guaranteed, Burstable, BestEffort) in Kubernetes? - Purpose & Use Cases

The Big Idea

What if your most important app suddenly lost all its resources during peak time?

The Scenario

Imagine running many apps on a shared computer without telling it which apps are more important. When the computer gets busy, all apps fight for resources like memory and CPU.

Some apps suddenly slow down or crash because they don't get enough resources.

The Problem

Manually guessing how much resource each app needs is slow and often wrong.

Without clear rules, important apps might starve, and less important ones hog resources.

This causes crashes, slow responses, and unhappy users.

The Solution

Kubernetes Quality of Service (QoS) classes fix this by grouping apps into three clear levels: Guaranteed, Burstable, and BestEffort.

This tells the system which apps must always get resources, which can share, and which get leftovers.

It keeps apps running smoothly even when resources are tight.

Before vs After

✗ Before

pod without resource requests and limits

✓ After

pod with resource requests and limits defining QoS class

What It Enables

This lets Kubernetes smartly manage resources so critical apps stay fast and reliable, while less important ones use spare capacity.

Real Life Example

In a busy online store, the payment service runs as Guaranteed to never slow down, while the product recommendation service runs as Burstable, and background data cleanup runs as BestEffort.

Key Takeaways

Manual resource management is error-prone and risky.

QoS classes clearly prioritize app resource needs.

This improves stability and user experience in shared environments.

Practice

1. Which Kubernetes Quality of Service (QoS) class guarantees that a pod will always get the resources it requests?

easy

A. Burstable

B. Guaranteed

C. BestEffort

D. Reserved

Solution

1. Step 1: Understand QoS classes in Kubernetes

   There are three main QoS classes: Guaranteed, Burstable, and BestEffort. Guaranteed means the pod has resource requests equal to limits.

2. Step 2: Identify which class ensures resource guarantees

   Guaranteed pods always get the resources they request because requests and limits match exactly.

3. Final Answer:

   Guaranteed -> Option B

4. Quick Check:

   Guaranteed = full resource guarantee [OK]

Hint: Guaranteed means requests equal limits, so full resource guarantee [OK]

Common Mistakes:

• Confusing Burstable with Guaranteed
• Thinking BestEffort has resource guarantees
• Assuming Reserved is a QoS class

2. Which of the following pod resource specifications will result in a Burstable QoS class?

easy

A. resources:\n requests:\n cpu: '200m'\n memory: '128Mi'\n limits:\n cpu: '500m'\n memory: '256Mi'

B. resources:\n requests:\n cpu: '500m'\n memory: '256Mi'\n limits:\n cpu: '500m'\n memory: '256Mi'

C. resources:\n requests:\n cpu: '0'\n memory: '0'\n limits:\n cpu: '0'\n memory: '0'

D. resources: {}

Solution

1. Step 1: Review resource requests and limits for QoS classes

   Guaranteed requires requests equal to limits. Burstable requires requests less than limits. BestEffort has no requests or limits.

2. Step 2: Match the given specs to Burstable

   resources:\n requests:\n cpu: '200m'\n memory: '128Mi'\n limits:\n cpu: '500m'\n memory: '256Mi' has requests less than limits, so it is Burstable.

3. Final Answer:

   requests less than limits -> Option A

4. Quick Check:

   Burstable = requests < limits [OK]

Hint: Burstable means requests less than limits, not equal [OK]

Common Mistakes:

• Choosing equal requests and limits as Burstable
• Assuming empty resources means Burstable
• Confusing zero requests with BestEffort

3. Given this pod spec snippet, what QoS class will Kubernetes assign?

resources:
  requests:
    cpu: '100m'
    memory: '128Mi'
  limits:
    cpu: '100m'
    memory: '128Mi'

medium

A. Burstable

B. BestEffort

C. null

D. Guaranteed

Solution

1. Step 1: Compare requests and limits in the pod spec

   Requests and limits for both CPU and memory are equal (100m CPU and 128Mi memory).

2. Step 2: Determine QoS class based on equality

   When requests equal limits for all resources, Kubernetes assigns Guaranteed QoS.

3. Final Answer:

   Guaranteed -> Option D

4. Quick Check:

   Requests = Limits means Guaranteed [OK]

Hint: Equal requests and limits means Guaranteed QoS [OK]

Common Mistakes:

• Choosing Burstable when requests equal limits
• Thinking BestEffort applies with resource specs
• Selecting null as a QoS class

4. Given this pod spec snippet, what QoS class will Kubernetes assign?

resources:
  requests:
    cpu: '0'
    memory: '0'

medium

A. BestEffort

B. Burstable

C. Guaranteed

D. It will cause a syntax error

Solution

1. Step 1: Analyze the resource specification

   Requests are set to cpu: '0' and memory: '0', with no limits defined.

2. Step 2: Apply QoS classification rules

   Requests are specified but zero values are treated as no requests. No limits are defined. This qualifies as BestEffort, since no resource requests or limits are effectively set.

3. Final Answer:

   BestEffort -> Option A

4. Quick Check:

   No requests or limits means BestEffort [OK]

Hint: Zero requests with no limits -> BestEffort (not Burstable) [OK]

Common Mistakes:

• Thinking it causes a syntax error
• Assuming zero requests means BestEffort
• Believing it qualifies as Guaranteed

5. You want to ensure a pod gets guaranteed CPU and memory resources but also allow it to burst beyond the requested amount when available. Which resource configuration and QoS class combination achieves this?

hard

A. Set no requests or limits; QoS class BestEffort

B. Set requests equal to limits; QoS class Guaranteed

C. Set requests less than limits; QoS class Burstable

D. Set limits less than requests; QoS class Burstable

Solution

1. Step 1: Understand resource guarantees and bursting

   Guaranteed QoS requires requests equal to limits, which means no bursting beyond limits. Burstable allows requests less than limits, enabling bursting.

2. Step 2: Match configuration to desired behavior

   Setting requests less than limits gives minimum guaranteed resources with ability to burst, which is Burstable QoS.

3. Final Answer:

   Set requests less than limits; QoS class Burstable -> Option C

4. Quick Check:

   Burstable = guaranteed minimum + burst capacity [OK]

Hint: Burstable = requests less than limits for bursting [OK]

Common Mistakes:

• Choosing Guaranteed which disallows bursting
• Setting limits less than requests (invalid)
• Assuming BestEffort provides guarantees