Why Resource requests and limits in Kubernetes? - Purpose & Use Cases

The Big Idea

What if one app could silently crash your whole system? Learn how to stop that with resource limits!

The Scenario

Imagine running many apps on a shared computer without telling it how much memory or CPU each app needs. Sometimes one app uses too much, making others slow or crash.

The Problem

Without setting resource limits, apps can hog all the CPU or memory. This causes slowdowns, crashes, and unpredictable behavior. Fixing this by guessing is slow and frustrating.

The Solution

Resource requests and limits let you tell Kubernetes exactly how much CPU and memory each app needs and the max it can use. This keeps apps running smoothly and fairly on shared machines.

Before vs After

✗ Before

containers:
  - name: app
    image: myapp:latest

✓ After

containers:
  - name: app
    image: myapp:latest
    resources:
      requests:
        memory: "256Mi"
        cpu: "500m"
      limits:
        memory: "512Mi"
        cpu: "1"

What It Enables

This makes your apps stable and your cluster efficient by preventing resource fights and crashes.

Real Life Example

In a busy online store, setting resource requests and limits stops one sale app from slowing down the whole site during big shopping days.

Key Takeaways

Manual resource management causes crashes and slowdowns.

Requests and limits tell Kubernetes how much CPU and memory each app needs.

This keeps apps stable and the system fair and efficient.

Practice

1. What does setting a resource request for a container in Kubernetes guarantee?

easy

A. The container will get at least the requested CPU and memory resources.

B. The container can never use more than the requested resources.

C. The container will be restarted if it uses more than the requested resources.

D. The container will share resources equally with all other containers.

Solution

1. Step 1: Understand resource requests in Kubernetes

   Resource requests specify the minimum CPU and memory a container needs to run properly.

2. Step 2: Clarify what is guaranteed by requests

   Kubernetes ensures the container gets at least the requested resources but does not limit usage to that amount.

3. Final Answer:

   The container will get at least the requested CPU and memory resources. -> Option A

4. Quick Check:

   Resource request = minimum guaranteed resources [OK]

Hint: Requests guarantee minimum resources, limits cap maximum [OK]

Common Mistakes:

• Confusing requests with limits
• Thinking requests cap usage
• Assuming requests cause restarts

2. Which of the following is the correct way to specify CPU and memory requests in a Kubernetes Pod YAML?

easy

A. resources:
requests:
cpu: "0.5"
memory: "256MB"

B. resources:
limits:
cpu: 500
memory: 256

C. resources:
requests:
cpu: 500m
memory: 256MiB

D. resources:
requests:
cpu: "500m"
memory: "256Mi"

Solution

1. Step 1: Review correct syntax for resource requests

   CPU requests use millicores with "m" suffix as string, memory uses Mi or Gi units as string.

2. Step 2: Identify correct option

   resources: requests: cpu: "500m" memory: "256Mi" uses "500m" for CPU and "256Mi" for memory correctly as strings.

3. Final Answer:

   resources: requests: cpu: "500m" memory: "256Mi" -> Option D

4. Quick Check:

   CPU in millicores and memory in Mi with quotes [OK]

Hint: Use quotes and correct units like "500m" and "256Mi" [OK]

Common Mistakes:

• Omitting quotes around resource values
• Using wrong units like MB instead of Mi
• Specifying CPU as plain number without m

3. Given this Pod spec snippet, what is the maximum CPU the container can use?

resources:
  requests:
    cpu: "200m"
  limits:
    cpu: "500m"

medium

A. 500 millicores

B. 200 millicores

C. 700 millicores

D. Unlimited CPU

Solution

1. Step 1: Understand CPU requests and limits

   Requests set minimum guaranteed CPU, limits set maximum allowed CPU usage.

2. Step 2: Identify maximum CPU from limits

   The limit is "500m", meaning 500 millicores is the max CPU the container can use.

3. Final Answer:

   500 millicores -> Option A

4. Quick Check:

   CPU limit = max CPU usage [OK]

Hint: Limits define max usage, requests define minimum [OK]

Common Mistakes:

• Confusing requests with limits
• Adding requests and limits incorrectly
• Assuming no max limit if requests set

4. You set a memory limit of 256Mi but forgot to set a memory request. What is the likely effect on your Pod scheduling?

medium

A. The Pod will use unlimited memory since request is missing.

B. The Pod may be scheduled with no guaranteed memory, risking eviction under pressure.

C. The Pod will be scheduled with 256Mi guaranteed memory automatically.

D. The Pod will fail to start due to missing memory request.

Solution

1. Step 1: Understand default behavior when requests are missing

   If memory request is not set, Kubernetes treats it as zero, meaning no guaranteed memory.

2. Step 2: Effect on scheduling and eviction

   Pod may be scheduled but can be evicted if node memory is low because it has no guaranteed reservation.

3. Final Answer:

   The Pod may be scheduled with no guaranteed memory, risking eviction under pressure. -> Option B

4. Quick Check:

   Missing request means no guaranteed resources [OK]

Hint: Always set requests to guarantee resources and avoid eviction [OK]

Common Mistakes:

• Assuming limits imply requests
• Thinking Pod fails without requests
• Believing missing requests mean unlimited resources

5. You want to ensure a container always has at least 100m CPU and 128Mi memory but never uses more than 300m CPU and 256Mi memory. Which is the correct resource configuration snippet?

hard

A. resources:
requests:
cpu: "300m"
memory: "256Mi"
limits:
cpu: "100m"
memory: "128Mi"

B. resources:
limits:
cpu: "100m"
memory: "128Mi"
requests:
cpu: "300m"
memory: "256Mi"

C. resources:
requests:
cpu: "100m"
memory: "128Mi"
limits:
cpu: "300m"
memory: "256Mi"

D. resources:
requests:
cpu: "100"
memory: "128"
limits:
cpu: "300"
memory: "256"

Solution

1. Step 1: Match minimum guaranteed resources to requests

   Requests must be set to 100m CPU and 128Mi memory to guarantee minimum.

2. Step 2: Match maximum allowed resources to limits

   Limits must be set to 300m CPU and 256Mi memory to cap usage.

3. Step 3: Verify correct syntax and order

   resources: requests: cpu: "100m" memory: "128Mi" limits: cpu: "300m" memory: "256Mi" correctly sets requests first, then limits with proper units and quotes.

4. Final Answer:

   resources: requests: cpu: "100m" memory: "128Mi" limits: cpu: "300m" memory: "256Mi" -> Option C

5. Quick Check:

   Requests = min, Limits = max with correct units [OK]

Hint: Requests are min, limits are max; use correct units and quotes [OK]

Common Mistakes:

• Swapping requests and limits
• Omitting units or quotes
• Using plain numbers without units