K6 Performance Testing for APIs – Quick Start Guide • Krishan Chawla

Introduction#

Performance testing is no longer optional for modern APIs. Even a perfectly functional API can fail under load if performance is not validated early.

K6 is a modern, open-source performance testing tool designed for developers, QA engineers, and DevOps teams. It allows you to write readable performance tests using JavaScript and run them locally or in CI/CD pipelines.

What is K6 and When Should You Use It#

K6 is best suited for:

REST and GraphQL API performance testing
Load testing microservices
Stress and spike testing
CI/CD performance gates
Pre-production validation

Core Performance Testing Concepts#

Term	Meaning
Virtual User (VU)	Simulated user executing your test
Iteration	One full execution of the test function
Load Test	Expected production traffic
Stress Test	Gradually increasing load to find limits
Spike Test	Sudden traffic increase
Threshold	SLA or pass/fail condition

Step 1: Install k6#

macOS#

brew install k6

bash

Windows#

choco install k6

bash

Linux (Debian / Ubuntu)#

sudo apt update
sudo apt install k6

bash

Docker (CI/CD Friendly)#

docker pull grafana/k6

bash

Verify installation:

k6 version

bash

Step 3: Your First K6 API Test#

Create api-test.js:

import http from 'k6/http'
import { check, sleep } from 'k6'

export const options = {
  vus: 10,
  duration: '30s',
}

export default function () {
  const response = http.get('https://jsonplaceholder.typicode.com/posts')

  check(response, {
    'status is 200': (r) => r.status === 200,
    'response time < 500ms': (r) => r.timings.duration < 500,
  })

  sleep(1)
}

javascript

What this test does#

Runs 10 virtual users
Executes for 30 seconds
Sends HTTP GET requests
Validates response status and performance

Step 4: Running the Test#

k6 run tests/api-test.js

bash

K6 will immediately start printing live metrics to the terminal.

Step 5: Understanding K6 Output#

Key Metrics to Watch

Metric	What It Tells You
http_req_duration	End-to-end request latency
http_req_failed	Percentage of failed requests
vus	Number of active virtual users
iterations	Total test executions
checks	Assertion success rate

Focus First On

✅ Error rate (must be near zero)
✅ P95 / P99 latency
✅ Consistency under sustained load

Load Testing Example#

export const options = {
  vus: 50,
  duration: '2m',
}

javascript

Use this to simulate normal expected traffic.

Stress Testing Example#

export const options = {
  stages: [
    { duration: '1m', target: 20 },
    { duration: '1m', target: 50 },
    { duration: '1m', target: 100 },
    { duration: '1m', target: 0 },
  ],
}

javascript

Use this to identify:

Performance degradation
Breaking points
Resource bottlenecks

API Testing with Headers and Auth#

const params = {
  headers: {
    Authorization: 'Bearer YOUR_TOKEN',
    'Content-Type': 'application/json',
  },
}

http.get('https://api.example.com/users', params)

javascript

Step 6: Adding Thresholds (SLAs)#

Thresholds fail the test when SLAs are violated.

export const options = {
  thresholds: {
    http_req_duration: ['p(95)<500'],
    http_req_failed: ['rate<0.01'],
  },
}

javascript

Perfect for CI/CD pipelines.

CI/CD Usage (Quick Example)#

k6 run tests/api-test.js --summary-export=summary.json

bash

Pipeline can fail if thresholds are breached.

Best Practices#

Start small, scale gradually
Keep tests deterministic
Version control test scripts
Avoid testing from unstable networks
Always define thresholds

When NOT to Use K6#

UI/browser testing
Visual performance testing
End-to-end user journey validation

Conclusion#

K6 enables teams to shift performance testing left and catch issues early.
With minimal setup and readable scripts, it becomes a powerful part of your DevTools ecosystem.

Key Takeaways#

K6 is lightweight and developer-friendly
Ideal for API and microservice testing
Easy CI/CD integration
Clear metrics and strong automation support