k6 Load Testing

Install and Setup

brew install k6                                    # macOS
sudo apt-get install k6                            # Debian/Ubuntu (after adding k6 repo)
choco install k6                                   # Windows
docker run --rm -i grafana/k6 run - <script.js     # Docker
k6 version                                         # verify

Basic Test Script

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

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

export default function () {
  const getRes = http.get('https://test-api.k6.io/public/crocodiles/');
  check(getRes, { 'GET status 200': (r) => r.status === 200 });

  const payload = JSON.stringify({ name: 'test', sex: 'M', date_of_birth: '2020-01-01' });
  const postRes = http.post('https://test-api.k6.io/anything', payload, {
    headers: { 'Content-Type': 'application/json' },
  });
  check(postRes, { 'POST status 2xx': (r) => r.status >= 200 && r.status < 300 });
  sleep(1);
}

Run with

k6 run script.js

.

Virtual Users and Duration

export const options = {
  vus: 50,           // 50 concurrent virtual users
  duration: '5m',    // run for 5 minutes
  iterations: 1000,  // or cap at 1000 total iterations across all VUs
};

Stages (Ramp Up, Steady State, Ramp Down)

export const options = {
  stages: [
    { duration: '2m', target: 50 },  // ramp up
    { duration: '5m', target: 50 },  // steady state
    { duration: '2m', target: 0 },   // ramp down
  ],
};

Test Types

// Load test -- typical expected traffic
stages: [{ duration: '5m', target: 100 }, { duration: '10m', target: 100 }, { duration: '5m', target: 0 }]

// Stress test -- beyond normal capacity, stepped increases
stages: [
  { duration: '2m', target: 100 }, { duration: '5m', target: 100 },
  { duration: '2m', target: 200 }, { duration: '5m', target: 200 },
  { duration: '2m', target: 300 }, { duration: '5m', target: 300 },
  { duration: '5m', target: 0 },
]

// Soak test -- sustained load over hours
stages: [{ duration: '5m', target: 100 }, { duration: '8h', target: 100 }, { duration: '5m', target: 0 }]

// Spike test -- sudden surge
stages: [
  { duration: '1m', target: 10 }, { duration: '10s', target: 500 },
  { duration: '3m', target: 500 }, { duration: '10s', target: 10 }, { duration: '2m', target: 0 },
]

// Breakpoint test -- find system limits
{ executor: 'ramping-arrival-rate', startRate: 1, timeUnit: '1s', preAllocatedVUs: 500,
  stages: [{ duration: '30m', target: 500 }] }

Checks (Assertions)

import { check } from 'k6';
check(res, {
  'status is 200': (r) => r.status === 200,
  'body contains expected text': (r) => r.body.includes('crocodile'),
  'response time < 500ms': (r) => r.timings.duration < 500,
  'content-type is JSON': (r) => r.headers['Content-Type'].includes('application/json'),
});

Thresholds (Pass/Fail Criteria)

export const options = {
  thresholds: {
    http_req_duration: ['p(95)<500', 'p(99)<1000'],  // 95th percentile < 500ms
    http_req_failed: ['rate<0.01'],                   // error rate < 1%
    checks: ['rate>0.99'],                            // 99% of checks pass
    http_reqs: ['rate>100'],                          // throughput > 100 req/s
    'http_req_duration{name:login}': ['p(95)<300'],   // threshold on tagged request
  },
};

k6 exits non-zero when any threshold fails, suitable for CI gating.

HTTP Methods

import http from 'k6/http';
export default function () {
  http.get('https://api.example.com/items');
  http.post('https://api.example.com/items',
    JSON.stringify({ name: 'widget' }), { headers: { 'Content-Type': 'application/json' } });
  http.put('https://api.example.com/items/1',
    JSON.stringify({ name: 'updated' }), { headers: { 'Content-Type': 'application/json' } });
  http.del('https://api.example.com/items/1');

  // Multipart file upload
  const file = open('/path/to/file.png', 'b');
  http.post('https://api.example.com/upload', {
    file: http.file(file, 'file.png', 'image/png'),
  });
}

Headers and Authentication

// Bearer token
http.get('https://api.example.com/protected', {
  headers: { Authorization: 'Bearer eyJhbGciOi...', 'Content-Type': 'application/json' },
});

// Login in setup, pass token to default function
export function setup() {
  const res = http.post('https://api.example.com/login',
    JSON.stringify({ username: 'user', password: 'pass' }),
    { headers: { 'Content-Type': 'application/json' } });
  return { token: res.json('token') };
}
export default function (data) {
  http.get('https://api.example.com/dashboard', {
    headers: { Authorization: `Bearer ${data.token}` },
  });
}

// Cookies
const jar = http.cookieJar();
jar.set('https://api.example.com', 'session_id', 'abc123');

Groups and Tags

import { group } from 'k6';
export default function () {
  group('user flow', function () {
    group('login', function () {
      http.post('https://api.example.com/login',
        JSON.stringify({ user: 'test', pass: 'test' }),
        { headers: { 'Content-Type': 'application/json' }, tags: { name: 'login' } });
    });
    group('browse', function () {
      http.get('https://api.example.com/items', { tags: { name: 'list-items' } });
    });
  });
}

Tags enable filtering in thresholds and outputs:

http_req_duration{name:login}

.

Parameterization

import { SharedArray } from 'k6/data';
import papaparse from 'https://jslib.k6.io/papaparse/5.1.1/index.js';

const users = new SharedArray('users', function () {
  return JSON.parse(open('./users.json'));  // [{username: "a", password: "b"}, ...]
});
const csvData = new SharedArray('csv', function () {
  return papaparse.parse(open('./data.csv'), { header: true }).data;
});

export default function () {
  const user = users[Math.floor(Math.random() * users.length)];
  http.post('https://api.example.com/login',
    JSON.stringify({ username: user.username, password: user.password }),
    { headers: { 'Content-Type': 'application/json' } });
}

SharedArray

loads data once and shares across VUs to reduce memory.

Custom Metrics

import { Counter, Gauge, Rate, Trend } from 'k6/metrics';

const errorCount = new Counter('custom_errors');    // cumulative count
const cacheHitRate = new Rate('cache_hits');         // percentage of true values
const pageLoadTime = new Trend('page_load_time');    // distribution (min/max/avg/p90/p95)
const activeConns = new Gauge('active_connections'); // last value

export default function () {
  const res = http.get('https://api.example.com/');
  pageLoadTime.add(res.timings.duration);
  cacheHitRate.add(res.headers['X-Cache'] === 'HIT');
  if (res.status !== 200) errorCount.add(1);
}

export const options = {
  thresholds: { page_load_time: ['p(95)<600'], custom_errors: ['count<10'] },
};

Output

k6 run script.js                                                  # stdout summary
k6 run --out json=results.json script.js                          # JSON file
k6 run --out csv=results.csv script.js                            # CSV file
k6 run --out influxdb=http://localhost:8086/k6 script.js          # InfluxDB
K6_CLOUD_TOKEN=token k6 run --out cloud script.js                 # Grafana Cloud
k6 run --out json=results.json --out influxdb=http://localhost:8086/k6 script.js  # multiple

// Custom summary handler
import { textSummary } from 'https://jslib.k6.io/k6-summary/0.0.2/index.js';
export function handleSummary(data) {
  return {
    stdout: textSummary(data, { indent: ' ', enableColors: true }),
    'summary.json': JSON.stringify(data),
  };
}

Browser Testing

import { browser } from 'k6/browser';
import { check } from 'k6';

export const options = {
  scenarios: {
    ui: { executor: 'shared-iterations', options: { browser: { type: 'chromium' } } },
  },
};

export default async function () {
  const page = await browser.newPage();
  try {
    await page.goto('https://test.k6.io/my_messages.php');
    await page.locator('input[name="login"]').type('admin');
    await page.locator('input[name="password"]').type('123');
    await page.locator('input[type="submit"]').click();
    const header = await page.locator('h2').textContent();
    check(header, { 'logged in': (h) => h === 'Welcome, admin!' });
  } finally { await page.close(); }
}

CI/CD Integration (GitHub Actions)

name: Load Test
on: [push, pull_request]
jobs:
  k6-test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: grafana/k6-action@v0.3.1
        with:
          filename: tests/load-test.js
        env:
          K6_CLOUD_TOKEN: ${{ secrets.K6_CLOUD_TOKEN }}

Thresholds serve as the quality gate -- the step fails when any threshold is breached.

Common Patterns

API Endpoint Test

import http from 'k6/http';
import { check, sleep } from 'k6';
export const options = {
  stages: [{ duration: '1m', target: 50 }, { duration: '3m', target: 50 }, { duration: '1m', target: 0 }],
  thresholds: { http_req_duration: ['p(95)<500'], http_req_failed: ['rate<0.01'] },
};
export default function () {
  check(http.get('https://api.example.com/health'), { 'status 200': (r) => r.status === 200 });
  sleep(1);
}

Login Flow Test

import http from 'k6/http';
import { check, sleep } from 'k6';
export default function () {
  const loginRes = http.post('https://api.example.com/auth/login',
    JSON.stringify({ email: 'user@test.com', password: 'password123' }),
    { headers: { 'Content-Type': 'application/json' }, tags: { name: 'login' } });
  check(loginRes, { 'login succeeded': (r) => r.status === 200 });
  const token = loginRes.json('access_token');
  const profileRes = http.get('https://api.example.com/me', {
    headers: { Authorization: `Bearer ${token}` }, tags: { name: 'profile' } });
  check(profileRes, { 'profile loaded': (r) => r.status === 200 });
  sleep(1);
}

File Upload Test

import http from 'k6/http';
import { check } from 'k6';
const testFile = open('/path/to/test-file.pdf', 'b');
export default function () {
  const res = http.post('https://api.example.com/upload', {
    file: http.file(testFile, 'report.pdf', 'application/pdf'),
  });
  check(res, { 'upload ok': (r) => r.status === 200, 'under 5s': (r) => r.timings.duration < 5000 });
}