Load Testing with k6: Scripts, Virtual Users, Thresholds, and Browser Testing

# Install k6 (macOS)
brew install k6

# Linux (Debian/Ubuntu)
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys C5AD17C747E3415A3642D57D77C6C491D6AC1D69
echo "deb https://dl.k6.io/deb stable main" | sudo tee /etc/apt/sources.list.d/k6.list
sudo apt-get update && sudo apt-get install k6

# Windows
winget install k6

// first-test.js
import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  vus: 10,           // 10 virtual users
  duration: '30s',   // Run for 30 seconds
};

export default function () {
  const res = http.get('https://test.k6.io');
  check(res, {
    'status is 200': (r) => r.status === 200,
    'response time < 500ms': (r) => r.timings.duration < 500,
  });
  sleep(1);
}

k6 run first-test.js

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

export const options = {
  stages: [
    { duration: '2m', target: 50 },   // Ramp up to 50 users
    { duration: '5m', target: 50 },   // Stay at 50 users
    { duration: '2m', target: 100 },  // Ramp up to 100
    { duration: '5m', target: 100 },  // Stay at 100
    { duration: '2m', target: 0 },    // Ramp down
  ],
};

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

export default function () {
  const res = http.get('https://api.example.com/users');

  check(res, {
    'status is 200': (r) => r.status === 200,
    'has users array': (r) => Array.isArray(r.json()),
    'response time < 300ms': (r) => r.timings.duration < 300,
  });
}

export const options = {
  thresholds: {
    http_req_duration: [
      'p(95)<500',     // 95% of requests under 500ms
      'p(99)<1500',    // 99% under 1.5s
    ],
    http_req_failed: ['rate<0.01'],  // Less than 1% failures
    checks: ['rate>0.95'],           // At least 95% checks pass
  },
};

k6 run script.js
# If thresholds are exceeded, k6 exits with non-zero code
# This can fail a CI pipeline

import http from 'k6/http';
import { check, sleep } from 'k6';
import { Rate, Trend, Counter } from 'k6/metrics';

const errorRate = new Rate('errors');
const loginDuration = new Trend('login_duration');
const ordersCreated = new Counter('orders_created');

export default function () {
  // Login
  const loginRes = http.post('https://api.example.com/login', {
    username: 'test_user',
    password: 'test_pass',
  });

  loginDuration.add(loginRes.timings.duration);

  if (loginRes.status === 200) {
    const token = loginRes.json().token;

    // Create order
    const orderRes = http.post(
      'https://api.example.com/orders',
      JSON.stringify({ product: 'book', quantity: 1 }),
      { headers: { Authorization: `Bearer ${token}` } }
    );

    if (orderRes.status === 201) {
      ordersCreated.add(1);
    }

    errorRate.add(orderRes.status !== 201);
  } else {
    errorRate.add(1);
  }

  sleep(1);
}

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

export const options = {
  scenarios: {
    ui: {
      executor: 'shared-iterations',
      options: {
        browser: {
          type: 'chromium',
        },
      },
    },
  },
  thresholds: {
    browser_web_vital_inp: ['p(75)<200'],  // Interaction to Next Paint
    browser_web_vital_lcp: ['p(75)<2500'], // Largest Contentful Paint
  },
};

export default async function () {
  const page = browser.newPage();

  try {
    await page.goto('https://example.com');

    // Measure real user metrics
    check(page, {
      'LCP is acceptable': () =>
        page.metrics().largestContentfulPaint < 2500,
    });

    // Interact with the page
    await page.fill('#search', 'performance testing');
    await page.click('button[type="submit"]');
    await page.waitForSelector('.results');

    check(page, {
      'results are visible': () =>
        page.locator('.results').isVisible(),
    });
  } finally {
    page.close();
  }
}

# .github/workflows/load-test.yml
name: Load Test
on:
  schedule:
    - cron: '0 6 * * 1'  # Every Monday at 6 AM
  workflow_dispatch:       # Manual trigger

jobs:
  load-test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Run k6 load test
        uses: grafana/k6-action@v0.2.0
        with:
          filename: tests/load/load-test.js
          flags: --out json=results.json

      - name: Upload results
        uses: actions/upload-artifact@v4
        with:
          name: k6-results
          path: results.json

      - name: Check thresholds
        run: |
          if jq -e '.metrics.http_req_duration.values["p(95)"] > 500' results.json; then
            echo "Performance threshold exceeded!"
            exit 1
          fi