k6 Load Testing

Overview

k6 is a modern, developer-centric load testing tool that helps you write and execute performance tests for HTTP APIs, WebSocket endpoints, and browser scenarios. This skill provides comprehensive guidance on writing realistic load tests, configuring test scenarios (smoke, load, stress, spike, soak), analyzing results, and integrating with CI/CD pipelines.

Use this skill when you need to validate system performance, identify bottlenecks, ensure SLA compliance, or catch performance regressions before deployment.

When to Use This Skill

• Use when you need to load test HTTP APIs, WebSocket endpoints, or browser scenarios
• Use when setting up performance regression tests in CI/CD
• Use when analyzing system behavior under various load conditions
• Use when comparing performance between code changes
• Use when validating SLA requirements and performance budgets

k6 Basics

Installation

# macOS
brew install k6

# Windows
choco install k6

# Linux
sudo gpg -k
sudo gpg --no-default-keyring --keyring /usr/share/keyrings/k6-archive-keyring.gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys C5AD17C747E3415A3642D57D77C6C491D6AC1D69
echo "deb [signed-by=/usr/share/keyrings/k6-archive-keyring.gpg] 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

Quick Start

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

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

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

Run with: k6 run simple-test.js

Test Configuration

Common Options

export const options = {
  // Virtual Users (concurrent users)
  vus: 100,
  
  // Test duration
  duration: '5m',
  
  // Or use stages for ramp-up/ramp-down
  stages: [
    { duration: '30s', target: 20 },   // Ramp up
    { duration: '1m', target: 100 },  // Stay at 100
    { duration: '30s', target: 0 },    // Ramp down
  ],
  
  // Thresholds (SLA)
  thresholds: {
    http_req_duration: ['p(95)<500'],  // 95% requests < 500ms
    http_req_failed: ['rate<0.01'],     // Error rate < 1%
  },
  
  // Load zones (distributed testing)
  ext: {
    loadimpact: {
      name: 'My Load Test',
      distribution: {
        'amazon:us:ashburn': { weight: 50 },
        'amazon:eu: Dublin': { weight: 50 },
      },
    },
  },
};

Test Types

HTTP Testing

Basic Requests

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

export default function () {
  // GET request
  const getRes = http.get('https://api.example.com/users');
  
  check(getRes, {
    'GET succeeded': (r) => r.status === 200,
    'has users': (r) => r.json('data.length') > 0,
  });

  // POST request with JSON body
  const postRes = http.post('https://api.example.com/users', 
    JSON.stringify({ name: 'Test User', email: 'test@example.com' }),
    {
      headers: {
        'Content-Type': 'application/json',
        'Authorization': 'Bearer ' + __ENV.API_TOKEN,
      },
    }
  );
  
  check(postRes, {
    'POST succeeded': (r) => r.status === 201,
    'user created': (r) => r.json('id') !== undefined,
  });

  sleep(1);
}

Request Chaining

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

export default function () {
  // Login and extract token
  const loginRes = http.post('https://api.example.com/login', 
    JSON.stringify({ email: 'test@example.com', password: 'password123' })
  );
  
  const token = loginRes.json('access_token');
  
  // Use token in subsequent requests
  const headers = {
    'Authorization': `Bearer ${token}`,
    'Content-Type': 'application/json',
  };
  
  const profileRes = http.get('https://api.example.com/profile', {
    headers: headers,
  });
  
  check(profileRes, {
    'profile loaded': (r) => r.status === 200,
  });
}

Parameterized Testing

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

const usernames = ['user1', 'user2', 'user3', 'user4', 'user5'];

export default function () {
  // Use shared array with VU-specific index
  const username = usernames[__VU % usernames.length];
  
  const res = http.get(`https://api.example.com/users/${username}`);
  
  check(res, {
    'user found': (r) => r.status === 200,
  });
}

Browser Testing (k6 Browser)

import { browser } from 'k6/browser';

export const options = {
  scenarios: {
    browser_test: {
      executor: 'constant-vus',
      vus: 5,
      duration: '30s',
      browser: {
        type: 'chromium',
      },
    },
  },
};

export default async function () {
  const page = await browser.newPage();
  
  try {
    await page.goto('https://example.com');
    
    const title = await page.title();
    console.log(`Page title: ${title}`);
    
    // Click and interact
    await page.click('button[data-testid="submit"]');
    
    // Wait for response
    await page.waitForSelector('.success-message');
    
  } finally {
    await page.close();
  }
}

Install browser support: k6 install chromium

WebSocket Testing

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

export default function () {
  const url = 'wss://echo.websocket.org';
  
  ws.connect(url, {}, function (socket) {
    socket.on('open', () => {
      console.log('WebSocket connected');
      socket.send('Hello WebSocket');
    });
    
    socket.on('message', (data) => {
      console.log(`Received: ${data}`);
      check(data, {
        'echo received': (d) => d.includes('Hello'),
      });
    });
    
    socket.on('close', () => {
      console.log('WebSocket closed');
    });
    
    // Send periodic messages
    socket.setInterval(function () {
      socket.send('ping');
    }, 1000);
    
    // Close after 5 seconds
    socket.setTimeout(function () {
      socket.close();
    }, 5000);
  });
}

Data Handling

CSV Data Source

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

// Option 1: Load once, shared across VUs
const users = new SharedArray('users', function () {
  return open('./users.csv').split('\n').slice(1).map(line => {
    const [email, password] = line.split(',');
    return { email, password };
  });
});

export default function () {
  const user = users[__VU % users.length];
  
  const res = http.post('https://api.example.com/login',
    JSON.stringify({ email: user.email, password: user.password })
  );
  
  check(res, { 'login successful': (r) => r.status === 200 });
}

JSON Data Source

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

const products = new SharedArray('products', function () {
  return JSON.parse(open('./products.json'));
});

export default function () {
  const product = products[Math.floor(Math.random() * products.length)];
  
  const res = http.get(`https://api.example.com/products/${product.id}`);
  
  check(res, { 'product found': (r) => r.status === 200 });
}

Thresholds & SLA

Basic Thresholds

export const options = {
  vus: 50,
  duration: '2m',
  
  thresholds: {
    // Response time thresholds
    http_req_duration: ['p(95)<500', 'p(99)<1000'],
    
    // Error rate threshold
    http_re