K6 Load Testing

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.

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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

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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  r.timings.duration  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

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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 });
}

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Thresholds & SLA

Basic Thresholds

export const options = {
  vus: 50,
  duration: '2m',
  
  thresholds: {
    // Response time thresholds
    http_req_duration: ['p(95)100'],
  },
};

Advanced Thresholds

export const options = {
  thresholds: {
    // Multiple thresholds on same metric
    http_req_duration: [
      'p(90) 500ms |
| http_req_failed | Error rate |  1% |
| http_reqs | Requests/sec | Meeting target | Near limit | At limit |
| vus | Virtual users | Stable | Gradual increase | Unexpected spike |

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## Examples

### Example 1: Basic API Load Test

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

export const options = {
  vus: 50,
  duration: '2m',
  thresholds: {
    http_req_duration: ['p(95) r.status === 200,
    'response time  r.timings.duration  r.status === 200 });
}

---

Best Practices

• Start with smoke test: Verify test works with 1-5 VUs before scaling up
• Use realistic data: Parameterize with real user data and behaviors
• Set meaningful thresholds: Match your SLA and business requirements
• Warm up systems: Include ramp-up time in stages
• Monitor external dependencies: Track not just your APIs but downstream services
• Use tags: Tag requests for granular analysis (tags: { endpoint: 'users' })
• Keep tests focused: One test file per scenario for clarity

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Common Pitfalls

• Problem: Tests pass locally but fail in CI

Solution: Ensure CI environment has similar resources and network conditions

• Problem: Inconsistent results between runs

Solution: Check for external dependencies, random data, or test data pollution

• Problem: k6 runs out of memory

Solution: Use SharedArray for large data, reduce VUs, or use --max-memory flag

• Problem: Thresholds too strict

Solution: Start with relaxed thresholds, tighten based on historical data

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Related Skills

• @performance-engineer - For broader performance optimization
• @api-testing-observability-api-mock - For API mocking during testing
• @application-performance-performance-optimization - For performance optimization

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Additional Resources

• k6 Documentation
• k6 Examples
• k6 Load Testing Guides
• k6 Cloud

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: LiHongwei-cn
• Source: LiHongwei-cn/lihongwei-cn
• License: MIT
• Homepage: https://lihongwei-cn.github.io/lihongwei-cn/

Install and usage instructions live in the source repository linked above.