K6 Testing Snippets

What is K6?

K6 is a modern, developer-friendly load testing tool built for testing the performance of APIs, microservices, and websites. Write tests in JavaScript and run them from the CLI or CI/CD pipeline.

---

📮 Testing - POST API Load Test

Load test a POST endpoint with JSON payload and performance thresholds.

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

export const options = {
  stages: [
    { duration: '1m', target: 50 },   // Ramp up to 50 users
    { duration: '3m', target: 50 },   // Stay at 50 users
    { duration: '1m', target: 0 },    // Ramp down
  ],
  thresholds: {
    http_req_duration: ['p(95)<500'], // 95% of requests under 500ms
    http_req_failed: ['rate<0.01'],   // Less than 1% errors
  },
};

export default function () {
  const url = 'https://api.example.com/users';
  const payload = JSON.stringify({
    name: 'John Doe',
    email: 'john@example.com',
    role: 'developer',
  });

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

  const res = http.post(url, payload, params);

  check(res, {
    'status is 201': (r) => r.status === 201,
    'response time < 500ms': (r) => r.timings.duration < 500,
    'has user id': (r) => JSON.parse(r.body).id !== undefined,
  });

  sleep(1);
}

Expected Output:

$ k6 run post-load-test.js

     ✓ status is 201
     ✓ response time < 500ms
     ✓ has user id

     checks.........................: 100.00% ✓ 9000    ✗ 0
     data_received..................: 2.1 MB  42 kB/s
     data_sent......................: 540 kB  11 kB/s
     http_req_blocked...............: avg=1.2ms    min=0s     med=1ms    max=45ms   p(90)=2ms    p(95)=3ms
     http_req_connecting............: avg=1ms      min=0s     med=1ms    max=42ms   p(90)=2ms    p(95)=2.5ms
   ✓ http_req_duration..............: avg=245ms    min=120ms  med=230ms  max=480ms  p(90)=350ms  p(95)=420ms
     http_req_failed................: 0.00%   ✓ 0       ✗ 3000
     http_req_receiving.............: avg=0.5ms    min=0.1ms  med=0.4ms  max=5ms    p(90)=1ms    p(95)=1.5ms
     http_req_sending...............: avg=0.3ms    min=0.1ms  med=0.2ms  max=3ms    p(90)=0.5ms  p(95)=0.8ms
     http_req_tls_handshaking.......: avg=0ms      min=0s     med=0s     max=0s     p(90)=0s     p(95)=0s
     http_req_waiting...............: avg=244ms    min=119ms  med=229ms  max=479ms  p(90)=349ms  p(95)=419ms
     http_reqs......................: 3000    60/s
     iteration_duration.............: avg=1.24s    min=1.12s  med=1.23s  max=1.48s  p(90)=1.35s  p(95)=1.42s
     iterations.....................: 3000    60/s
     vus............................: 50      min=0     max=50
     vus_max........................: 50      min=50    max=50

✓ Thresholds passed

When to use:

• Testing user registration endpoints
• Load testing POST APIs
• Validating API performance under load
• Setting performance baselines

---

🔍 Testing - GET API Stress Test

Stress test a GET endpoint to find breaking points.

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

export const options = {
  stages: [
    { duration: '2m', target: 100 },  // Ramp up to 100 users
    { duration: '5m', target: 100 },  // Stay at 100 users
    { duration: '2m', target: 200 },  // Ramp up to 200 users
    { duration: '5m', target: 200 },  // Stay at 200 users
    { duration: '2m', target: 0 },    // Ramp down
  ],
  thresholds: {
    http_req_duration: ['p(99)<1000', 'p(95)<800', 'p(50)<400'],
    http_req_failed: ['rate<0.05'],
  },
};

export default function () {
  const url = 'https://api.example.com/products?page=1&limit=20';

  const params = {
    headers: {
      'Accept': 'application/json',
      'Authorization': 'Bearer YOUR_TOKEN_HERE',
    },
  };

  const res = http.get(url, params);

  check(res, {
    'status is 200': (r) => r.status === 200,
    'has products array': (r) => {
      const body = JSON.parse(r.body);
      return Array.isArray(body.products);
    },
    'response size ok': (r) => r.body.length < 100000,
  });

  sleep(0.5);
}

Expected Output:

$ k6 run get-stress-test.js

     ✓ status is 200
     ✓ has products array
     ✓ response size ok

     checks.........................: 100.00% ✓ 24000   ✗ 0
     data_received..................: 48 MB   60 kB/s
     data_sent......................: 3.2 MB  4.0 kB/s
     http_req_blocked...............: avg=0.8ms    min=0s     med=0.5ms  max=35ms   p(90)=1.5ms  p(95)=2ms
     http_req_connecting............: avg=0.7ms    min=0s     med=0.5ms  max=32ms   p(90)=1.2ms  p(95)=1.8ms
   ✓ http_req_duration..............: avg=320ms    min=95ms   med=285ms  max=980ms  p(90)=520ms  p(95)=680ms  p(99)=890ms
     http_req_failed................: 0.00%   ✓ 0       ✗ 8000
     http_req_receiving.............: avg=2ms      min=0.2ms  med=1.5ms  max=25ms   p(90)=4ms    p(95)=6ms
     http_req_sending...............: avg=0.2ms    min=0.1ms  med=0.2ms  max=2ms    p(90)=0.4ms  p(95)=0.6ms
     http_req_waiting...............: avg=318ms    min=94ms   med=283ms  max=978ms  p(90)=518ms  p(95)=678ms
     http_reqs......................: 8000    100/s
     iteration_duration.............: avg=820ms    min=595ms  med=785ms  max=1.48s  p(90)=1.02s  p(95)=1.18s
     iterations.....................: 8000    100/s
     vus............................: 200     min=0     max=200
     vus_max........................: 200     min=200   max=200

✓ All thresholds passed

When to use:

• Finding system capacity limits
• Stress testing read-heavy APIs
• Performance regression testing
• Capacity planning

---

⚡ Testing - Spike Test (Sudden Traffic)

Test how your API handles sudden traffic spikes.

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

export const options = {
  stages: [
    { duration: '10s', target: 10 },   // Normal load
    { duration: '30s', target: 500 },  // Sudden spike!
    { duration: '1m', target: 500 },   // Sustained spike
    { duration: '10s', target: 10 },   // Recovery
  ],
  thresholds: {
    http_req_duration: ['p(95)<2000'], // More lenient during spikes
    http_req_failed: ['rate<0.1'],     // Allow 10% errors during spike
  },
};

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

  check(res, {
    'status is 200': (r) => r.status === 200,
    'response time OK': (r) => r.timings.duration < 3000,
  });

  sleep(Math.random() * 2); // Random think time
}

Expected Output:

$ k6 run spike-test.js

     ✓ status is 200
     ✗ response time OK
       ↳  92% — ✓ 4140 / ✗ 360

     checks.........................: 96.00%  ✓ 8640    ✗ 360
     data_received..................: 1.8 MB  18 kB/s
     data_sent......................: 450 kB  4.5 kB/s
     http_req_blocked...............: avg=2.5ms    min=0s     med=1ms    max=120ms  p(90)=5ms    p(95)=8ms
     http_req_connecting............: avg=2ms      min=0s     med=1ms    max=115ms  p(90)=4ms    p(95)=7ms
   ✓ http_req_duration..............: avg=850ms    min=180ms  med=650ms  max=2800ms p(90)=1500ms p(95)=1850ms
     http_req_failed................: 8.00%   ✓ 360     ✗ 4140
     http_req_receiving.............: avg=1.2ms    min=0.1ms  med=0.8ms  max=45ms   p(90)=2.5ms  p(95)=4ms
     http_req_sending...............: avg=0.4ms    min=0.1ms  med=0.3ms  max=8ms    p(90)=0.8ms  p(95)=1.2ms
     http_req_waiting...............: avg=848ms    min=179ms  med=649ms  max=2798ms p(90)=1498ms p(95)=1848ms
     http_reqs......................: 4500    45/s
     iteration_duration.............: avg=1.85s    min=1.18s  med=1.65s  max=4.8s   p(90)=2.5s   p(95)=3.2s
     iterations.....................: 4500    45/s
     vus............................: 10      min=10    max=500
     vus_max........................: 500     min=500   max=500

✓ Thresholds passed (degraded performance during spike expected)

When to use:

• Testing auto-scaling behavior
• Validating rate limiting
• Black Friday / traffic surge preparation
• DDoS resilience testing

---

🔐 Testing - Authentication Flow

Test authenticated POST requests with token refresh.

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

export const options = {
  vus: 20,
  duration: '2m',
  thresholds: {
    http_req_duration: ['p(95)<600'],
  },
};

// Login once per VU
export function setup() {
  const loginRes = http.post('https://api.example.com/auth/login', JSON.stringify({
    username: 'testuser',
    password: 'testpass123',
  }), {
    headers: { 'Content-Type': 'application/json' },
  });

  return { token: JSON.parse(loginRes.body).token };
}

export default function (data) {
  const params = {
    headers: {
      'Content-Type': 'application/json',
      'Authorization': `Bearer ${data.token}`,
    },
  };

  // Create a post
  const payload = JSON.stringify({
    title: 'Test Post',
    content: 'This is a test post content',
  });

  const res = http.post('https://api.example.com/posts', payload, params);

  check(res, {
    'post created': (r) => r.status === 201,
    'has post id': (r) => JSON.parse(r.body).id !== undefined,
  });

  sleep(2);
}

Expected Output:

$ k6 run auth-flow-test.js

     ✓ post created
     ✓ has post id

     checks.........................: 100.00% ✓ 2400    ✗ 0
     data_received..................: 960 kB  8.0 kB/s
     data_sent......................: 384 kB  3.2 kB/s
     http_req_blocked...............: avg=0.6ms    min=0s     med=0.5ms  max=12ms   p(90)=1ms    p(95)=1.5ms
     http_req_connecting............: avg=0.5ms    min=0s     med=0.4ms  max=10ms   p(90)=0.9ms  p(95)=1.2ms
   ✓ http_req_duration..............: avg=285ms    min=145ms  med=270ms  max=580ms  p(90)=390ms  p(95)=450ms
     http_req_failed................: 0.00%   ✓ 0       ✗ 1200
     http_req_receiving.............: avg=0.8ms    min=0.2ms  med=0.6ms  max=8ms    p(90)=1.5ms  p(95)=2ms
     http_req_sending...............: avg=0.3ms    min=0.1ms  med=0.2ms  max=4ms    p(90)=0.5ms  p(95)=0.8ms
     http_req_waiting...............: avg=284ms    min=144ms  med=269ms  max=579ms  p(90)=389ms  p(95)=449ms
     http_reqs......................: 1200    10/s
     iteration_duration.............: avg=2.28s    min=2.14s  med=2.27s  max=2.58s  p(90)=2.39s  p(95)=2.45s
     iterations.....................: 1200    10/s
     vus............................: 20      min=20    max=20
     vus_max........................: 20      min=20    max=20

✓ All thresholds passed

When to use:

• Testing login/signup flows
• Validating JWT token handling
• Session-based API testing
• Auth service load testing

---

🔄 Testing - Multi-Endpoint Scenario

Test realistic user journey across multiple endpoints.

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

export const options = {
  stages: [
    { duration: '1m', target: 30 },
    { duration: '3m', target: 30 },
    { duration: '1m', target: 0 },
  ],
  thresholds: {
    'group_duration{group:::User Journey}': ['p(95)<5000'],
    http_req_duration: ['p(95)<500'],
  },
};

export default function () {
  group('User Journey', function () {
    // 1. Browse products
    let res = http.get('https://api.example.com/products');
    check(res, { 'products loaded': (r) => r.status === 200 });
    sleep(1);

    // 2. View product details
    res = http.get('https://api.example.com/products/123');
    check(res, { 'product details loaded': (r) => r.status === 200 });
    sleep(2);

    // 3. Add to cart
    const cartPayload = JSON.stringify({ productId: 123, quantity: 1 });
    res = http.post('https://api.example.com/cart', cartPayload, {
      headers: { 'Content-Type': 'application/json' },
    });
    check(res, { 'added to cart': (r) => r.status === 201 });
    sleep(1);

    // 4. Checkout
    const checkoutPayload = JSON.stringify({
      paymentMethod: 'card',
      address: '123 Main St',
    });
    res = http.post('https://api.example.com/checkout', checkoutPayload, {
      headers: { 'Content-Type': 'application/json' },
    });
    check(res, { 'checkout success': (r) => r.status === 200 });
  });

  sleep(3);
}

Expected Output:

$ k6 run multi-endpoint-test.js

     ✓ products loaded
     ✓ product details loaded
     ✓ added to cart
     ✓ checkout success

     checks.........................: 100.00% ✓ 4800    ✗ 0
     data_received..................: 9.6 MB  32 kB/s
     data_sent......................: 1.2 MB  4.0 kB/s
     group_duration.................: avg=2.8s     min=2.1s   med=2.7s   max=4.5s   p(90)=3.5s   p(95)=3.9s
   ✓ group_duration{group:::User Journey}: avg=2.8s     min=2.1s   med=2.7s   max=4.5s   p(90)=3.5s   p(95)=3.9s
     http_req_blocked...............: avg=0.7ms    min=0s     med=0.5ms  max=18ms   p(90)=1.2ms  p(95)=1.8ms
     http_req_connecting............: avg=0.6ms    min=0s     med=0.4ms  max=15ms   p(90)=1ms    p(95)=1.5ms
   ✓ http_req_duration..............: avg=298ms    min=125ms  med=280ms  max=485ms  p(90)=410ms  p(95)=450ms
     http_req_failed................: 0.00%   ✓ 0       ✗ 4800
     http_req_receiving.............: avg=1.2ms    min=0.2ms  med=0.9ms  max=12ms   p(90)=2.5ms  p(95)=3.5ms
     http_req_sending...............: avg=0.4ms    min=0.1ms  med=0.3ms  max=5ms    p(90)=0.7ms  p(95)=1ms
     http_req_waiting...............: avg=296ms    min=124ms  med=278ms  max=483ms  p(90)=408ms  p(95)=448ms
     http_reqs......................: 4800    16/s
     iteration_duration.............: avg=9.8s     min=9.1s   med=9.7s   max=11.5s  p(90)=10.5s  p(95)=10.9s
     iterations.....................: 1200    4/s
     vus............................: 30      min=0     max=30
     vus_max........................: 30      min=30    max=30

✓ All thresholds passed

When to use:

• E-commerce flow testing
• User journey validation
• Complex workflow testing
• Integration testing under load

---

⏱️ Testing - Rate Limiting Validation

Test API rate limiting behavior.

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

const rateLimitHit = new Rate('rate_limit_hit');

export const options = {
  scenarios: {
    rate_limit_test: {
      executor: 'constant-arrival-rate',
      rate: 100,           // 100 requests per timeUnit
      timeUnit: '1s',
      duration: '1m',
      preAllocatedVUs: 50,
      maxVUs: 100,
    },
  },
};

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

  check(res, {
    'success': (r) => r.status === 200,
    'rate limited': (r) => r.status === 429,
  });

  // Track rate limiting
  rateLimitHit.add(res.status === 429);

  if (res.status === 429) {
    const retryAfter = res.headers['Retry-After'] || 1;
    console.log(`Rate limited. Retry after ${retryAfter}s`);
  }

  sleep(0.1);
}

Expected Output:

$ k6 run rate-limit-test.js

INFO[0015] Rate limited. Retry after 60s                source=console
INFO[0018] Rate limited. Retry after 60s                source=console
INFO[0022] Rate limited. Retry after 60s                source=console

     ✓ success
     ✓ rate limited

     checks.........................: 100.00% ✓ 12000   ✗ 0
     data_received..................: 2.4 MB  40 kB/s
     data_sent......................: 1.2 MB  20 kB/s
     http_req_blocked...............: avg=0.5ms    min=0s     med=0.4ms  max=15ms   p(90)=1ms    p(95)=1.5ms
     http_req_connecting............: avg=0.4ms    min=0s     med=0.3ms  max=12ms   p(90)=0.8ms  p(95)=1.2ms
     http_req_duration..............: avg=45ms     min=25ms   med=42ms   max=250ms  p(90)=68ms   p(95)=85ms
     http_req_failed................: 15.00%  ✓ 900     ✗ 5100
     http_req_receiving.............: avg=0.3ms    min=0.1ms  med=0.2ms  max=3ms    p(90)=0.5ms  p(95)=0.8ms
     http_req_sending...............: avg=0.2ms    min=0.1ms  med=0.1ms  max=2ms    p(90)=0.3ms  p(95)=0.4ms
     http_req_waiting...............: avg=44ms     min=24ms   med=41ms   max=248ms  p(90)=67ms   p(95)=84ms
     http_reqs......................: 6000    100/s
     iteration_duration.............: avg=145ms    min=125ms  med=142ms  max=350ms  p(90)=168ms  p(95)=185ms
     iterations.....................: 6000    100/s
     rate_limit_hit.................: 15.00%  ✓ 900     ✗ 5100
     vus............................: 50      min=50    max=100
     vus_max........................: 100     min=100   max=100

Rate limiting triggered at ~85 req/s (expected: 100/s)

When to use:

• Validating rate limit configuration
• Testing throttling mechanisms
• API quota testing
• DDoS protection validation

---

🔮 Testing - GraphQL Query Load Test

Load test GraphQL endpoints with complex queries.

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

export const options = {
  vus: 30,
  duration: '2m',
  thresholds: {
    http_req_duration: ['p(95)<800'],
  },
};

export default function () {
  const query = `
    query GetUserProfile($userId: ID!) {
      user(id: $userId) {
        id
        name
        email
        posts(limit: 10) {
          id
          title
          comments {
            id
            text
            author {
              name
            }
          }
        }
      }
    }
  `;

  const variables = {
    userId: Math.floor(Math.random() * 1000) + 1,
  };

  const payload = JSON.stringify({ query, variables });

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

  const res = http.post('https://api.example.com/graphql', payload, params);

  check(res, {
    'query successful': (r) => r.status === 200,
    'no errors': (r) => !JSON.parse(r.body).errors,
    'has user data': (r) => JSON.parse(r.body).data.user !== null,
  });

  sleep(1);
}

Expected Output:

$ k6 run graphql-test.js

     ✓ query successful
     ✓ no errors
     ✓ has user data

     checks.........................: 100.00% ✓ 3600    ✗ 0
     data_received..................: 7.2 MB  60 kB/s
     data_sent......................: 1.8 MB  15 kB/s
     http_req_blocked...............: avg=0.9ms    min=0s     med=0.6ms  max=22ms   p(90)=1.5ms  p(95)=2.2ms
     http_req_connecting............: avg=0.7ms    min=0s     med=0.5ms  max=18ms   p(90)=1.2ms  p(95)=1.8ms
   ✓ http_req_duration..............: avg=425ms    min=220ms  med=395ms  max=780ms  p(90)=590ms  p(95)=680ms
     http_req_failed................: 0.00%   ✓ 0       ✗ 1200
     http_req_receiving.............: avg=2.5ms    min=0.5ms  med=2ms    max=18ms   p(90)=4.5ms  p(95)=6ms
     http_req_sending...............: avg=0.5ms    min=0.1ms  med=0.4ms  max=6ms    p(90)=0.9ms  p(95)=1.3ms
     http_req_waiting...............: avg=422ms    min=218ms  med=393ms  max=778ms  p(90)=588ms  p(95)=678ms
     http_reqs......................: 1200    10/s
     iteration_duration.............: avg=1.42s    min=1.22s  med=1.39s  max=1.78s  p(90)=1.59s  p(95)=1.68s
     iterations.....................: 1200    10/s
     vus............................: 30      min=30    max=30
     vus_max........................: 30      min=30    max=30

✓ All thresholds passed

When to use:

• GraphQL API load testing
• Complex nested query testing
• Resolver performance testing
• N+1 query problem detection

---

📤 Testing - File Upload Performance

Test file upload endpoints with multipart data.

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

const file = open('./test-file.pdf', 'b'); // Binary file

export const options = {
  vus: 10,
  duration: '1m',
  thresholds: {
    http_req_duration: ['p(95)<3000'], // Uploads can be slower
  },
};

export default function () {
  const formData = {
    file: http.file(file, 'test-file.pdf', 'application/pdf'),
    description: 'Test file upload',
    category: 'documents',
  };

  const res = http.post('https://api.example.com/upload', formData, {
    headers: {
      'Authorization': 'Bearer YOUR_TOKEN_HERE',
    },
  });

  check(res, {
    'upload successful': (r) => r.status === 201,
    'has file id': (r) => JSON.parse(r.body).fileId !== undefined,
    'upload time reasonable': (r) => r.timings.duration < 5000,
  });

  sleep(2);
}

Expected Output:

$ k6 run file-upload-test.js

     ✓ upload successful
     ✓ has file id
     ✓ upload time reasonable

     checks.........................: 100.00% ✓ 900     ✗ 0
     data_received..................: 180 kB  3.0 kB/s
     data_sent......................: 30 MB   500 kB/s
     http_req_blocked...............: avg=1.5ms    min=0s      med=1ms    max=45ms   p(90)=3ms    p(95)=5ms
     http_req_connecting............: avg=1.2ms    min=0s      med=0.8ms  max=38ms   p(90)=2.5ms  p(95)=4ms
   ✓ http_req_duration..............: avg=1.85s    min=850ms   med=1.72s  max=2.95s  p(90)=2.45s  p(95)=2.68s
     http_req_failed................: 0.00%   ✓ 0       ✗ 300
     http_req_receiving.............: avg=0.8ms    min=0.2ms   med=0.6ms  max=8ms    p(90)=1.5ms  p(95)=2.2ms
     http_req_sending...............: avg=125ms    min=45ms    med=118ms  max=350ms  p(90)=188ms  p(95)=225ms
     http_req_waiting...............: avg=1.72s    min=795ms   med=1.59s  max=2.82s  p(90)=2.32s  p(95)=2.55s
     http_reqs......................: 300     5/s
     iteration_duration.............: avg=3.85s    min=2.85s   med=3.72s  max=4.95s  p(90)=4.45s  p(95)=4.68s
     iterations.....................: 300     5/s
     vus............................: 10      min=10    max=10
     vus_max........................: 10      min=10    max=10

✓ All thresholds passed

When to use:

• Testing document upload endpoints
• Image upload performance
• Large file handling
• Multipart form data testing

---

📊 Performance Metrics Guide

Understanding K6 output metrics:

Response Time Percentiles:

• P50 (Median): 50% of requests faster than this
• P90: 90% of requests faster than this
• P95: 95% of requests faster than this - common SLA target
• P99: 99% of requests faster than this - tail latency

Request Timing Breakdown:

• http_req_duration: Total request time (waiting + receiving)
• http_req_waiting: Time to first byte (TTFB)
• http_req_receiving: Time to download response
• http_req_sending: Time to send request
• http_req_blocked: Time waiting for connection
• http_req_connecting: TCP connection time

Success Metrics:

• checks: Assertion pass rate
• http_req_failed: HTTP error rate
• http_reqs: Total requests per second

When to use different thresholds:

• API endpoints: p(95) < 500ms
• Database queries: p(95) < 200ms
• File uploads: p(95) < 3000ms
• Complex reports: p(95) < 2000ms

---