k6 Performance Testing

Docs: https://grafana.com/docs/k6/latest/

Quick Start

# Install
brew install k6                          # macOS
sudo apt install k6                      # Ubuntu/Debian
choco install k6                         # Windows

# Run a test
k6 run script.js
k6 run --vus 10 --duration 30s script.js
k6 run --out json=results.json script.js

Basic Script Structure

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

export const options = {
  vus: 10,
  duration: '30s',
  thresholds: {
    http_req_duration: ['p(95)<500'],   // 95% of requests under 500ms
    http_req_failed: ['rate<0.01'],     // error rate under 1%
  },
};

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

Test Lifecycle

export function setup() {
  // Runs once before all VUs — return data shared with VUs
  const token = getAuthToken();
  return { token };
}

export default function (data) {
  // Runs for each VU iteration — receives setup() return value
  http.get('https://api.example.com', {
    headers: { Authorization: `Bearer ${data.token}` },
  });
}

export function teardown(data) {
  // Runs once after all VUs finish
  console.log('Test complete');
}

HTTP Requests

import http from 'k6/http';

// GET
const res = http.get('https://api.example.com/users');

// POST with JSON body
const payload = JSON.stringify({ name: 'test', value: 42 });
const params = { headers: { 'Content-Type': 'application/json' } };
const res = http.post('https://api.example.com/users', payload, params);

// PUT / PATCH / DELETE
http.put(url, body, params);
http.patch(url, body, params);
http.del(url, null, params);

// Batch requests (parallel)
const responses = http.batch([
  ['GET', 'https://api.example.com/users'],
  ['GET', 'https://api.example.com/posts'],
]);

// With auth
const res = http.get(url, {
  headers: { Authorization: 'Bearer token123' },
});

// Form data
const res = http.post(url, { username: 'user', password: 'pass' });

// File upload
const binFile = open('./image.png', 'b');
const res = http.post(url, {
  file: http.file(binFile, 'image.png', 'image/png'),
  field: 'value',
});

Checks and Thresholds

import { check } from 'k6';

// Checks (soft assertions — don't stop test on failure)
const res = http.get(url);
check(res, {
  'status 200': (r) => r.status === 200,
  'body contains ID': (r) => r.body.includes('"id"'),
  'duration < 500ms': (r) => r.timings.duration < 500,
});

// Thresholds (hard pass/fail criteria)
export const options = {
  thresholds: {
    // HTTP metrics
    http_req_duration: ['p(90)<400', 'p(95)<800', 'avg<200'],
    http_req_failed: ['rate<0.05'],

    // Custom metric thresholds
    my_errors: ['count<10'],

    // Tag-based thresholds (specific endpoints)
    'http_req_duration{name:homepage}': ['p(95)<500'],

    // Abort test if threshold breached
    http_req_duration: [{ threshold: 'p(99)<3000', abortOnFail: true }],
  },
};

Custom Metrics

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

const myErrors = new Counter('my_errors');
const activeUsers = new Gauge('active_users');
const successRate = new Rate('success_rate');
const reqDuration = new Trend('req_duration', true); // true = display in ms

export default function () {
  const res = http.get(url);
  if (res.status !== 200) myErrors.add(1);
  successRate.add(res.status === 200);
  reqDuration.add(res.timings.duration);
}

Scenarios and Executors

See references/scenarios-executors.md for full reference.

export const options = {
  scenarios: {
    // Constant VUs
    steady_load: {
      executor: 'constant-vus',
      vus: 50,
      duration: '5m',
    },
    // Ramping VUs (most common)
    ramp_up: {
      executor: 'ramping-vus',
      startVUs: 0,
      stages: [
        { duration: '2m', target: 100 },
        { duration: '5m', target: 100 },
        { duration: '2m', target: 0 },
      ],
    },
    // Constant arrival rate (requests per second)
    constant_rps: {
      executor: 'constant-arrival-rate',
      rate: 100,             // 100 iterations/sec
      timeUnit: '1s',
      duration: '5m',
      preAllocatedVUs: 50,
      maxVUs: 200,
    },
    // Per-VU iterations
    per_vu: {
      executor: 'per-vu-iterations',
      vus: 10,
      iterations: 100,
    },
  },
};

Test Types

See references/test-types.md for full examples.

Type	Purpose	Load Pattern
Smoke	Verify script works, minimal load	1-2 VUs, short duration
Load	Normal expected load	Ramp up → steady → ramp down
Stress	Beyond normal capacity	Increasing stages until breaking
Spike	Sudden traffic surge	Instant spike then drop
Soak	Extended duration for memory leaks	Moderate load, hours-long
Breakpoint	Find system limits	Continuously increasing load

Environment Variables and CLI Flags

# Pass variables to script
k6 run -e MY_VAR=value script.js     # access as __ENV.MY_VAR
k6 run --env API_URL=https://api.example.com script.js

# Override options
k6 run --vus 50 --duration 60s script.js
k6 run --iterations 1000 script.js
k6 run --stage 30s:10,1m:50,30s:0 script.js   # ramping stages

# Output formats
k6 run --out json=result.json script.js
k6 run --out csv=result.csv script.js
k6 run --out influxdb=http://localhost:8086/k6 script.js
k6 run --out statsd script.js

Groups and Tags

import { group } from 'k6';

export default function () {
  group('homepage', () => {
    const res = http.get('/');
    check(res, { 'status 200': (r) => r.status === 200 });
  });

  group('login flow', () => {
    group('load login page', () => {
      http.get('/login');
    });
    group('submit credentials', () => {
      http.post('/login', { user: 'test', pass: 'test' });
    });
  });
}

// Custom tags on requests
http.get(url, { tags: { name: 'homepage', version: 'v2' } });

WebSocket

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

export default function () {
  const url = 'ws://echo.websocket.org';
  const res = ws.connect(url, {}, function (socket) {
    socket.on('open', () => {
      socket.send('Hello!');
      socket.setInterval(() => socket.ping(), 10000);
    });
    socket.on('message', (data) => {
      check(data, { 'message received': (d) => d.length > 0 });
    });
    socket.on('error', (e) => console.error(e.error()));
    socket.setTimeout(() => socket.close(), 30000);
  });
  check(res, { 'status 101': (r) => r.status === 101 });
}

Browser Testing

import { browser } from 'k6/browser';
import { check } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';

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 Promise.all([
      page.waitForNavigation(),
      page.locator('input[type="submit"]').click(),
    ]);
    check(page, { 'header': p => p.locator('h2').textContent() == 'Welcome, admin!' });
  } finally {
    await page.close();
  }
}

Modules and Imports

// Built-in modules
import http from 'k6/http';
import { check, group, sleep, fail } from 'k6';
import { Counter, Gauge, Rate, Trend } from 'k6/metrics';
import { SharedArray } from 'k6/data';
import { scenario, vu } from 'k6/execution';
import ws from 'k6/ws';
import grpc from 'k6/net/grpc';
import { browser } from 'k6/browser';
import crypto from 'k6/crypto';
import encoding from 'k6/encoding';
import { htmlReport } from 'https://raw.githubusercontent.com/benc-uk/k6-reporter/main/dist/bundle.js';

// jslib utilities
import { uuidv4 } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';
import { randomItem, randomIntBetween } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';

SharedArray (efficient data loading)

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

// Load CSV once, shared across all VUs
const users = new SharedArray('users', function () {
  return papaparse.parse(open('./users.csv'), { header: true }).data;
});

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

export default function () {
  const user = users[Math.floor(Math.random() * users.length)];
  http.post('/login', { username: user.username, password: user.password });
}

k6 Cloud (Grafana Cloud k6)

export const options = {
  cloud: {
    projectID: 123456,
    name: 'My Load Test',
    note: 'Release candidate',
    distribution: {
      distributionLabel1: { loadZone: 'amazon:us:ashburn', percent: 50 },
      distributionLabel2: { loadZone: 'amazon:eu:dublin', percent: 50 },
    },
  },
};

# Authenticate
k6 cloud login --token <your-token>

# Run in cloud
k6 cloud script.js

# Run locally but stream results to cloud
k6 run --out cloud script.js

CLI Flags Summary

Flag	Description
`--vus N`	Number of virtual users
`--duration Xs/Xm/Xh`	Test duration
`--iterations N`	Total iterations
`--stage Xm:N`	Add ramp stage
`--env KEY=value`	Environment variable
`--out FORMAT`	Output destination
`--quiet`	Suppress progress output
`--no-summary`	Skip end-of-test summary
`--summary-export FILE`	Export summary as JSON
`--http-debug`	Print HTTP request/response details
`--insecure-skip-tls-verify`	Skip TLS verification
`--tag KEY=VALUE`	Add test-wide tag

Key Built-in Metrics

Metric	Type	Description
`http_req_duration`	Trend	Total request time
`http_req_failed`	Rate	Failed request rate
`http_req_waiting`	Trend	Time to first byte (TTFB)
`http_req_connecting`	Trend	TCP connection time
`http_req_tls_handshaking`	Trend	TLS handshake time
`http_reqs`	Counter	Total HTTP requests
`vus`	Gauge	Current active VUs
`vus_max`	Gauge	Max VUs allocated
`iterations`	Counter	Total VU iterations
`iteration_duration`	Trend	Time to complete one iteration
`data_sent`	Counter	Data sent
`data_received`	Counter	Data received
`checks`	Rate	Check success rate

k6

k6 Performance Testing

Quick Start

Basic Script Structure

Test Lifecycle

HTTP Requests

Checks and Thresholds

Custom Metrics

Scenarios and Executors

Test Types

Environment Variables and CLI Flags

Groups and Tags

WebSocket

Browser Testing

Modules and Imports

SharedArray (efficient data loading)

k6 Cloud (Grafana Cloud k6)

CLI Flags Summary

Key Built-in Metrics

References

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