k6

k6 is a developer-centric, open-source load testing tool suitable for testing APIs, microservices, and websites. It is written in Go but you script tests in JavaScript.

When to Use

• API Load Testing: The gold standard for modern API performance testing.
• CI/CD Integration: Very lightweight binary (or Docker), easy to gate capabilities ("fail if p95 > 500ms").
• Developer Friendly: Uses JS (ES6) for scripting, so backend/frontend devs can write tests.

Quick Start

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

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

export default function () {
  const res = http.get("http://test.k6.io");
  check(res, {
    "status was 200": (r) => r.status == 200,
  });
  sleep(1);
}

Run with k6 run script.js.

Core Concepts

Virtual Users (VUs)

Simulated users that run your script in a loop. They are concurrent but not browser-based (unless you use xk6-browser), so they are CPU efficient.

Checks & Thresholds

• Check: Boolean assertion (like an assert). Doesn't fail the test, just reports pass/fail % at end.
• Threshold: Pass/Fail criteria for the CI pipeline.

export const options = {
  thresholds: {
    http_req_duration: ["p(95)<500"], // 95% of requests must complete below 500ms
  },
};

Best Practices (2025)

Do:

• Modularize: Split logic into folders. k6 supports ES modules (import { ... } from './utils.js').
• Use Scenarios: Mix different patterns (ramping up, constant arrival rate) in one test.
• Correlate specific data: Ensure you are not just hitting cache. Use dynamic data (random IDs).

Don't:

• Don't treat it like a browser: Standard k6 http does not parse HTML or execute JS on the page. It just hits endpoints. Use k6-browser module if you strictly need browser rendering (but it's heavier).

References

• k6 Documentation