Load Testing: k6 vs Locust vs Gatling — Which Should You Pick?

Why Load Testing Is Non-Negotiable

Every production system needs load testing. Without it, you discover your performance limits when real users hit them — and that means downtime, slow responses, and lost revenue.

Load testing answers critical questions:

• How many concurrent users can your system handle?
• What is the response time at peak load?
• Where are the bottlenecks (database, API, memory, CPU)?
• Does your system degrade gracefully or crash suddenly?

k6: The Developer-Friendly Choice

k6 (by Grafana Labs) uses JavaScript for test scripts and Go for the engine. It is fast, developer-friendly, and integrates beautifully with CI/CD.

// load-test.js
import http from 'k6/http';
import { check, sleep } from 'k6';
import { Rate, Trend } from 'k6/metrics';

// Custom metrics
const errorRate = new Rate('errors');
const apiDuration = new Trend('api_duration');

export const options = {
  stages: [
    { duration: '1m', target: 50 },    // Ramp up to 50 users
    { duration: '3m', target: 50 },    // Stay at 50 users
    { duration: '1m', target: 200 },   // Ramp up to 200 users
    { duration: '3m', target: 200 },   // Stay at 200
    { duration: '1m', target: 0 },     // Ramp down
  ],
  thresholds: {
    http_req_duration: ['p(95)<500', 'p(99)<1000'],  // 95th < 500ms, 99th < 1s
    errors: ['rate<0.01'],                              // Error rate < 1%
    api_duration: ['avg<300'],                           // Average API < 300ms
  },
};

export default function () {
  // Simulate user browsing
  const homeRes = http.get('https://api.example.com/');
  check(homeRes, {
    'home status 200': (r) => r.status === 200,
    'home response < 500ms': (r) => r.timings.duration < 500,
  });

  sleep(1);

  // API call with authentication
  const loginRes = http.post('https://api.example.com/auth/login', JSON.stringify({
    email: '[email protected]',
    password: 'testpass',
  }), {
    headers: { 'Content-Type': 'application/json' },
  });

  const token = loginRes.json('token');
  errorRate.add(loginRes.status !== 200);

  // Authenticated API call
  const apiRes = http.get('https://api.example.com/api/dashboard', {
    headers: { Authorization: 'Bearer ' + token },
  });

  apiDuration.add(apiRes.timings.duration);
  check(apiRes, {
    'dashboard status 200': (r) => r.status === 200,
    'has data': (r) => r.json('data') !== null,
  });

  sleep(Math.random() * 3 + 1); // Random think time 1-4 seconds
}

# Run locally
k6 run load-test.js

# Run with Grafana Cloud output
k6 run --out cloud load-test.js

# Run with InfluxDB output
k6 run --out influxdb=http://localhost:8086/k6 load-test.js

Locust: The Python-Powered Option

Locust uses Python for test scripts, making it accessible to backend developers. It has a built-in web UI for real-time monitoring.

# locustfile.py
from locust import HttpUser, task, between, events
import json

class WebsiteUser(HttpUser):
    wait_time = between(1, 5)
    host = "https://api.example.com"

    def on_start(self):
        """Login when user starts"""
        response = self.client.post("/auth/login", json={
            "email": "[email protected]",
            "password": "testpass",
        })
        self.token = response.json()["token"]
        self.headers = {"Authorization": f"Bearer {self.token}"}

    @task(3)
    def view_dashboard(self):
        """Most common action - weighted 3x"""
        self.client.get("/api/dashboard", headers=self.headers)

    @task(2)
    def list_orders(self):
        """Second most common - weighted 2x"""
        self.client.get("/api/orders?page=1&limit=20", headers=self.headers)

    @task(1)
    def create_order(self):
        """Least common - weighted 1x"""
        self.client.post("/api/orders", json={
            "items": [{"product_id": 1, "quantity": 2}],
            "total": 49.99,
        }, headers=self.headers)


class AdminUser(HttpUser):
    wait_time = between(5, 15)
    weight = 1  # 1 admin per 10 regular users

    def on_start(self):
        response = self.client.post("/auth/login", json={
            "email": "[email protected]",
            "password": "adminpass",
        })
        self.token = response.json()["token"]

    @task
    def view_analytics(self):
        self.client.get("/api/admin/analytics",
                       headers={"Authorization": f"Bearer {self.token}"})

# Run with web UI
locust -f locustfile.py --host=https://api.example.com

# Run headless (CI mode)
locust -f locustfile.py --headless \
  --users 200 --spawn-rate 10 \
  --run-time 5m \
  --host=https://api.example.com \
  --csv=results

k6 in Gitea Actions:

name: Load Test
on:
  push:
    branches: [main]

jobs:
  load-test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Install k6
        run: |
          curl -s https://dl.k6.io/key.gpg | sudo apt-key add -
          echo "deb 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

      - name: Run load test
        run: k6 run --out json=results.json tests/load-test.js

      - name: Check thresholds
        run: |
          if k6 run tests/load-test.js --quiet; then
            echo "Load test passed"
          else
            echo "Load test FAILED - thresholds exceeded"
            exit 1
          fi

Our Recommendation

Choose k6 when: Your team knows JavaScript, you want CI/CD integration, you need high performance from a single machine, or you use Grafana for monitoring.

Choose Locust when: Your team knows Python, you want a built-in web UI for manual testing, you need distributed testing without Kubernetes, or you want the simplest setup.

Choose Gatling when: Your team uses Scala/Java, you need beautiful HTML reports for stakeholders, or you are in an enterprise environment with JVM infrastructure.

At TechSaaS, we use k6 for all our load testing. The JavaScript scripting is familiar to our full-stack team, the Go engine handles high concurrency on a single machine, and the Grafana integration gives us beautiful dashboards. For a quick smoke test, we run k6 in CI after every deployment to ensure we have not introduced performance regressions.