Load Testing: k6 vs Locust vs Gatling — Which Should You Pick?
Compare k6, Locust, and Gatling for load testing. Scripting languages, distributed testing, CI/CD integration, and real-world benchmarking strategies explained.
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.
<div style="margin:2.5rem auto;max-width:600px;width:100%;text-align:center;"><svg viewBox="0 0 600 180" xmlns="http://www.w3.org/2000/svg" style="width:100%;height:auto;"><rect width="600" height="180" rx="12" fill="#1a1a2e"/><rect x="30" y="55" width="90" height="50" rx="8" fill="#6366f1" opacity="0.9"/><text x="75" y="85" text-anchor="middle" fill="#ffffff" font-size="12" font-family="system-ui">Code</text><rect x="150" y="55" width="90" height="50" rx="8" fill="#3b82f6" opacity="0.9"/><text x="195" y="85" text-anchor="middle" fill="#ffffff" font-size="12" font-family="system-ui">Build</text><rect x="270" y="55" width="90" height="50" rx="8" fill="#a855f7" opacity="0.9"/><text x="315" y="85" text-anchor="middle" fill="#ffffff" font-size="12" font-family="system-ui">Test</text><rect x="390" y="55" width="90" height="50" rx="8" fill="#2dd4bf" opacity="0.9"/><text x="435" y="85" text-anchor="middle" fill="#1a1a2e" font-size="12" font-family="system-ui">Deploy</text><rect x="510" y="55" width="60" height="50" rx="8" fill="#f59e0b" opacity="0.9"/><text x="540" y="85" text-anchor="middle" fill="#1a1a2e" font-size="12" font-family="system-ui">Live</text><path d="M122,80 L148,80" stroke="#e2e8f0" stroke-width="2" marker-end="url(#arrow1)"/><path d="M242,80 L268,80" stroke="#e2e8f0" stroke-width="2" marker-end="url(#arrow1)"/><path d="M362,80 L388,80" stroke="#e2e8f0" stroke-width="2" marker-end="url(#arrow1)"/><path d="M482,80 L508,80" stroke="#e2e8f0" stroke-width="2" marker-end="url(#arrow1)"/><defs><marker id="arrow1" markerWidth="8" markerHeight="6" refX="8" refY="3" orient="auto"><path d="M0,0 L8,3 L0,6" fill="#e2e8f0"/></marker></defs><text x="300" y="145" text-anchor="middle" fill="#94a3b8" font-size="11" font-family="system-ui">Continuous Integration / Continuous Deployment Pipeline</text></svg><p style="margin-top:0.75rem;font-size:0.85rem;color:#94a3b8;font-style:italic;line-height:1.4;">A typical CI/CD pipeline: code flows through build, test, and deploy stages automatically.</p></div>
Load testing answers critical questions:
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.jsLocust: 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<div style="margin:2.5rem auto;max-width:600px;width:100%;text-align:center;"><svg viewBox="0 0 600 190" xmlns="http://www.w3.org/2000/svg" style="width:100%;height:auto;"><rect width="600" height="190" rx="12" fill="#0d1117"/><rect x="0" y="0" width="600" height="28" rx="12" fill="#1c2333"/><rect x="0" y="12" width="600" height="16" fill="#1c2333"/><circle cx="18" cy="14" r="5" fill="#ef4444"/><circle cx="34" cy="14" r="5" fill="#f59e0b"/><circle cx="50" cy="14" r="5" fill="#2dd4bf"/><text x="300" y="18" text-anchor="middle" fill="#94a3b8" font-size="10" font-family="monospace">Terminal</text><text x="20" y="50" fill="#2dd4bf" font-size="11" font-family="monospace">$</text><text x="35" y="50" fill="#e2e8f0" font-size="11" font-family="monospace">docker compose up -d</text><text x="20" y="70" fill="#94a3b8" font-size="11" font-family="monospace">[+] Running 5/5</text><text x="20" y="88" fill="#2dd4bf" font-size="10" font-family="monospace"> ✓</text><text x="38" y="88" fill="#94a3b8" font-size="10" font-family="monospace">Network app_default Created</text><text x="20" y="106" fill="#2dd4bf" font-size="10" font-family="monospace"> ✓</text><text x="38" y="106" fill="#94a3b8" font-size="10" font-family="monospace">Container web Started</text><text x="20" y="124" fill="#2dd4bf" font-size="10" font-family="monospace"> ✓</text><text x="38" y="124" fill="#94a3b8" font-size="10" font-family="monospace">Container api Started</text><text x="20" y="142" fill="#2dd4bf" font-size="10" font-family="monospace"> ✓</text><text x="38" y="142" fill="#94a3b8" font-size="10" font-family="monospace">Container db Started</text><text x="20" y="165" fill="#2dd4bf" font-size="11" font-family="monospace">$</text><rect x="35" y="155" width="8" height="14" fill="#e2e8f0" opacity="0.7"/></svg><p style="margin-top:0.75rem;font-size:0.85rem;color:#94a3b8;font-style:italic;line-height:1.4;">Docker Compose brings up your entire stack with a single command.</p></div>
Gatling: The Scala-Based Heavyweight
Gatling uses Scala DSL and generates beautiful HTML reports. It is popular in enterprise Java/Scala shops.
// BasicSimulation.scala
import io.gatling.core.Predef._
import io.gatling.http.Predef._
import scala.concurrent.duration._
class LoadTestSimulation extends Simulation {
val httpProtocol = http
.baseUrl("https://api.example.com")
.acceptHeader("application/json")
.contentTypeHeader("application/json")
val loginAndBrowse = scenario("User Journey")
.exec(
http("Login")
.post("/auth/login")
.body(StringBody("""{"email":"[email protected]","password":"testpass"}"""))
.check(jsonPath("$.token").saveAs("token"))
)
.pause(1)
.exec(
http("Dashboard")
.get("/api/dashboard")
.header("Authorization", "Bearer #{token}")
.check(status.is(200))
.check(responseTimeInMillis.lte(500))
)
.pause(2, 5)
.exec(
http("List Orders")
.get("/api/orders?page=1&limit=20")
.header("Authorization", "Bearer #{token}")
.check(status.is(200))
)
setUp(
loginAndBrowse.inject(
rampUsersPerSec(1).to(50).during(1.minute),
constantUsersPerSec(50).during(3.minutes),
rampUsersPerSec(50).to(200).during(1.minute),
constantUsersPerSec(200).during(3.minutes),
)
).protocols(httpProtocol)
.assertions(
global.responseTime.percentile3.lt(500), // 95th percentile < 500ms
global.successfulRequests.percent.gt(99), // > 99% success rate
)
}Comparison
|---------|-----|--------|---------|
CI/CD Integration
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<div style="margin:2.5rem auto;max-width:600px;width:100%;text-align:center;"><svg viewBox="0 0 600 200" xmlns="http://www.w3.org/2000/svg" style="width:100%;height:auto;"><rect width="600" height="200" rx="12" fill="#1a1a2e"/><path d="M100,30 L500,30 L460,65 L140,65 Z" fill="#3b82f6" opacity="0.8"/><text x="300" y="53" text-anchor="middle" fill="#ffffff" font-size="11" font-family="system-ui">Unoptimized Code — 2000ms</text><path d="M140,70 L460,70 L420,105 L180,105 Z" fill="#6366f1" opacity="0.8"/><text x="300" y="93" text-anchor="middle" fill="#ffffff" font-size="11" font-family="system-ui">+ Caching — 800ms</text><path d="M180,110 L420,110 L380,145 L220,145 Z" fill="#a855f7" opacity="0.8"/><text x="300" y="133" text-anchor="middle" fill="#ffffff" font-size="11" font-family="system-ui">+ CDN — 200ms</text><path d="M220,150 L380,150 L350,175 L250,175 Z" fill="#2dd4bf" opacity="0.9"/><text x="300" y="168" text-anchor="middle" fill="#1a1a2e" font-size="11" font-family="system-ui" font-weight="bold">Optimized — 50ms</text><text x="530" y="53" text-anchor="start" fill="#94a3b8" font-size="10" font-family="system-ui">Baseline</text><text x="445" y="93" text-anchor="start" fill="#2dd4bf" font-size="10" font-family="system-ui">-60%</text><text x="405" y="133" text-anchor="start" fill="#2dd4bf" font-size="10" font-family="system-ui">-90%</text><text x="365" y="168" text-anchor="start" fill="#2dd4bf" font-size="10" font-family="system-ui" font-weight="bold">-97.5%</text></svg><p style="margin-top:0.75rem;font-size:0.85rem;color:#94a3b8;font-style:italic;line-height:1.4;">Performance optimization funnel: each layer of optimization compounds to dramatically reduce response times.</p></div>
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.
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