Load Testing

Functionality is pointless if the server crashes at 10 users.

Types of Performance Tests

1. Load Test: Simulate expected usage (e.g., 100 users). Verify response time stays low.
2. Stress Test: Increase load until it breaks. Find the breaking point (e.g., "We die at 5000 users").
3. Soak Test: Run load for 24 hours. Finds memory leaks.
4. Spike Test: Sudden burst (Black Friday). Does autoscaling kick in fast enough?

Tools: k6

The modern standard. Written in Go, scripted in JavaScript.

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

export const options = {
  vus: 100, // Virtual Users
  duration: '30s',
};

export default function () {
  const res = http.get('https://test-api.com');
  check(res, { 'status was 200': (r) => r.status == 200 });
  sleep(1);
}

Interpreting Results

• Throughput: Requests per second (RPS).
• Latency (P95/P99): "99% of users saw a response faster than X".
  • Average is useless. If 50% get 100ms and 50% get 10s, the average is 5s, which looks okay, but half your users are angry. Look at P99.