Load & Performance Testing

Your tests prove the app is correct; load tests prove it survives a thousand people showing up at once. Here's the mental model, the metrics that actually matter (percentiles, not averages), and how to run one test and read where it breaks.

1. Why Load-Test Correctness tests ask 'is it right?' for one user; load tests ask 'does it still work when a crowd arrives at once?' Here's the mental model, the launch-day scenario, and what you're trying to learn before your users learn it for you.
2. The Metrics That Matter Three numbers tell the story: throughput (requests/sec), latency (and why you read percentiles p50/p95/p99, not averages - the slow tail is what users feel), and error rate under load. Plus the four test types: load, stress, soak, and spike.
3. Running One & Reading It The workflow end to end: pick a realistic scenario, ramp up virtual users, watch throughput/latency/errors, and find the knee - the point where latency spikes and errors climb. With an illustrative readout, and the traps (test like production, or the numbers lie) that fool first-timers.