Why load test?

Load testing means deliberately hammering your API to find out what happens when traffic spikes. Most teams discover their bottleneck on launch day from a screaming Slack channel. This course is the alternative: find it on Tuesday afternoon, with coffee, and fix it.

What load testing answers

• How many concurrent users can my API handle? 50? 5000? You should be able to answer this with a number.
• Where does it break? DB CPU, connection pool, app CPU, memory, downstream service?
• What does the user experience look like under load? p95 = 200ms? 2s? 20s? The difference matters.
• Does my change make things WORSE? Run before + after; the numbers tell you.

The five test types

• Smoke — 10 VUs for 1 min. "Does this basically work?" Run on every PR.
• Load — expected production traffic for 5-10 min. "Does it hold up at planned scale?"
• Stress — ramp until it breaks. "Where's the cliff?"
• Spike — sudden 10x surge. "Does a marketing email crash us?"
• Soak — moderate load for hours. "Do we leak memory / connections over time?"

When NOT to load test

Load testing has a cost: time to write, infrastructure to run, signal to interpret. Skip it if:

• You have <100 daily active users. Your DB isn't the bottleneck — feature velocity is. Build first.
• You haven't shipped anything. "What if a million users come?" is a great problem to have later. Don't pre-optimise.
• The endpoint is rarely called. A quarterly-reporting endpoint doesn't need to handle 1000 rps. Test the hot paths, ignore the cold ones.

What we'll build together

• A k6 suite in tests/k6/ with one script per scenario type.
• Thresholds that fail the build on regression.
• A workflow that runs k6 on every PR via GitHub Actions.
• One real measured optimisation against your own API.

The mindset shift

Most performance work is vibes-based: "feels faster", "should be quicker". Load testing is the discipline of replacing vibes with numbers. You make a change, you run the test, the number went down — or it didn't. The graph is brutal honest. That's the whole point.

What's next

Next lesson — Install k6. One binary, no Node, no deps. We'll have you running a test by the end of the next 3 minutes.