Load Testing: k6 and JMeter
You shipped a feature. It works on your laptop, it works in staging, it works for the ten people in the demo. Then a launch, a sale, or a link goes wide, and the thing that ran in 80 milliseconds is now timing out, the database is on fire, and you are reading logs at midnight trying to figure out what the limit actually was. The whole point of load testing is to learn that limit on a Tuesday afternoon instead of during the incident.
This guide gives you two tools that do the same job from opposite ends. k6 is load testing as code: you write a small JavaScript file, run it from the terminal, and drop it into CI. JMeter is the older, GUI-driven heavyweight that speaks more protocols than you will ever need. By the end you will know which to reach for, how to design a test that mimics real traffic instead of a meaningless flood, and how to read the numbers so you trust them.
How to read this
Read it in order the first time. Phase 1 builds the mental model: what load, stress, and soak tests actually measure, and why the average response time lies to you. Phase 2 is the everyday core, the same realistic scenario written in both k6 and JMeter so you can compare them on equal footing. Phase 3 is the part people skip and regret: the gotchas that quietly invalidate a test and what production reality does to your tidy numbers.
If you already run load tests and only want the comparison, skim Phase 1 and live in Phase 2. If you have never written one, start at the top.
The phases
- What load testing actually measures - load vs stress vs soak, virtual users, and why p95 beats the average.
- Writing the same test in k6 and JMeter - a realistic ramping scenario, side by side, plus thresholds and CI.
- When the numbers lie and the system breaks - the traps that fake a passing test and what breaks under real load.
For a wider view of where load testing fits, see /guides/load-and-performance-testing and /guides/what-performance-means.