Virtual User (VU)
// Definition
A simulated user in a load test — an independent thread that follows a scripted journey (log in, browse, check out) with its own session and think-time, mimicking one real person's load. "1,000 VUs" means a thousand concurrent simulated journeys. VUs are the unit you scale to model traffic.
// Why it matters
Virtual users are how QA expresses load realistically — modelling 1,000 journeys (each with think-time and varied data) is far truer than firing 1,000 identical requests. QA cares because getting VU behaviour right (concurrency, pacing, data variety) is the difference between a load test that predicts production and one that proves nothing.
// How to test
VUs are configured in the load tool (k6 example, conceptually): • each VU runs the full scripted journey, not a single request • give VUs think-time between steps (real users pause; back-to-back is unrealistic) • vary each VU's data (different accounts/items) to defeat caching • ramp VUs up gradually to model arrival, then hold at target concurrency
// Common mistakes
- VUs that fire requests with no think-time (unrealistic, overstates capacity)
- All VUs using identical data (all cache hits — unrealistically fast)
- Confusing VUs (concurrent journeys) with requests-per-second (a throughput measure)
// Related terms
Concurrent Users
The number of users actively interacting with the system at the same moment. A primary input to load and stress test design.
Ramp-up Period
The duration over which virtual users gradually start hitting the system at the beginning of a load test. A short ramp-up can mimic a flash crowd; a longer one models steady traffic growth.
Throughput
The amount of work completed per unit of time — requests per second, transactions per second. Higher is better.