Locust is an open-source load and performance testing tool where scenarios are defined directly in Python : you inherit from classes like HttpUser or FastHttpUser and decorate actions with @task, rather than manipulating a heavy interface or XML. Its event-driven architecture based on gevent runs each virtual user in its own lightweight coroutine, allowing tens of thousands to be simulated per process.

Locust offers a real-time web interface (throughput, response time, error rates, graphs) with live load adjustment, as well as a headless mode ideal for CI/CD pipelines. Although HTTP-oriented, it can test almost any protocol via custom clients, and its pluggable architecture supports extensions (Grafana/Prometheus, load profiles).

Designed for scaling, it runs in distributed master/worker mode: the master hosts the interface and drives the campaign, while workers generate the load (one worker per CPU core). It is used by Google, Microsoft, Mozilla, and EA, remains fully stateless, and naturally self-hosts in Docker containers.