Locust

Locust is an open source load testing framework written in Python that enables developers to define user behavior with plain Python code and simulate millions of concurrent users to test system performance and scalability. Unlike traditional load testing tools that use domain-specific languages or XML configuration, Locust uses pure Python scripts where each simulated user is represented by a class inheriting from the User base class, with tasks defined as methods decorated with the task decorator. This approach makes test scenarios infinitely flexible and easy to version control alongside application code. The distributed architecture runs tests across multiple machines using a master-worker model, where a master node coordinates worker nodes that each generate a configurable number of user instances. This enables horizontal scaling to simulate traffic levels far beyond what a single machine could produce. The web UI provides real-time statistics including requests per second, response time percentiles, failure rates, and active user counts, with the ability to start, stop, and adjust user counts dynamically during a running test. The headless mode enables integration into CI/CD pipelines via command line arguments, with results exportable in CSV format for automated reporting. Custom client classes can wrap any protocol including HTTP, WebSocket, gRPC, and Thrift, enabling load testing of non-HTTP systems. The framework is used by thousands of companies including Cloudflare, Humana, and Mozilla, and is recommended by AWS and Microsoft Azure documentation for performance validation. The BSD-licensed project maintains active development on GitHub with a vibrant community contributing plugins, integrations with monitoring tools like StatsD and Datadog, and custom reporting extensions.