Gatling

Gatling is a high-performance, open-source load testing tool: it uses an asynchronous architecture (Akka/Netty) to simulate thousands of concurrent users with minimal resources.

This is Gatling: the premier 'Load-Test-As-Code' platform, built for modern DevOps and CI/CD pipelines. It's an open-source framework, primarily written in Scala, designed to stress-test web applications and APIs at scale. The key is its asynchronous, non-blocking architecture, which allows a single machine to efficiently simulate massive user loads (thousands of virtual users) without the thread-per-user overhead of older tools. You define scenarios using an expressive DSL (Domain-Specific Language) via SDKs for Java, Scala, JavaScript, and Kotlin. Post-execution, you get detailed, interactive HTML reports for immediate bottleneck analysis and performance validation.

https://gatling.io/