How can I perform a stress test?

Hi dear community!

I’m a new member from Germany :slight_smile: So don’t be angry with me, when
sometimes my english sounds terrible.

Currently I’m working on a huge project, a web application, which is
coded with Ruby on Rails. My request to you is how can I do a stress
test, so I can see how this web application behaves when e.g. 100
persons are online.

I found a commercial software (Scapa StressTest) and an open-source
Project for Eclipse (hyades), but I don’t see how it will works.

Regards from Paderborn