Harp is a lightweight HTTP reverse proxy allowing to proxy incoming requests to different upstream servers depending on the IP address of the visitor. It can also proxy requests to the same server but adding tags in the HTTP headers. Both techniques are a simple way to implement A/B testing with as little invasion in the application's code as possible.
A simple configuration file sending 10% of the requests to localhost:3000 and the remaining 90% to localhost:3001.
In order to install harp from sources, you can use the following commands which should work on most systems.