The Dark Siphon is a simple Goliath app which allows you to duplicate a fraction of your live traffic from your production app to a test environment.
Tools like nginx and so on allow you to load balance requests between multiple servers but they don't let you duplicate a given event across two different backend servers.
Dark Siphon lets you do this simply:
$ ruby dark_siphon.rb --mainline=http://production.myapp.com --duplicate=http://test.myapp.com
This will spin up a Goliath::API listener on port 9000 which will send every request to production.myapp.com and return the response and will silently send 10% of events to test.myapp.com, ignoring the response.
The production app will continue to receive and respond to all requests while the test app will receive 10% of the real production traffic.
Try running `dark_siphon.rb –help' to see other options (both built-in to Goliath as well as special to Dark Siphon like `–fraction').