Mongrel is an open-source software HTTP library and web server written in Ruby by Zed Shaw.

This makes layering other servers in front of it possible using a web proxy, a load balancer, or a combination of both, instead of having to use more conventional methods employed to run scripts such as FastCGI or SCGI to communicate.

[3] Mongrel was capable of serving Ruby on Rails powered sites without requiring any other web servers, though as a single-threaded application this configuration is unsuitable for all but light loads.

[citation needed] After the original author Zed Shaw left the Ruby on Rails-scene[4] the releases of Mongrel stopped.

The current release 1.1.5 is no longer able to install in Ruby versions higher than 1.9.2 which is recommended for Rails 3, the fix is to use the much older 1.2.0-pre2[5][unreliable source?]