Deployment
One popular configuration was to run Apache HTTP Server 2.2 as a load balancer using mod_proxy_balancer in conjunction with several Mongrel instances. Each Mongrel instance would run on a separate TCP port, configured via the mongrel_cluster management utility. Until recently, Twitter was a notable instance of this configuration.
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.
Read more about this topic: Mongrel (web Server)