Classic Ruby event-driven networking/reactor library, popular in Ruby's early days when the community was looking to build reactive applications around the time Rails was breaking into the scene. David Halasz considered it for a remote-console proxy but discarded it because ManageIQ uses PostgreSQL in async mode, which breaks EventMachine. Has been unmaintained for several years, which is the explicit motivation for UringMachine (whose name pays homage to EventMachine); Sharon Rosner maintains industrial process-control applications still running on EventMachine and plans to migrate them to UringMachine.