It's multi-process. The core logic is a Qt application, but it delegates the external protocol I/O to separate processes. Mongrel2 handles inbound and Zurl handles outbound (Zurl is a project of ours that is basically the inverse of Mongrel2).
Parts of mongrel2 were sadly a solution in search of a problem. Mostly the "let's redo FastCGI via ZMQ". Was still immensely fun working on and with it.
