Other servers assume that every request is received by a browser, then sent to a backend, and then directly sent out to the client and that’s it. Mongrel2 assumes that there is a connected client, and it sends requests to backends, but it makes no assumptions about how those backends respond to the clients. All it requires is that the backend application send messages addressed to the client and it will write them on the socket.

Because of this design, Mongrel2 can easily house both classic HTTP clients, keep-alive style HTTP client, chunked encoding responses, JSSockets, or WebSockets using the same code.