To recap for the non-Java reader: the problem was that the Servlet specification did not initially support suspending and resuming without suspending and resuming the thread handling the request. The Servlet 3.0 specification supports it, but before you could not use multiplexed IO within containers that required the servlet interface, even though IO multiplexing worked perfectly fine (and, as you said, some containers allowed it via a custom interface, since it is pretty much trivial to add).