[phamilton]

This is not a very accurate comment. Async code, cooperative scheduling, threading, and shared memory are all completely independent. You can have any combination of those.

It comes down to this: Mutual exclusion is a requirement for concurrent operations.

This can be accomplished with mutexes (usually how thread-based implementations work), cooperative scheduling (how some async implementations work, but not all), shared nothing architectures (one request per process, actor/CSP model), among other approaches.

There are plenty of hybrids out there. libevhtp is a threaded async web server. It screams. Erlang presents a no shared memory model along with an aggressive scheduler, allowing for one request per process designs, except in this case a process is extremely lightweight. Golang does something similar with goroutines, similarly lightweight units of execution.