While I'm arguing that modern libraries can avoid threads... Since you have Haskell and Actors, but worrying thread-safety of some other libraries. You can con

While I'm arguing that modern libraries can avoid threads... Since you have Haskell and Actors, but worrying thread-safety of some other libraries. You can consider a single-threaded/multi-process deploy. By setting the thread pool to 1, Actors in a single process can still handle tens of thousands of concurrent connections, while there is surely no thread-safety problem.