@eolach It's eventually a global state and then some smaller per-connection state that provides a little REPL for sending interactions up to affect the global state. I don't think it would be too hard in almost any language, as long as you can reason about game logic and stop the individual connections from trampling each other too badly. With a MUD, you can probably get away with a queue: that kind of thing tends not to be too CPU-intensive (and you can fit way more into RAM than you could back in the heyday of MUDs; you can probably keep the entire thing in RAM nowadays), so even at 100k users, responses should be instantaneous.

NecroKvntPuke has a MUSH, I don't remember what it's written in. dkmush.cyou, port 6250.