You're right that it can do async io. But you compared a framework to a language. Rust has Tokio for async - Iron is just not using it.

It's similar with Ruby/RoR - yeah, they can do async. But not on their own. With unicorn server, you still get no threading and just a bunch of processes. With puma you can do threading (async cooperative really) - as long as you keep the configuration/code within the limits of what's allowed.

And due to the extra care needed whenever you do caching/storage things, I expect unicorn is still the king in RoR deployments. (GH uses that for example)

It's definitely preemptive rather than cooperative. Ruby/Puma is actually using one OS thread per Ruby thread, so when one hits a DB call and blocks on sync IO, it releases the GVL and another Ruby thread can proceed. There's a timer thread Ruby runs and pre-empts each thread to schedule them.

Yeah, I guess I moved to Puma long enough ago that I forgot about this. Good points, thank you.

Yeah, that's pretty much what happens. There's also non-blocking IO you can use with EventMachine but the DB drivers are a bit of an issue AFAIK.