[Matthias247]

Don't be focused on "requests". Requests (where most people mean HTTP requests) are one layer where you need concurrency, but in principal you need it on multiple layers.

E.g. at first you have a server that accepts multiple connections and each must be handled -> Thread per connection or one thread for all connections? If you go for threads you might even need multiples, e.g. a reader thread, a writer thread which processes a write queue and a third one which maintains the state for the connection and coordinates reads and writes.

Then on a higher layer you might have multiple streams per connection (e.g. in HTTP/2), where you again have to decide how these should be represented.

Depending on the protocol and application there might be even more or other layers that need concurrency and synchronization.

But the general approaches that you mention do still apply here: You can either use a thread for each concurrent entity and using blocking operations. Or you can multiplex multiple concurrent entities on a single thread with async operations and callbacks. Coroutines are a mix which provide an API like the first approach with an implementation that looks more like the second approach.