I agree with what you (and the others) say. Regarding green threads and state machines I would however add that while green threads allow to ditch some state m

I agree with what you (and the others) say. Regarding green threads and state machines I would however add that while green threads allow to ditch some state machines (e.g. ones that wait until a complete packet was received) you won't get rid of all of them, e.g. you will still need state machines for lots of protocol implementations. E.g. if you are implementing the websocket protocol or HTTP/2, you have some interaction between incoming data, data that must be sent out and requests from the [library] user. The state of the connection depends on all of these, so you need some kind of connection state machine which would run most likely in it's own green thread (in addition to a sending thread and a receiving thread and a user thread). And now you have to implement clean synchronization between all of those threads. The lack of necessary synchronization (and the ability to observe a huge amount of state at once without danger) can make the implementation a little bit more easy in my experience. But here I also have to agree with strmpnk that a good design is necessary for both approaches, and accessing lots of shared mutable data in multiple callbacks is probably also not the cleanest approach.