How flexible is Netty framework compared to functional languages frameworks ? For example- Erlang has OTP(gen_server) module where you can create your custom protocol and its implementation. I think these languages have built in support for multicores and immutability which i think are selling points for them in writing server apps.
Netty is a general purpose non-blocking / async network framework, which allows you to implement "any" protocol on top of it. While netty includes many different implementations for various protocols you are free to write your own if you need too.
Netty is very flexible and scale very well and so is used by many high-scale projects and companies that operate in environments which can be called "massive scale". For more details about projects and companies that are using netty see: