Yeah, I get that it may be non-trivial to avoid head-of-line blocking in that context, but I doubt that it's impossible. If it is impossible, maybe Erlang just isn't the best tool for the job.

From what I've read, I think you could create a separate process dedicated to sending stuff to B, and have other processes deal with other clients. It's probably not possible, but it would also be nice if you could detect these hung processes and start more workers to deal with other clients, each dealing with potentially more then one client. (yes, I may be totally wrong)