It's not impossible. You just pick CP from the CAP theorem (and give up availability). i.e., the message will be delivered exactly once if the system makes pr

It's not impossible. You just pick CP from the CAP theorem (and give up availability). i.e., the message will be delivered exactly once if the system makes progress. If you want an existence proof, NFSv3 had this working back in the 1980's. I doubt it was the first.