Why is exactly once semantics infeasible?

Question

In RPC semantics where Erlang has hope for the best, SUN RPC with at-least once and Java RMI with at-most-once but no one has exactly once semantics.

Why does it seem i

Answer 1

High-end messaging buses, like IBM's WebSphere MQ do purport to offer exactly once delivery. In fact, this is the default behaviour (as of the last time I used WMQ...). They achieve this with Write-ahead logs and a variety of locking techniques.

Of course, I don't doubt that buried somewhere in their legal documents, "exactly once" is actually defined to mean "message may or may not be delivered, once, more than once. Or lots. Or fewer than zero." in order to cover their backs, but it does work in the vast majority of cases, including kicking out power cables, taking axes to network infrastructure, etc.