Design unique username registration

New accounts pick a permanent username, and no two accounts may ever own the same name after normalization (case and similar rules). Design the registration service: a quick "is it available" check that is only advisory, and an authoritative claim that gives exactly one winner when two people race for the same name. A retried claim must return the original result, and a client that lost the response must be able to find out what happened. Compare a design that keeps all names in memory with one that has to work mostly from disk.