save      -> id=1 name=Asha Rao
findById  -> Customer[id=1, name=Asha Rao, email=asha@example.com, tier=GOLD]
update    -> name=Asha Rao-Iyer (same id=1, SQL UPDATE)
count     -> 1
deleteById-> count now 0
findById(9999) -> empty Mono (completes with no value, no exception)