What you're not supposed to allow is letting both succeed, because then the clients would assume everything went fine. If one errored before the commit, both tr

What you're not supposed to allow is letting both succeed, because then the clients would assume everything went fine. If one errored before the commit, both transactions didn't execute successfully and you're fine. Pretty much every approach to isolation has a good chunk of errors that aren't strictly necessary (deadlocks, spurious serialization errors, ...).