For Wait-Or-Die, do you really need to fetch_and_add to get a transaction ID? Do you need a transaction ID at all? It sounds like the goal is just to have an arbitrary-but-consistent ordering of active transactions so that different transactions can agree on who should wait and who should die in the event of a conflict. So why not just use the thread ID? Or even a random number might work (if ties are treated as “die”, so the worst that happens is that both transactions unnecessarily abort, and then retry with new random numbers).

While it’s not mentioned, I suppose you want to prioritize older transactions in order to prevent long-running transactions from being starved by shorter-running transactions. (If one long transaction conflicts with an average of, say, three short transactions, and on each conflict it’s effectively random who wins, then each long transaction has only a 1/8 chance of winning all three conflicts and being able to commit.)

But preventing starvation only requires older transactions to be prioritized most of the time, not every single time, especially not if the transaction is only slightly older. So some kind of timestamp / cycle counter should work fine, even if there’s skew between threads or other sources of inaccuracy. Ties could be broken by thread ID, or again by having both sides abort.