[cheradenine_uk]

Yes, this is a hard problem. Not even partitioning by tenant will always help you.

This is fundamentally equivalent to database ACID constraints, and the other modes described are great in the same way that if you're able to relax some of the ACID constraints in your code (say, by not being SERIALIZABLE), you get in return nice things (like reads never deadlocking).

If you can't know if message N will change the outcome of processing message N+M, then you have to resolve that before you can proceed - as surely as a serializable database will wait for the outcome of transaction N before being able to proceed with N+M.