I understand, but you can't just throw a DB at it and walk away. For instance, which DB? Setup how? Running on what type of hosts? What topological requirements does this have? How much does of a multiplier does it place on your data load.

I'd definitely use a trusted DB for storing bank accounts. The consistency is pretty much the first requirement and the data maps perfectly to tables. And 7B checking accounts isn't that big, compared to some problems so it'd probably scale pretty well even worst-case.

But I probably wouldn't for an MMO. Or at least, it wouldn't be where I stored every little thing going on around them - just the events (xp and gold earned) that they'd freak out about if we lost. But even just a log-structured DB would work well for that.

If there's no contention for a resource (in the bank case - the value in the account) there's much less reason for a transaction. I want the system to make its best effort but I don't want to wait around for the message if there isn't anything I can do on a failure anyways.