Well, I think what makes it interesting is that the simultaneous write issues don't just apply to ZODB, but the indexes (and/or balancing data) inside the encrypted buckets being maintained by the clients to maintain their own bsts, per my question above. You can't just put a transaction around one bucket because they have to, together, form a consistent tree.

That's especially true when you consider that to maintain log n the tree will have to be a self-balancing tree, so any given modification can touch a larger number of nodes during rotation.

I might be missing something obvious, but if the client has to maintain the tree I have a hard time seeing how you wouldn't have to queue access into the tree for modifying operations. That seems like it could be a pretty significant issue for some applications.