[stephen]

A nit, but the remark of "auditing == everything is written down twice == double-entry accounting" is cute, but doesn't seem applicable to availability.

Double-entry is more of an internal (financial) system implementation detail, and so an orthogonal concern to intra-system audits.

(It's not like one side of the entry is in one bank's IT system, and the other side of the entry is in the other bank's IT system.)

(...speculating further, I really doubt the OLTP systems of banks are double-entry anyway.)

[PeterisP]

Funny, but "It's not like one side of the entry is in one bank's IT system, and the other side of the entry is in the other bank's IT system." is actually false in interbank deals such as correspondent accounts, money market or forex deals.

If a deal involves two banks, then the authoritative "other side" of the entry will be in the other bank's IT system. Of course, you'll maintain some records of what the entry should be in your opinion, but they won't always match as you don't have full info and you'll reconcile with data you get from the other bank's IT system by (for example) SWIFT network.

And I've seen only double-entry OLTP's for the core system that includes general ledger. Maybe no all of them are built that way, but I haven't seen such examples.