Banks are eventually consistent. Online transaction processing creates "pending" transactions, and the data is often inconsistent. Your charge may exist in the

Banks are eventually consistent. Online transaction processing creates "pending" transactions, and the data is often inconsistent. Your charge may exist in the merchant's database but not post to your online banking for several hours. Or it may be a wildly different amount - i.e. gas stations will place a $100 hold on your debit card and it will stay that way for days until it's settled for the actual (lesser) amount. If you were accidentally double-charged, than rather than processing a separate refund, the merchant may simply not settle the duplicate charge, and it will drop off your pending transactions... eventually. The lag time may be several days or weeks. If it's a debit card, you can't spend the money during that time and you may be temporarily broke because of it. If you make an ACH transfer, money will disappear from your account one night, spend a business day in the aether, and then post to the recipient's account on the third day. The system is in inconsistent state (i.e. money is missing) for at least a day, possibly a whole weekend. The actual transaction is settled and goes into "posted" state with a lag of 2.5 * 10^8 ms - i.e. 3 business days. That's if you're lucky. Banks do need strong consistency, but not in anything approaching realtime. Even ZooKeeper could probably handle the U.S.'s financial transactions faster than current infrastructure.