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Comment on Eric Brewer on Why Banks are BASE, Not ACID – Availability Is Revenueparent

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In most cases when processing a withdrawal, ATMs or POS terminals don't know your exact balance and how much money you've taken out already today. A withdrawal tends to involve a request for $X which gets a yes/no response from the bank or from the chip of the card if the terminal is not online connected - the bank or chipcard logic will check the balance and daily limits, not the ATM.

You can think of the (rare nowadays, but possible) offline, paper-based card transactions (physical imprint of the card number + signature) as a very strong example of eventual consistency - your card will be billed for that amount sometime later when the documents are processed and all the balances will be correct, but for many days the "online visible" card balance as known to the bank will be different from the "real" legal/accounting balance of that card.

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