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Comment by julienfr112 | Hacker News Reader

And how can you at a bank run a transaction if you don't know if there is money or not in the account ???

1) Risk management. Accept some losses if necessary.

2) Overdraft. Take the money, putting the account below zero, and fine the customer for letting their account go below zero.

Just make sure you can cache the last known state of the database and use a separate queue for all non-database-applied transactions. This way you can do database maintenance or suffer database outages without affecting your service.

I heard that around 10 years ago, in The Netherlands a major bank still only wrote transactions to their system in a batch once a day. And all transactions of _today_ showing up in their e-banking applications came from their queue and caching systems on the side of their big database. It's weird but also has some benefits

One benefit to the bank is that they can apply the transactions in whatever order they want to your account. Banks like to apply the largest debit first, so that maybe all of the smaller ones will cause an overdraft, thus maximizing the overdraft fees they can charge. The banks say they do this so your mortgage payment for example doesn't bounce, but it doesn't hurt that they make a ton of money from it - not that a bank would ever do anything just for the money.

While processing a payment a psp/acquirer is mostly receiving money, so no need to check if there is balance available. Only for cases like refunds and card payouts you need to know if a merchant has sufficient balance to process the request.

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