Ask HN: How are account balances updated in a real bank?

[kul_]

To folks who worked with financial organisations, I wonder how accurate the Account Balance updated examples are with respect to transaction isolation and concurrent updates? Or do banks just updates ledger and balances in a literally serialized manner with one thead avoiding any complexity altogether?

[b20000]

it all just lives in a bunch of csv files on a few people’s PCs and every night a 62 year old man copy pastes it together and checks everything and then uploads it to some old mainframe. he is the only one who knows how to do that job.

[kul_]

I cannot tell if this is a serious comment.

[0xBDB]

It isn't, but it's not that far from being a serious comment.

The old man has an ancient PC choking on dust under his desk that runs a QWS3270 terminal that updates the mainframe directly. If it's a big bank, the bank owns the mainframe. If it's a small bank, a vendor (probably Fiserv) does.

Source: I've consulted for a lot of banks.

[sloaken]

I remember one bank used to, and it was big news, process all deductions from accounts, then issue overdrawn penalties, then any deposits. So if you had $500 in your account, then deposited $500 in cash, then withdrew $501, you would be overdrawn and assessed a penalty. Oh and it was a very major bank (in the US).

[twunde]

If you're interested in this topic, you may want to read through some of the Bits about Money essays/newsletters: https://www.bitsaboutmoney.com/

[throwaway828]

Look into Flexcube docs to more fully answer your questions. It's a reasonably well established core banking product, or series of products.

[gitgud]

It’s all an immutable ledger, where the “account balance” is just a sum of all transactions on the account…

disclaimer: I don’t work in finance at all

[stop50]

In the past it was mostly the second, but now its the first.