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

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 · 2 years ago

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_ · 2 years ago

I cannot tell if this is a serious comment.

0xBDB · 2 years ago

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 · 2 years ago

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 · 2 years ago

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 · 2 years ago

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

gitgud · 2 years ago

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 · 2 years ago

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