[bko]

Does this mean there's a central server / database every ATM for a particular bank queries?

I'm thinking if there are multiple servers you could theoretically do it at the exact same time. Or if there are multiple servers but one database, is the database locked for your account when performing the request?

[retrac]

Yes. Each bank provides an access point to the interbank network. Which is a fairly centralised service run collectively by the participating banks. There's one database (approximately) per bank. That holds your account figures, etc. Atomic transactions are used, so yes, the account is locked on both ends until the funds go through or the transaction fails.

In the early days (1980s, 90s) this all tended to crash on Black Friday and Boxing Day, taking the whole thing down for entire banks and, a few times, nationwide. Been a good 20 years since that happened regularly though.