[mneil]

ATMs do not communicate with each other. They communicate with a central server cluster. Transactions are atomic and consistent.

CAP theorem gives the database of money transfers the ability to be consistent, partition tolerant, or available. Pick 2. All banks choose consistency, they have to. And partition tolerant is the obvious second choice so you don't magically lose all you money during a server crash.

If you're thinking someone can withdraw money in one country and do the same in another country would it figure out the correct balance or could you trick it? The answer is you cannot trick the ATM.