I wounder how the world-wide ATM-systems are architected. It must be pretty hard for the banks to design a consistent system world wide. Do they use eventual consistency for this or do they use a great ACID system?
I can be in Sweden one day, where my bank is and use the ATM, then take the airplane to USA or Thailand and use the ATM abroad a few hours later. If the system is consistent, then they need a very reliable connection to anywhere in the world.
The systems are all different. Many are based on PCs (when one crashes, you can see BIOS messages on the screen). Some run Windows or Linux (couldn't find a crash image for those ... hmmm...).
What happens is that your credit card is the common key. All those systems can read it and check that it's valid. Then, they give you some of their money.
Eventually, the system will collect all that data and send it to the company which issued the card. This can be on- or offline. The accounts will be updated and after some time, the company will reimburse the bank.