>If an ATM is disconnected from the network and when the partition eventually heals, the ATM sends sends a list of operations to the bank and the end balance will still be correct.
I don't think so. I support ATM client software for a large bank in the US and we certainly don't do this. This may be true for "remote" ATMs that are installed in convenience stores on POTS. I can't say I've ever actually heard of it though - the main problem with this idea is that cards cannot be authenticated without network access, and just spewing out money to every piece of plastic calling itself a card when your network connection has been dropped isn't really a recipe for success. Fraud is a real problem.
The ATM client software I support cannot do any transactions without a connection with its authorization system. That authorization system though, can stand-in for the various accounting systems and external networks up to pre-defined limits. So for example if for some reason we can't reach the checking account system we'll authorize up to $xxx total for the day on a stand-in basis. The transaction with the authorization system is definitely ACID; the ATM will not get a response code authorizing a withdrawal unless the transaction has been recorded in the authorization system. The account system may well be caught up later. The funny thing is, ACID is a property of individual database systems and it has absolutely nothing to with a question of whether two separate ledgers are guaranteed to be changed together or not at all. That would be the job of a distributed transaction coordinator - and those really are not used very much in banking. Instead there is a protocol of credits and debits and a settlement process to work out the exceptions. Maybe this is what the article was trying to say up to a point but they sort of confused the issue between the point of view of the ATM and the accounting systems of record.
I don't think so. I support ATM client software for a large bank in the US and we certainly don't do this. This may be true for "remote" ATMs that are installed in convenience stores on POTS. I can't say I've ever actually heard of it though - the main problem with this idea is that cards cannot be authenticated without network access, and just spewing out money to every piece of plastic calling itself a card when your network connection has been dropped isn't really a recipe for success. Fraud is a real problem.
The ATM client software I support cannot do any transactions without a connection with its authorization system. That authorization system though, can stand-in for the various accounting systems and external networks up to pre-defined limits. So for example if for some reason we can't reach the checking account system we'll authorize up to $xxx total for the day on a stand-in basis. The transaction with the authorization system is definitely ACID; the ATM will not get a response code authorizing a withdrawal unless the transaction has been recorded in the authorization system. The account system may well be caught up later. The funny thing is, ACID is a property of individual database systems and it has absolutely nothing to with a question of whether two separate ledgers are guaranteed to be changed together or not at all. That would be the job of a distributed transaction coordinator - and those really are not used very much in banking. Instead there is a protocol of credits and debits and a settlement process to work out the exceptions. Maybe this is what the article was trying to say up to a point but they sort of confused the issue between the point of view of the ATM and the accounting systems of record.