jeremyjh did a good job explaining this. I work for a company that supports the card issuance side and previous to that I worked for one of the largest drivers

jeremyjh did a good job explaining this. I work for a company that supports the card issuance side and previous to that I worked for one of the largest drivers of ATM's for small and regional banks. This has nothing to do with transactions in the sense of database transactions or a transaction manager managed by an app server, these may still be used behind the scenes. jeremyjh did a good job explaining this. I work for a company that supports the card issuance side and previous to that I worked for one of the largest drivers of ATM's for small and regional banks. This has nothing to do with transactions in the sense of database transactions or a transaction manager managed by an app server, these may still be used behind the scenes. What happens here is the ATM sent a withdrawal request over the network, that was recorded by the issuing bank - a memo to that account - the bank recorded a debit. Subsequently, the ATM/network/something failed and a second 'reversing' request was sent over the network to the issuing bank, this reversal credit rollsback the initial credit - this may or may not get memo posted to the bank account in real time because reversal transactions can be processed at a lower priority. During the end of day settlement process, the ATM network 'settles' up with each Bank, during this time the debit/credit markup transactions are dropped and replaced by the REAL activity posted by the ATM network (STAR/Pulse/Cirrus/etc). ATM systems are a lot more complicated than a simple database transaction, and even I left out several of the steps and possible events that could occur during the process. During the end of day settlement process, the ATM network 'settles' up with each Bank, during this time the debit/credit markup transactions are dropped and replaced by the REAL activity posted by the ATM network (STAR/Pulse/Cirrus/etc). ATM systems are a lot more complicated than a simple database transaction, and even I left out several of the steps and possible events that could occur during the process.