Almost every article ever written about database transactions includes the example of an ATM machine dispensing money. The articles will describe how the bank uses a transaction to prevent people from overdrawing the account during simultaneous access, or how the bank can rollback a transaction if an error occurs at the ATM.
They are all wrong.
About 7 years ago I went to a local ATM, punched in my PIN, and asked the ATM to withdraw $80 from my account. The machine thought about the request for a bit, and then started to make the happy fluttering noise ATM machines will do just before delivering cash. The happy fluttering noise was followed by a fateful POP sound in the distance, and then a total blackout. Power failure.
When power came back seconds later I still had no money, and the ATM had an OS/2 boot screen.
Cool! I remember thinking. The bank’s database just rolled back my transaction! I drove off in search of a working machine. Of course I was a bit younger then and had not learned the secret mission statement of every major financial institution: “Customer inconvenience is our first priority”. When my bank statement arrived the next month I was genuinely surprised to see the bank had committed the $80 transaction against my account.
One might think ATM machines would be equipped with some sort of UPS to keep them running in the event of a power failure during an open transaction, but one would be forgetting the second mission statement of all financial institutions: “Mistakes are acceptable, as long as we screw the customer and not ourselves”.
For the curious, here is how the story ends.
A month later I went to the bank, approached a teller, and told her if she gave me $80 in unmarked bills nobody would get hurt.
Well, not really, but it felt that way.
I showed the teller my statement and told her my story. She asked me to stand by a door to the left and walked through a back door behind the counter.
While waiting in the lobby I thought about the possibility of the teller returning to tell me the bank records were never wrong. Not only would I not get my $80 back, but they’d deduct a $10 service fee for seeing me in person. I’m always optimistic like that when dealing with banks. Thieves.
I was surprised when a woman burst out from the door beside the counter, ran over and practically hugged me. The bank had noticed an $80 discrepancy in the ATM records and this lady’s sole purpose was to find out what happened. I made her day and got my money back.