Java Threads, Second Edition

Return to the Banking Example

So, we just survived the ATM withdrawal problem. It turns out that this problem occurred so infrequently that the total cash involved with the problem transactions was only a few thousand dollars. Luckily, the bank kept records that were good enough to recover the cash. While our manager did not like the fact that we caused a major panic among the upper-level managers, she was somewhat impressed that we were able to track down the problem. While she still does not trust us completely, we still have a job and are able to design and enhance different parts of the ATM system.

The first thing we do is to look at our existing ATM code: we check and double check every piece of code for race conditions, using the synchronized mechanisms that we’ve learned so far to resolve the problems. Everything seems to be going well until one day the president of the bank receives a phone call from an irate customer. This customer did a balance inquiry at the ATM that showed a balance of $300. Immediately, he attempted to withdraw $290, but could not.

It turns out that in the very short period of time between when the customer checked his balance and attempted to withdraw the money, his wife withdrew $100 from another ATM. Even though the “correct” thing happened, it turned into a big political problem for the bank when the husband threatened to remove his $1 million business account from the bank if the bank “couldn’t keep their records straight.” So the bank established ...