A medical radiation machine operator types the letter ‘x’, realizes it’s an error, backspaces, types ‘e’, and continues. Consequences of the error and related defects lead to the patient’s untimely death.
Ten years later, a Japanese stock broker mistakenly switches the share price and amount on the sell order of a new stock. He tries to cancel the order, but fails. His employers lose $225 million, and are involved in lawsuits for years.
A blogger clicks the “back arrow” by mistake, gets a warning message, concludes he meant to continue writing, and clicks “OK” to continue. He loses his work, and has to rewrite it. Here, the consequences are annoying, if trivial by comparison.
What do all these cases have in common? Canceling a request. If it’s not hard enough to program computers to do what we want them to do, who would have thought that telling them not to do it would be hard too?
The cancellation scenario — or “use case” as it’s called in software design — is the silent partner of every positive request supported by a piece of software. It has to cover giving the user clear options, executing the cancellation, and rolling back any partial results. Things get more complicated if authorization is required, or if the transaction has already gone through (both of those requirements figured into the Japanese broker error story). Cancellation and rollback are also part of automatic requests that may occur if one software module (the “server”) cannot complete a request by another (the “client”) and has to make sure to put everything back the way it was, and send the proper response code.
So the next time you’re designing a piece of software, no matter how simple, think what it’s supposed to do, but also what it will do if the user or client calls out, “Cancel that!”