the code they’re testing has painful hardwired dependencies on expensive external resources
I’ve told this story elsewhere, but I had a coworker who wrote an app to remote-control a baseball-throwing machine from a PDA (running WinCE). These machines cost upwards of $50K so he only very rarely had physical access to one. He loved to write tests, which did him no good when his code fired a 125 mph knuckleball a foot over a 10-year-old kid’s head. This resulted in the only occasion in my career when I had to physically restrain a client from punching a colleague.
Wow. I love that story and I’m glad nobody was hurt.
I wonder whether that happened as a result of unexpected behavior by the pitching machine or an incorrect assumption about the pitching machine in that coworker’s tests.
I find this story compelling because it illustrates the points about managing risk and the limits of testing, but it doesn’t sound like the typical story that’s obviously hyperbole and could never happen to me.
Thank you for sharing it.
It happened because the programmer changed the API from a call that accepted integer values between 0 and 32767 (minimum and maximum wheel speeds) to one that accepted float values between 0.0 and 1.0. A very reasonable change to make, but he quick-fixed all the compiler errors that this produced by casting the passed integer parameters all through his code to float and then clamping the values between 0.0 and 1.0. The result was that formerly low-speed parameters (like 5000 and 6000, for example, which should have produced something like a 20 mph ball with topspin) were instead cast and clamped to 1.0 - maximum speed on both throwing wheels and the aforesaid 125 mph knuckleball. He rewrote his tests to check that passed params were indeed between 0.0 and 1.0, which was pointless since all input was clamped to that range anyway. And there was no way to really test for a “dangerous” throw anyway since the machine was required to be capable of this sort of thing if that’s what the coach using it wanted.
API from a call that accepted integer values between 0 and 32767 (minimum and maximum wheel speeds) to one that accepted float values between 0.0 and 1.0.
This would cause alarm bells to ring in my head for sure. If I did something like that I would make a new type that was definitely not implicitly castable to or from the old type. Definitely not a raw integer or float type.
Yikes! That’s also a great cautionary tale for Primitive Obsession/Whole Value as well as a bunch of other design principles.
I’m thinking about how I’d have done that refactoring and now I wish I had the code base to try it on. It sounds like it would make a really good real-life exercise in a workshop. “Remember folks, you have to get this right. There’s not really a way to check this with the real hardware, and if you get it wrong, someone’s going to get hurt.”
Thanks again.
