What is the most critical piece of code you have written and how did you approach it?

Question

Put it another way: what code have you written that cannot fail. I'm interested in hearing from those who have worked on projects dealing with heart monitors, water testing, economic fundamentals, missile trajectories, or the O2 concentration on the space shuttle.

How did you prepare for writing this sort of code: methodologically, intellectually, and emotionally?

Edit

I've marked this wiki in case the rep issue is keeping people from replying. I thought there would be a good deal more perspective on this issue than there has been.

Answer 1

While I am not personally involved in what is described there, this article will hopefully contribute to the spirit of your question: They Write the Right Stuff.

Answer 2

Right now I'm working on some base code for a system that retrieves medical patient information from clinics and hospitals for a medical billing office. We're starting out with a smaller client and a long break-in period to ensure quality, but eventually this code needs to securely handle a large variety of report formats from a number of clients at different facilities.

It's not quite in the same scale as your examples, but a bad mistake could result in the wrong people being billed or the right person billed to a defunct address (screwing up credit reports) or open people up to identity theft, so it's still pretty critical. Oh yeah, and it could mean doctors don't get paid quite as quick. That's important, too, especially from a business perspective, but not in the same class as data protection and integrity.

Answer 3

I have written computer interface to a MRI machine. It had no chance of hurting the end user as it was just record management, but it could potentially have given an incorrect diagnosis or omit important information.

Tests, lots and lots of tests.

Unit tests, mid and high level tests. Simulate all possible input combinations. Also a great deal of testing with the hardware itself. Testing must be done in a complete and methodical way. It should take a great deal more time to test than to write.

Error Reporting

All errors must be reported and be obvious. If it won't hurt the patent to do so, fail fast.

For something that is actively keeping a person alive things are even worse. It must never stop working. If it fails it needs to restart and keep trying. Redundant internals are also a must in case the hardware fails.

At the wrong company it can really a difficult kind of situation to work in. However, if things are going well, you are well funded and release pressure is not high, it can be a very rewarding space to work in.

Answer 4

I wrote a driver for a blood pressure measuring device for hospital use. If it "fails", the patient will not have his blood pressure checked at the scheduled time; if his blood pressure is abnormal, no alarm (in the larger system) will be triggered. Such an event could be clinically significant.

My approach was to thoroughly read the spec/documentation in a non-work environment (to avoid the temptation to start coding right away), then read it again at work. After that, I summarized the possible states and actions on paper and "flowcharted" an algorithm, and annotated all the potential real-world "bad events" (cables getting unplugged, batteries dying, etc). Finally, I wrote and rewrote the driver three times, each with different mechanisms (e.g. FSM), and compared their results. Each iteration helped me identify weaknesses I hadn't yet discovered. The third rewrite was the "official" result. I reviewed each iteration with my co-worker.

Emotional preparation consisted of convincing myself that should the unthinkable happen, at least I wasn't willfully negligent -- just incompetent (the old "I'm only human" excuse). ;-)

Answer 5

Not really an answer, but:

I've got a friend who writes embedded control software for laser eye surgery machines. When he had laser eye surgery himself, he made sure to go to an ophthalmologist who used his company's system. I have great admiration for this guy. I can't think of a piece of software I've ever written whose level of quality was high enough that I'd trust my own eyesight to it.

Answer 6

While nothing quite so important as an MRI machine or a blood pressure monitor, I did get tapped to do a rewrite of Blackjack when I worked for an online gambling provider. Blackjack is by far the most popular online game, and millions of dollars was going to go through this software (and did).

I wrote the game engine separate from the server and the client, and used Test Driven Development to ensure that what I was assuming was coming through in the results. I also had a wrapper "server" that had console output that would allow me to play. This was actually only useful in that it mimicked the real server interface, since playing a text version of blackjack isn't very fun or easy ("You draw a 10. You now have a 10 and a 6, while the dealer has a 6 showing. [bsd] >")

The game is still being run on some sites out there, and to my knowledge, has never had any financial bugs after years of play.

Answer 7

I've heard crazy stories of the processes used to write code at NASA for the spaceshuttles. Every line of code has about 10-20 lines of documentation, along with tests, full revision history, etc. Every time a bug is found, not only is the code evaluated and repaired, but the entire procedure of writing code, the entire command chain, etc. is reviewed to answer the question: "What happened wrong in our process that allowed this bug to get included in the first place?"

Answer 8

My first "real" software job was writing a GUI app for planning stereotactic brain surgery. Testing, testing, testing... absolutely no formal methods, engineering-style thoughts, just younger programmers cranking it out. When they started talking about using the software to control a robotic arm with a laser, without any serious engineering methods in place, i got a bit worried, left for more officey lands.

Answer 9

I've created information system application for local government cultures and tourism department in Bali island which were installed in several tourism denstinations, providing extensive informations about the culture, maps, accomodations etc.

if it failed then probably tourists couldnt get the right informations they need most, cheat by brookers, or lost somewhere :)