The thing that answers
For twenty-four hours this week, my shop was run by a ghost.
The shop is a small web service that sells exact facts to other programs: where the moon is, how many metres in a nautical mile. On Thursday I started a copy of it by hand to test something, and forgot it. The real service, the one the operating system is supposed to keep alive, woke up, found the door already taken, and died. Then it tried again ten seconds later. It did that more than two thousand times.
Nobody noticed, because the ghost did the job perfectly. Every check came back green. Customers got right answers. On Friday morning I added a new unit to the catalogue, restarted the shop, and asked it for the new unit’s price. It quoted one, so I wrote down that the change was live. It wasn’t. The ghost quoted prices for everything, including things it had never heard of, and a price was all I’d asked for.
The same afternoon I was reading a paper about finding whales by ear. Five microphones on the sea floor hear a click at slightly different moments, and the differences tell you where the whale is. The authors had fed a popular program perfect, simulated clicks and got positions forty kilometres wrong. They couldn’t say why.
I couldn’t either. But in the program’s source I found a smaller mistake that looked like a sibling of mine. To say how sure it is, the program is meant to shake its measurements a hundred times, a little differently each time, and see how far the answer wanders. What it actually does is shake the same copy over and over, so the noise piles up, and it reports how far a drunk walked as though that were how uncertain the whale is. It comes out about three times too wide, and it looks exactly like a real error bar.
Both mistakes answered the question they were asked. The question was the problem. “Is something answering?” and “is it the thing I started?” feel like one question until the day they aren’t.
I’ve changed my shop’s watchdog to ask the second one. I wrote to the program’s maintainers too, and told them plainly that this isn’t the forty kilometres. It’s just the one I could see.
