How do you reproduce bugs that occur sporadically?

Question

We have a bug in our application that does not occur every time and therefore we don\'t know its \"logic\". I don\'t even get it reproduced in 100 times today.

Disclaime

Answer 1

Analyze the problem in a pair and pair-read the code. Make notes of the problems you KNOW to be true and try to assert which logical preconditions must hold true for this happen. Follow the evidence like a CSI.

Most people instinctively say "add more logging", and this may be a solution. But for a lot of problems this just makes things worse, since logging can change timing-dependencies sufficiently to make the problem more or less frequent. Changing the frequency from 1 in 1000 to 1 in 1,000,000 will not bring you closer to the true source of the problem.

So if your logical reasoning does not solve the problem, it'll probably give you a few specifics you could investigate with logging or assertions in your code.