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

Let’s say I’m starting with a production application.

1. I typically add debug logging around the areas where I think the bug is occurring. I setup the logging statements to give me insight into the state of the application. Then I have the debug log level turned on and ask the user/operator(s) notify me of the time of the next bug occurrence. I then analyze the log to see what hints it gives about the state of the application and if that leads to a better understanding of what could be going wrong.

2. I repeat step 1 until I have a good idea of where I can start debugging the code in the debugger

3. Sometimes the number of iterations of the code running is key but other times it maybe the interaction of a component with an outside system (database, specific user machine, operating system, etc.). Take some time to setup a debug environment that matches the production environment as closely as possible. VM technology is a good tool for solving this problem.

4. Next I proceed via the debugger. This could include creating a test harness of some sort that puts the code/components in the state I’ve observed from the logs. Knowing how to setup conditional break points can save a lot of time, so get familiar with that and other features within your debugger.

5. Debug, debug , debug. If you’re going nowhere after a few hours, take a break and work on something unrelated for awhile. Come back with a fresh mind and perspective.

6. If you have gotten nowhere by now, go back to step 1 and make another iteration.

7. For really difficult problems you may have to resort to installing a debugger on the system where the bug is occurring. That combined with your test harness from step 4 can usually crack the really baffling issues.