Gremlins in the code

Monday 23 February 2004

Not only is error-handling code hard to write, it is hard to test. One technique is to intentionally introduce errors into your system. I did this again recently, and called it the Gremlin.

If you’ve done your job, your code has been written to properly handle exceptions at any point in the code. But most error conditions are hard to simulate. You could identify particular failure modes, and figure out how to make them happen, then meticulously test your software while creating those failures. That’s a lot of work, and still would miss many failure points.

The Gremlin is both simpler and more thorough, though with some randomness thrown in. The idea is to identify a few high traffic points in the code, and at those places, randomly throw exceptions. I wrote a function called PokeGremlin, and called it from those high-traffic points. PokeGremlin decides randomly whether to return or throw an exception.

For example, your memory allocator is called many times during any particular operation. Throwing exceptions randomly from there will test all sorts of failure handling that would normally never be invoked. There are probably other similar low-level utility methods that would make good call points for PokeGremlin.

If you try this, you will learn two things:

  1. There are a half-dozen or so places that need to be fortified immediately to make your software robust against this sort of onslaught.
  2. It is nearly impossible to make your software completely safe in this mode of operation.

I found that PokeGremlin had to lie quietly for some number of calls, just so the system could initialize itself to the point of performing. Make PokeGremlin configurable, both in the likelihood of an exception being thrown, and also in the number of initial opportunities it sits dormant. Then you’ll be able to experiment with different rhythms of torment for your system.


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