Thursday, June 10, 2010

Is your software dependable enough?

Most embedded system software has to be reasonably dependable. For example, customers are likely to be unhappy if their software crashes once per minute. But how dependable is good enough can be a slippery subject. For example, is it OK if your software crashes once every 10 minutes? Every 10 hours? Every 10 days? Every 10 years? Is that number written down anywhere? Or is it just a guess as to what might be acceptable?

We suggest that every product have written dependability requirements. This probably has two parts: Mean Time Between Failures (MTBF) for hardware, and mean time between crashes for software. (You can add a lot more if you like, but if you are missing either of these you have a big hole in your requirements.)

Once you have set your requirements, how do you know you meet them? For hardware you can use well established reliability calculation approaches that ultimately rest upon an assumption of random independent failures. But for software there is no reasonable failure rate to make predictions with. So that leaves you with testing to determine software dependability.

Testing to determine your software crashes less often than once per minute is pretty easy. But when your dependability target is many years between software crashes, then testing longer than that is likely to be a problem. So, for most systems we recommend defining not only a target operational dependability, but also a concrete acceptance test for dependable that is easily measurable.

For example, set a requirement that the system has to survive 1 week of intense stress testing without a crash before it ships. This certainly doesn't guarantee you'll get 10 years between crashes in the field, but at least it is a concrete, measurable requirement that everyone can discuss and agree upon during the requirements process. It's far better to have a concrete, defined dependability acceptance test than to just leave dependability out of the requirements and hope things turn out OK. Chapter 26 of my book discusses embedded system dependability in more detail.
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