Software reliability is difficult to specify. Don't make the problem even more difficult by being vague. For example, "The system shall be 99.999% reliable" means nothing.
Does it mean that the system cannot be "down" more often than 5 minutes every year but that it is okay to occasionally make a mistake (for example, a hotel reservation service may occasionally mix up hotel details). Or does it mean that it must make no more than one mistake every 100,000 transactions (for instance, a patient monitoring system cannot cause the death of more than one out of every 100,000 patients)?
When writing requirements, differentiate between:
- Failure on demand.
- What is the likelihood, measured as a percentage of requests, that the system will fail to respond correctly? For example, "The system shall correctly display 99.999% of hotel information."
- Rate of failure.
- This is the same as "failure on demand" but it is measured as a percentage of time. For example, "The system may fail to report hotel information correctly no more than twice a year."
- Availability.
- What percentage of time may the system be unavailable for use? For example, "The hotel reservation system shall be available 99.99% of the time in any given calendar year."
Reference:
Sommerville, I., "Software Engineering," Addison-Wesley, Reading, MA, 1992.
