A formal approach to software error removal

There is a dichotomy of opinion on the use of software testing versus formal verification in software development. Testing has been the accepted method for detecting and removing errors and has played a significant error removal role. Formal verification has only recently matured into accepted practice but shows the potential for playing an even more significant error prevention role. The Cleanroom software development process which has been developed by the IBM Federal Systems Division combines both ideas into an effective development tool. Software engineering methods based on functional verification support the production of software with sufficient quality to forego traditional unit or structural testing. Statistical methods are introduced that define objective and formal strategies for product or functional testing. The synergism between the two ideas results in software with fewer errors which are both easier to find and to fix and in products with exceptional operating characteristics. Error prevention, not removal, is the key and the only viable approach to any sustained software quality growth. The Cleanroom development method and its impact on the error prevention and removal processes are covered in this paper. The results from its use for software development are also discussed.