Here, Parnas steps aside from software correctness and formal proof of programs and discusses another problem: Is a program that outputs correct useful if we cannot rely on it when we demand?
He introduces the term Reliability “a measure of the extent to which the system can be expected to deliver usable services when those services are demanded”. In other words, a system is considered to be highly reliable, if it is highly probable that, when we demand a service from the system, it will perform to our satisfaction.
Software structure may harm reliability when build upon the wrong assumption that nothing can go wrong. Parnas consider some situations that can influence reliability, among them the influence of external dependencies and the correctness of the software itself.
The author explains that the error detection and handling mechanism is often neglected or poorly done. It is important that the interface between modules enable communication about errors as well. The means to express this possibility of errors between interrelated modules seems to be well solved, e.g., by Exceptions and try-catch blocks in modern languages such as Java. Still, the way to use them correctly is easily overlooked.
Nowadays, software architecture studies contemplate a whole bunch of other attributes of software architectures. Yet, the influence of software structure on reliability is still a hot topic in software architecture. And although some of the early questions can be answered, new ones arrive.
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