On the relation between design contracts and errors: a software development strategy

When designing a software module or system, a systems engineer must consider and differentiate between how the system responds to external and internal errors. External errors cannot be eliminated and must be tolerated by the system, while the number of internal errors should be minimized and the resulting faults should be detected and removed. This paper presents a development strategy based on design contracts and a case study of an industrial project in which the strategy was successfully applied. The goal of the strategy is to minimize the number of internal errors during the development of a software system while accommodating external errors. A distinction is made between weak and strong contracts. These two types of contracts are applicable to external and internal errors, respectively. According to the strategy, strong contracts should be applied initially to promote the correctness of the system. Before releasing, the contracts governing external interfaces should be weakened and error management of external errors enabled. This transformation of a strong contract to a weak one is harmless to client modules.