Formal justification in requirements engineering

One of the many difficult tasks in requirements engineering for software-based systems is validation. The aspect of validation dealt with in this thesis is the justification that a proposed system, once implemented, will lead to the system in its environment displaying required behaviour. The work reported in this thesis is motivated by two problems with attempts at justification: • the need to reason about dependencies in composite systems • the need to carry out the re-justification of proposals as changes are made Solutions to both of these problems are found in the definition of a temporal logic and compositional reasoning method. These are based on the assumption/commitment approach to modelling, which was originally developed for reasoning about interference in concurrent systems. The approach is applied in this thesis in the area of requirements engineering through the provision of a highly modular style of object description in which: • the achievement of each object’s commitments can be deduced from its assumptions on its environment and its assumptions on lower-level refining objects, so allowing reasoning about dependencies within composite systems, with varying degrees of rigour • the use of a compositional reasoning method ensures that the amount of work involved in re-justification is proportional to the severity of the changes that have been made The work is evaluated through a case study of a requirements engineering process. The main contribution of this work is a demonstration that the above two problems in justification can be addressed using what are effectively extensions of current formal requirements engineering languages and reasoning methods, creating a qualitativelyimproved approach to justification in requirements engineering, and so to improving confidence in a proposed system.