Losartan One contribution to knowledge of that research is t

One contribution to knowledge of that research is the POE Safety Pattern (PSP; [25]), shown on the right of Fig. 6, a process pattern for capturing high-level descriptions of system requirements and domain properties and assumptions through detailed problem models coupled with their traceable and justifiable step-wise Losartan to specifications and high-level architectural design artefacts, with the essential quality that those problem models are amenable to various forms of safety analysis. The steps of the PSP are described in Table 1. The use of the PSP will be discussed in detail in the case study. POE specialises Rogers\' definition of engineering [37] to systems engineering as: As such, systems engineering becomes a problem solving exercise, the problem being, given a physical environment E, to find the system S that meets a real-world need N to the satisfaction of a group of stakeholders K, written . Each of E, S and N are typically complex objects: E (resp. S) being formed from a collection of domains (resp. components), with N being, perhaps, a collection of use cases, user stories, requirements clauses, etc. We thus use a number of notations, graphical and otherwise, to represent and illustrate problems, from natural language, causal calculi, program code, to a problem diagram-like notation [19] (see Fig. 5). A design is a sequence of solvability preserving transformations that move a problem to known solved problems . Problem transformations relate a conclusion problem P to a collection of premise problems, P, , (), via a step rationale J. By identifying premise and conclusion problems, such transformations build into design trees. Fig. 7 shows the whole design tree for the case study, to be explicated in the sequel. During design, POE interleaves analysis in and of the problem space with synthesis in and of the solution space: Within the POE ‘toolkit’, the PSP is a form of Assurance-Driven Design (ADD; [11]) through which assurance is seen as a driving force in the design of a system rather than as a ‘bolt-on’. ADD results from the interpretation of Eq. (1) not as a relation between a conclusion problem and a set of premise problems mediated by a step rationale, but as a relationship between a (conclusion, step rationale) pair and a set of premises. This places the step rationale, and so the safety case which will be derived from it, on a par with the solution artefact: any step towards a solution must consider both assurance and product needs. Pressing assurance concerns, discovered during the exploration of J0, are then allowed to drive problem solving.
Case study Given the emerging importance of COTS within safety-critical system development and that little is known about the construction of safety cases that involve them, we speculatively investigated the application of ADD (through the PSP) to evaluate its benefits and limitations. The full development can be read in the technical report that accompanies cells paper [34]. Here, we give some highlights of the development that form the basis of the evaluation. The reader may wish to refer to Fig. 7 throughout this section, together with accompanying domain descriptions (Fig. 8), phenomena descriptions (Fig. 9) and requirements (Fig. 10).