A discussion of practices for enhancing diversity in software designs

By Lorenzo Strigini and Bev Littlewood

ABSTRACT
This report discusses the practices which have been used or recommended for increasing the degree of diversity between redundant implementations of software or software-based systems. Its purpose is to give useful indications for designers, project managers and safety/reliability assessors in deciding about how great an advantage should be expected from the use of these practices, in absolute and in comparative terms. Existing knowledge does not allow one to state any strong general recommendations, but it is possible to improve on the intuitive justifications usually given for these various practices. This report clarifies the ways the various practices are conjectured to aid system reliability, the factors that should affect their efficacy, and thus, for a practitioner, the aspects of a specific project situation that need to be considered to inform decisions.
Thus this report is meant to improve on the many recommendations available in the literature by a more rigorous analysis of the support available for individual recommendations and for decision between them, on the basis of existing known evidence about diversity, of general experience in software engineering and of the result of our reliability modelling work. An executive summary gives the highlights of the report and a guide to the topics treated. The other sections are an introduction giving the scope and background of the work; a general analysis of the factors affecting the achievement of useful diversity and the criteria for choosing among possible "diversity-seeking decisions" to this end, and a summary table of the considerations applying to each category of "diversity-seeking decisions", with explanations of detail in an appendix.
This report updates and supersedes the previous DISPO project report AT_DI-D-01-v1.7, "A list of intuitive diversity enhancing measures/practices", 20 February 1998, which was produced at the beginning of the DISPO project, to reflect our changed understanding at the end of the project. Parts of the old report have been eliminated as the corresponding topics are now covered by separate DISPO documents.

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