This volume summarizes the results obtained by the groupworking on softwarefault tolerance within the REQUEST(Reliability and Quality of European Software Technology)project of the ESPRIT programme of the European Communities. It should be read by anyone with a professional interest insafety-critical and fault-tolerant computing. A generic model is developed for evaluating the reliabilityof fault-tolerant software systems. Emphasis is put onidentification of problem areas in the development andassessment of fault-tolerant software systems and in thecomponents. Examples of crucial failures are those ofdiverse versions due to a common cause, or failures in theadjudicator which acts on outputs of diverse versions. Thecauses for common failures of versions are similarities inthe solutions of specified problems. Methods were developedto determine similarity among versions by means ofwell-known software engineering methods. Concerningadjudicators, the influences of several factors on failuredetection capability are discussed and guidelines are givenfor optimal design. A methodology is developed to determinedissimilarity on the level of diverse specifications.
Cost-based support is given for deciding whether diversityshould be used in a software system or a single programshouldbe enhanced by additional verification effort.