Prospective development 2: software-fault tolerance

This is a preliminary study on the topic of the tolerance of design faults, and in particular of software bugs, in Delta-4 systems. At the beginning of the Delta-4 project, a design assumption was made to the effect that only physical faults were to be taken into account when providing fault-tolerant mechanisms: the possibility of design faults could therefore be neglected. Now that the project has produced a viable scheme for the treatment of physical faults, it is worthwhile to explore how the schema can (or needs to) be extended to take into account design faults as well. The most common type of design faults are software bugs, and therefore the main topic of this study is software-fault tolerance, although we expect that hardware design faults can be dealt with by mechanisms very similar to those studied here. The components of this study, in this preliminary version, are: 1) a brief history of software-fault tolerance, and motivations for its importance; 2) motivations for studying software-fault tolerance for Delta-4. A further part of this work will be a description of the current state of the art, and the open technical problems in the field. The study will be completed with a discussion of the specific problems arising in the application of software-fault tolerance in Delta-4 systems, the techniques that appear most promising in this environment, and the extensions they require to the current Delta-4 specifications.