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4 Fault Tolerance Mechanisms
The only fault tolerance approach for combating common-mode design errors is design diversity--the implementation of more than one variant of the function to be performed [Avizienis 84]. For computer-based applications, it is generally accepted that it is more effective to vary a design at higher levels of abstraction (i.e., by varying the algorithm or physical principles used to obtain a result) than to vary implementation details of a design (i.e. by using different programming languages or low level coding techniques). Since diverse designs must implement a common system specification, the possibility for dependencies always arises in the process of refining the specification to reflect difficulties uncovered in the implementation process. Truly diverse designs would eliminate dependencies on common design teams, design philosophies, software tools and languages, and even test philosophies. Many approaches attempt to achieve the necessary independence through randomness, by creating separate design teams that remain physically separate throughout the design, implementation, and test process. Recently, some projects have attempted to create diversity by enforcing differing design rules for the multiple teams.
A Conceptual Framework for Systems Fault Tolerance - 30 MAR 95
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