Benign failure models for shared memory

This paper introduces two benign failure models for shared memory in distributed systems, crash-omission and crash-eventual. These models are of intermediate power between the crash model and omission models of [JCT92]. (They exhibit more faulty behaviors than crash and fewer than omission.) Unlike the crash model, crash-omission is shown to have universal, gracefully-degrading constructions. That is, for any integer k, any shared object may be constructed from shared registers and consensus objects, so that: (1) if no more than k of the components suffer crash-omission failures, the constructed object exhibits no failures, and (2) if more than k of the components suffer crash-omission failures, the constructed object exhibits crash-omission failures. Simple constructions also demonstrate that registers and consensus objects in the (apparently) less benign crash-eventual model can be used to construct corresponding objects in the crash-omission model. These results are cited as evidence that the crash-omission failure model may be an appropriate choice to consider in the formulation of a more extensive theory of fault-tolerant shared objects.