A VOLTAN node is composed of a number of conventional processors on which application-level processes are replicated to achieve fault-tolerance. The architecture of a family of such nodes with differing functionalities is presented. These include failure-masking, fail-signal, and fail-silent nodes. The software architectures of a three-processor failure-masking and a two-processor fail-silent node are discussed in detail. The suitability of VOLTAN nodes as building blocks of reliable distributed systems is also discussed.>

Replicated execution of distributed programs, which provides a means of masking hardware (processor) failures in a distributed system, is discussed. Application-level entities (processes, objects) are replicated to execute on distinct processors. Such replica entities communicate by message passing. Nondeterminism within the replicas could cause messages to be processed in nonidentical order, producing a divergence of state. Possible sources of nondeterminism are identified, and a generic mechanism for ensuring that nonfaulty replicas process messages in identical order, thereby preventing state divergence among such replicate entities, is presented.>

Replicated processing with majority voting is a well known method of achieving fault tolerance. We consider the problem of constructing a distributed system composed of an arbitrarily large number of N-modular redundant (NMR) nodes, where each node itself is composed of N, N = 2m + 1 and m ≥ 1, processing and voting elements. Advanced microprocessors, such as Inmos Transputers, provide fast serial communication links for inter-processor communication, making it possible to construct large networks of processors. We describe how replicated processing with majority voting can be achieved for such processor networks. This paper will present the overall systems architecture, including voting and NMR synchronization algorithms specially developed to exploit fast point to point communication facilities.