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Lookup NU author(s): Dr Alan Tully
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N-Modular Redundancy (NMR) is a form of active replication in which eaC!l processor is replicated to form a node and each processor replica within the node executes the same set of software component replicas. Communication between nodes, in the form of messages, passes through a voting mechanism by which processor failures are masked. When the degree of replication is three, the technique is known as Triple Modular Redundancy (TMR) and can tolerate the failure of a single node processor. For voting to be successful, non-faulty software component replicas must output identical messages in an identical order. If we assume that software components are deterministic, then we need only ensure that the replicas process identical input messages in an identical order. Such software components conform to the well understood and researched state machine model of active replication. However, most distributed programs employ mechanisms not incorporated in the state machine model such as timeouts and prioritized messages. These potential sources of non-determinism could lead to a divergence of state among software component replicas which could then produce inconsistent responses to identical input messages, thereby defeating the NMR voting mechanism.
Author(s): Tully A
Publication type: Report
Publication status: Published
Series Title: Computing Laboratory Technical Report Series
Year: 1991
Report Number: 328
Institution: Computing Laboratory, University of Newcastle upon Tyne
Place Published: Newcastle upon Tyne
