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Slides by Prof. Jennifer Welch

Slides by Prof. Jennifer Welch. Consensus Problem. Every processor has an input. Termination: Eventually every nonfaulty processor must decide on a value. decision is irrevocable! Agreement: All decisions by nonfaulty processors must be the same.

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Slides by Prof. Jennifer Welch

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  1. Set 9: Fault Tolerant Consensus Slides by Prof. Jennifer Welch

  2. Consensus Problem • Every processor has an input. • Termination: Eventually every nonfaulty processor must decide on a value. • decision is irrevocable! • Agreement: All decisions by nonfaulty processors must be the same. • Validity: If all inputs are the same, then the decision of a nonfaulty processor must equal the common input. Set 9: Fault Tolerant Consensus

  3. Agreement Condition • The agreement condition above requires “exact agreement” • The output must be exactly equal Set 9: Fault Tolerant Consensus

  4. FLP Impossibility Result • Fischer, Lynch, Paterson (FLP) showed that consensus (satisfying above conditions) is impossible in an asynchronous system where any one process may fail • No bound on message delay Set 9: Fault Tolerant Consensus

  5. Circumventing FLP To circumvent FLP, we must relax one of the conditions required above for exact consensus: Examples: • Approximate agreement (instead of exact) • Sometimes algorithm may not terminate in finite time Set 9: Fault Tolerant Consensus

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