Chapter8 - Chapter 8 Reasoning with Knowledge Ajay...

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Chapter 8: Reasoning with Knowledge Ajay Kshemkalyani and Mukesh Singhal Distributed Computing: Principles, Algorithms, and Systems Cambridge University Press Reasoning with Knowledge CUP 2008 1 / 29
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Distributed Computing: Principles, Algorithms, and Systems Muddy Children Puzzle (Scenario A) n children, all intelligent, can see others but not their own faces k ( n ) have mud on their forehead Scenario A: Father says:: ψ : ”At least one of you has mud on the forehead.” Father then repeatedly asks (i.e., broadcasts) in rounds (to model synchronous operation) to the assembled children: I Do you have mud on your forehead? How does each child respond in each round, r = 1 , 2 ,... k - 1 , k , k + 1 n , n + 1 ? An answer is ”broadcast” in that round. Let c = clean child, d = dirty child k = 0: contradicts ψ k = 1: In r = 1, the d answers ”Yes”. For r = 2, the c answer ”No”. k = 2: In r = 1, no responses. In r = 2, both d answer ”Yes”. In r = 3, the c answer ”No” k = 3: In r = 1 , 2, no responses. In r = 3, the 3 d answer ”Yes”. In r = 4, the n - 3 c answer ”No”. k n : In r < k , no responses. In r = k , the k d answer ”Yes”. In r = k + 1, the n - k c answer ”No”. Reasoning with Knowledge CUP 2008 2 / 29
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Distributed Computing: Principles, Algorithms, and Systems Muddy Children Puzzle (Scenario A) n children, all intelligent, can see others but not their own faces k ( n ) have mud on their forehead Scenario A: Father says:: ψ : ”At least one of you has mud on the forehead.” Father then repeatedly asks (i.e., broadcasts) in rounds (to model synchronous operation) to the assembled children: I Do you have mud on your forehead? How does each child respond in each round, r = 1 , 2 ,... k - 1 , k , k + 1 n , n + 1 ? An answer is ”broadcast” in that round. Let c = clean child, d = dirty child k = 0: contradicts ψ k = 1: In r = 1, the d answers ”Yes”. For r = 2, the c answer ”No”. k = 2: In r = 1, no responses. In r = 2, both d answer ”Yes”. In r = 3, the c answer ”No” k = 3: In r = 1 , 2, no responses. In r = 3, the 3 d answer ”Yes”. In r = 4, the n - 3 c answer ”No”. k n : In r < k , no responses. In r = k , the k d answer ”Yes”. In r = k + 1, the n - k c answer ”No”. Reasoning with Knowledge CUP 2008 2 / 29
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Distributed Computing: Principles, Algorithms, and Systems Muddy Children Puzzle (Scenario A) n children, all intelligent, can see others but not their own faces k ( n ) have mud on their forehead Scenario A: Father says:: ψ : ”At least one of you has mud on the forehead.” Father then repeatedly asks (i.e., broadcasts) in rounds (to model synchronous operation) to the assembled children: I Do you have mud on your forehead? How does each child respond in each round, r = 1 , 2 ,... k - 1 , k , k + 1 n , n + 1 ? An answer is ”broadcast” in that round. Let c = clean child, d = dirty child k = 0: contradicts ψ k = 1: In r = 1, the d answers ”Yes”. For r = 2, the c answer ”No”.
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Chapter8 - Chapter 8 Reasoning with Knowledge Ajay...

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