Consensus Queries

Configuration process robot failures communications

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Unformatted text preview: cess (robot) failures • communications failures Two Flavors: Communication: • bounded quantity: “At most one robot will fail” • probabilistic “Messages arrive with probability p” • total number of messages • messages per robot per round (bandwidth) 2008-01-06 CSE481C wi09 - Robotics Capstone, Lec3: Consensus 16 2008-01-06 CSE481C wi09 - Robotics Capstone, Lec3: Consensus 17 Leader Election Requirements: • one process becomes leader • other processes become not-leader Bonus Requirement: • all processes know which one is the leader leader election 2008-01-06 CSE481C wi09 - Robotics Capstone, Lec3: Consensus 19 2008-01-06 CSE481C wi09 - Robotics Capstone, Lec3: Consensus approaches Problems 1. All processes start with same initial state 20 How to deal with removal of leader? • If you have two identical processes, design an algorithm to elect one of them a leader. • But only one execution possible on both processes • Can’t break symmetry Impossibility proof – not possible to elect leader How to deal with multiple leaders? How to elect two leaders? • Running time and communications complexity? k leaders? 2. Randomized Algorithm • • • • • Running time and communications complexity? 1 random bit 50/50 change of electing leader on each flip How long will it take if graph is fully connected? How long will it take if graph is not fully connected? 3. Unique IDs • break symmetry with deterministic algorithm • can elect leader in bounded time how long will it take? 2008-01-06 CSE481C wi09 - Robotics Capstone, Lec3: Consensus 21 2008-01-06 CSE481C wi09 - Robotics Capstone, Lec3: Consensus 22 3 What is consensus? Simple: • All processes agree on a quantity • All processes know that they agree Formal: • Agreement: no two processes decide on different values • Validity consensus 1. If all processes start with 0, then 0 is the only possible decision value 2. If all processes start with 1 and all messages are delivered, then 1 is the only possible decision value • Termination All processes eventually decide 2008-01-06 CSE481C wi09 - Robotics Capstone, Lec3: Consensus 24 2008-01-06 CSE481C wi09 - Robotics Capstone, Lec3: Consensus 25 Whoa… Anothe...
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This document was uploaded on 03/02/2014 for the course CS 481 at Rice.

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