Paxos and Raft Distributed Consensus Algorithm

Paxos and Raft Distributed Consensus Algorithm

• 1.
  Paxos & Raft Distributed Consensus Algorithm Eric FU 2017-5-11
• 2.
  Leslie Lamport • Lamport is best known for his seminal work indistributed systems and as the initial developer of the document preparation system LaTeX • Microsoft Research • New York
• 3.
  Paxos • Processors operate at arbitrary speed. • Processors may experience failures. •Processors can send messages to any other processor. • Messages are sent asynchronously and may take arbitrarily long to deliver. • Messages may be lost, reordered, or duplicated. • Messages are delivered without corruption
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  There is onlyone consensus protocol, and that's Paxos – all other approaches are just broken versions of Paxos – Mike Burrows
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  Concept Roles • Proposer • Acceptor •Learner Quorums • {A,B,C}, {A,C,D}, {A,B,D}, {B,C,D} while U = {A,B,C,D} Proposal Number: N Agreed Value: V
• 6.
  Phase 1 a) A proposer selects a proposal number n and sends a prepare request with number n to a majority of acceptors. b)If an acceptor receives a prepare request with number n greater than that of any prepare request to which it has already responded, then it responds to the request with a promise not to accept any more proposals numbered less than n and with the highest- numbered proposal (if any) that it has accepted.
• 7.
  Phase 1
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  Phase 2 a) If the proposer receives a response to its prepare requests (numbered n) from a majority of acceptors, then it sends an accept request to each of those acceptors for a proposal numbered n with a value v , where vis the value of the highest-numbered proposal among the responses, or is any value if the responses reported no proposals. b) If an acceptor receives an accept request for a proposal numbered n, it accepts the proposal unless it has already responded to a prepare request having a number greater than n.
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  Phase 2
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  Demo! https://github.com/renquinn/Paxos-Demo
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  Raft • Raft is a consensus algorithm that is designed to be easy to understand • equivalent to Paxosin fault-tolerance and performance • Decomposed into relatively independent subproblems • cleanly addresses all major pieces needed for practical systems.
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  Replicated State Machine • Replicated state machines are typically implemented using a replicated log • The consensus module on a server receives commands from clients and adds them to its log •Once commands are properly replicated, each server’s state machine processes them in log order
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  Subproblems • Leader election: a new leader must be chosen when an existing leader fails. • Log replication: the leader must accept log entries •Safety: if any server has applied a particular log entry to its state machine, then no other server may apply a different command for the same log index.
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  Leader Election
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  Log Replication
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  Demo! http://thesecretlivesofdata.com/raft/
• 17.
  Thanks!