Distributed consensus

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Distributed consensus ( distributed-consensus )

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CHAPTER 2. CONSENSUS & CLASSIC PAXOS 35 p1 a1 a2 a3 p2 epro:0 eacc:0 epro:0 vacc :A epro:0 epr eacc :1 vacc :B o:1 (1, B) decided epro:1 eacc :1 vacc :B epro:1 eacc :1 vacc :B e : 1, E : {3, . . . } prepare(1) promise(1,nil,nil) QP : {a3} promise(1,nil,nil) QP :{a2,a3} propose(1,B) accept(1) QP :{a3} accept(1) QP :{a2,a3} Figure 2.4: Example run of Classic Paxos where proposer p1 stops during phase two prior to reaching the commit point. Proposer p2 does not observe the proposal from p1. Initially, in Figure 2.3, proposer p2 has executed Classic Paxos and the proposal (1, B) has been decided and accepted by all acceptors. Subsequently the proposer p1 executes phase one for epoch 0, however this phase is unsuccessful. The proposer p1 retries Classic Paxos and the proposal (2,B) is decided. Unlike before, proposer p1 in this example required three phases to learn the decided value. Figures 2.4 and 2.5 illustrate two possible outcomes if a proposer (in this case p1) stops after making a proposal (in this case (0, A)) but prior to reaching the commit point. In Figure 2.4, proposer p2 does not observe the proposal (0, A) during its phase one thus the proposal (1,B) is subsequently decided. In contrast, in Figure 2.5 the proposer p2 does observe the proposal (0, A) during its phase one thus the proposal (1, A) is subsequently decided. The examples thus far have demonstrated proposers executing Classic Paxos in serial. In Figure 2.6, we observe the worst case scenario of Classic Paxos when concurrent proposers

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