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CHAPTER 3. KNOWN REVISIONS 53 p1 a1 a2 a3 p2 epro:0 eacc :0 vacc :A epro:0 eacc :0 vacc :A epro:1 epro:0 eacc :0 vacc :A epr epro:1 e : 1, E : {3, . . . } prepare(1) promise(1,0,A) QP :{a3},QV :{a3},emax :0,v:A promise(1,0,A) QP : {a2, a3}, QV : {a2, a3} o:1 Figure 3.2: Classic Paxos with bypass (Alg. 4,7) There are therefore three possible outcomes of the proposer’s phase one: Decision not reached - No proposals were received with promises during phase one, therefore no value has yet been decided. The proposer will propose its candidate value in phase two. Decision reached - All promises received in phase one agreed on a value. This value has been decided and the proposer has learned the chosen value. No further action is necessary. Uncertainty - Some proposals were returned in phase one. The proposer is uncertain if commit point has been reached. If reached, then the decided value is the value returned with the highest epoch so the proposer therefore proposes this value. Algorithm 7 gives a version of the proposer algorithm which bypasses phase two when it learns that a decision has been reached. This is achieved by maintaining a set of acceptors, QV , who have promised and returned the proposal (emax, v) with their promise (lines 3,12,15,16). Once phase one is completed, if QV includes the majority of acceptors then phase two can be bypassed (lines 18-19). Note that the acceptor algorithm is unchanged. Figure 3.2 demonstrates how our first Classic Paxos example (Figure 2.2) could be improved using phase two bypass. Proposer p2 is able to skip over phase two as it learns that the proposal (0,A) has already been decided since it has been accepted by a majority of acceptors.PDF Image | Distributed consensus
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