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Chapter 5 Promises revised Classic Paxos (Chapter 2) requires proposers to wait until they have received promises from a majority of acceptors before proposing a value in phase two of the algorithm. In the last chapter (Chapter 4), we refined this to require proposers to wait until they have received promises from a phase two quorum of acceptors for each previous epoch before proceeding. Classic Paxos, and our revisions thus far, all require a proposer to wait for sufficient promises before proceeding, regardless of the content of the promises received1. In this chapter, we will demonstrate that the information learned from the promises received can be utilised to improve the flexibility of these algorithms. We will prove that proposers can safely proceed to phase two early depending on the content of the promises received in phase one. 5.1 Intuition Paxos revision B requires that a proposer’s phase one quorum must intersect with all possible phase two quorums for each previous epoch. This is because the proposer has no knowledge of which phase two quorums were used by other proposers. Consider what happens when a proposer receives promise(e,f,v) from an acceptor during phase one for epoch e. This proposer has learned that if a decision was reached in epoch f then the value chosen was v. This proposer need not wait for promises from all phase two quorums of f, Qf2 , as they will not return a promise with same epoch but a different value (Corollary 9.1). Moreover, now that the proposer knows that value v was proposed in epoch f then the proposer does not need to intersect with phase two quorums associated with previous epochs < f. 1The exception to this statement is bypassing phase two when a majority of proposers promise with the same proposal (§3.2). 91PDF Image | Distributed consensus
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