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118 7.3. EPOCHS BY RECOVERY Algorithm 23: Acceptor algorithm for binary decision 1 while true do 2 3 4 5 6 7 8 9 10 11 switch do case prepare(e) received from proposer if epro =nil∨e≥epro then epro ← e send promise(e,eacc) to proposer case propose(e) received from proposer if epro =nil∨e≥epro then epro ← e eacc ← e send accept(e) to proposer only one phase (phase one) by any proposer (Algorithm 22, lines 4-5). Likewise, we are able to skip the remainder of phase one if a promise is received with a proposal from the predecessor epoch (Algorithm 22, line 13). As the epochs directly correspond to values, v = e mod 2, the proposed/accepted value can also be omitted. Figure 7.1 shows an example of this in practice where proposer p1 wishes to commit and p2 wishes to abort. 7.3 Epochs by recovery In this chapter so far, we have proposed various techniques to maintain value uniqueness (lemma 9). In this section, however, we will consider how to remove the requirement that values are unique to epochs. Our approach which we will refer to as epochs by recovery, allows proposers to use any epoch by adding mechanisms to recover if multiple values are proposed for the same epoch. 7.3.1 Intuition We will now derive an algorithm with shared epochs by considering what will go wrong if we were to simply permit epochs to be shared in Classic Paxos. To maintain generality, we use Paxos revision B as our starting point5. Problem 1: Firstly, it is possible that multiple values are committed by different proposers with the same epoch, since each value can be accepted by non-intersecting phase two quorums. 5Later, we will consider whether we can also apply revision C as it directly uses the value uniqueness lemma.PDF Image | Distributed consensus
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