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CHAPTER 3. KNOWN REVISIONS 57 Algorithm 9: Acceptor algorithm for Classic Paxos with termination 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 state : • vdec: decided value while true do switch do case prepare(e) received from proposer if vdec ̸= nil then /* notify proposer that decision has been reached */ send decided( vdec) to proposer else if epro = nil ∨ e ≥ epro then epro ← e send promise(e,eacc,vacc) to proposer case propose(e,v) received from proposer if vdec ̸= nil then /* notify proposer that decision has been reached */ send decided( vdec) to proposer else if epro = nil ∨ e ≥ epro then epro←e vacc ←v,eacc ←e send accept(e) to proposer case decided(v) received from proposer /* save decided value */ vdec ← v been notified of decision is up. As a result, the proposer may return a decided value after communicating with just one acceptor. Algorithms 8 and 9 provide an example of how this could be implemented into Classic Paxos. Algorithm 9 adds vdec, the decided value state to acceptors. In algorithm 8, once a proposer learns that a value v is decided, it sends decided(v) to all acceptors5. Upon receipt of decided(v), an acceptor can set the decided value vdec to v and henceforth reply to incoming messages (regardless of message type or epoch) with decided(v). All other state on the acceptor can now be safely discarded. This approach is taken by algorithms such as Mencius [MJM08, §4.2]. Figure 3.3 demonstrates how this additional phase can allow future proposers (in this case p2) to learn the decided value after communicating with just one acceptor a3. Figure 3.3 uses the same scenario as our first Classic Paxos example (Figure 2.2). This approach requires the proposer to send the value, which could be large, to all acceptors, 5This message is sometimes called learn.PDF Image | Distributed consensus
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