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28 2.2. CLASSIC PAXOS the proposer can propose the candidate value γ. If the outcome of phase one is that a value might already be decided, then that value must be proposed in phase two instead. Each of these two phases requires a majority of acceptors to agree in order to proceed. We now define the terms epoch and proposal and then use these to summarise the Classic Paxos algorithm. Definition 2. An epoch e is any member of the set of epochs E. E is any infinite totally ordered set such that the operators <, > and = are always defined 9. Definition 3. A proposal (e,v) is any epoch and value pair10. Classic Paxos Phase 1 1. A proposer chooses a unique epoch e and sends prepare(e) to the acceptors. 2. Each acceptor stores the last promised epoch and last accepted proposal. When an acceptor receives prepare(e), if e is the first epoch promised or if e is equal to or greater than the last epoch promised, then e is written to storage and the acceptor replies with promise(e,f,v). (f,v) is the last accepted proposal (if present) where f is the epoch and v is the corresponding proposed value. 3. Once the proposer receives promise(e, , ) from the majority of acceptors, it proceeds to phase two. Promises may include a last accepted proposal which will be used by the next phase. 4. Otherwise if the proposer times out, it will retry with a greater epoch. Classic Paxos Phase 2 1. The proposer must now select a value v using the following value selection rules: i If no proposals were returned with promises in phase one, then the proposer will choose its candidate value γ. ii If one proposal was returned, then its value is chosen. iii If more than one proposal was returned then the proposer must choose the value associated with the greatest epoch. The proposer then sends propose(e,v) to the acceptors. 9Epochs are also referred to as terms [OO14, §5.1], view numbers [LC12, §3], round numbers [MPSP10, §3], instance values/epoch [HKJR10, §1] or ballot numbers in the literature 10Proposals are also referred to as ballots in the literaturePDF Image | Distributed consensus
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