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Chapter 6 Value selection revised In Classic Paxos and our revisions, the value v proposed in phase two is the value associated with the highest epoch, emax received from the acceptors. Initially, emax and v were set to nil and they were updated each time a promise was received which included a proposal with a higher epoch. Once phase one was completed, v was proposed provided it was not nil, otherwise, the proposer’s candidate value was proposed. For now on, we refer to this approach as classic value selection. In this section, however, we generalise over the classic value selection rules, by exploiting the additional insight that a proposer gains from each promise it receives. We refer to our revised technique as Quorum-based value selection and it can give proposers more flexibility when choosing a value to propose. We divide our discussion into two sections, firstly we consider the simpler case of epoch agnostic quorums (§6.1) before generalising to epoch dependent quorums (§6.1). 6.1 Epoch agnostic algorithm Algorithm 16 shows an alternative version of Paxos revision A proposer algorithm (Algo- rithm 13). The acceptor algorithm (Algorithm 4) is unchanged. Unlike the original algorithm, our new algorithm tracks the promises received from each acceptor in response to prepare(e) using R. R is a mapping from each acceptor a ∈ A to either no, meaning that no promise has yet been received or to a proposal (f, w), meaning that promise(e,f,w) has been received. Note that as per usual, (f, w) may be nil. Initially, R is set to no for all acceptors (line 5, Algorithm 16) and is updated each time a promise is received (line 10, Algorithm 16). Phase one is completed when the proposer has received a promise from at least one acceptor in each phase two quorum (line 7, Algorithm 16). After which possibleValues is passed the set of promises, R, and it returns the set of values 97PDF Image | Distributed consensus
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