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Chapter 3 Known revisions Thus far, we have considered Classic Paxos as a single concrete algorithm to solve single valued distributed consensus. Instead however, Paxos is a broad family of algorithms for distributed consensus. In this systematisation of knowledge chapter, we survey some of the most commonly used refinements to the Classic Paxos algorithm. 3.1 Negative responses (NACKs) Classic Paxos as has been detailed so far, could be described as following the idea that “If you can’t say something nice, don’t say nothing at all”1. More specifically, acceptors will not reply to proposers whose epoch e is less than their last promised epoch epro. The result is that proposers must wait for their prepare to timeout and retry with a new epoch. This can be improved by adding negative responses, such as no-promise(e) and no-accept(e). These negative responses would be sent by acceptors to proposers upon receipt of prepare or propose messages where e < epro. When a proposer receives negative responses, it can opt to restart the proposal with a higher epoch. Otherwise, the proposer can ignore the negative responses and wait to see if they receive positive responses from a majority of participants. If a proposer receives negative responses from a majority of acceptors, then its proposal will not be successful and the proposer should restart the proposal. It is safe for a proposer to abandon or restart a proposal at any stage, since this is functionally equivalent to a proposer failing and restarting. The acceptors can include additional information in the negative responses such as the no-promise(e,f) and no-accept(e,f), where f is the acceptor’s last promised epoch or even no-promise(e,f,g,v) and no-accept(e,f,g,v), where (g,v) is the acceptor’s last accepted proposals2. 1Quote from Thumper in the Disney film Bambi. 2For example, in the Raft algorithm acceptors include their last promised epoch (referred to as cur- 49PDF Image | Distributed consensus
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