Exercise 19: Is safety violated? (Consensus)

[dag7dev]

Exercise 19: Assume that all proposers are learners as well. Let even rounds be assigned to proposers with the rules that we know. Moreover, If round $2i$ is assigned to proposer $p$, then also round $2i+1$ is assigned to proposer $p$. Odd rounds are “recovery” rounds. If round $2i$ is a fast round and if the proposer of round $2i$ sees a conflict (it is also a learner), then the proposer immediately sends an accept for round $2i+1$ with the value that has been most voted in round $2i$, without any prepare and any promise. Is safety violated? If yes, show an example. If not, demonstrate safety.

[Tommaso-Sgroi]

It doesn’t violate safety. In Paxos, the safety theorem states that if an acceptor $a$ has voted for value $v$ in round $i$, then no value $v' \neq v$ can be chosen in rounds that are smaller than $i$. In Fast Paxos, when the coordinator recognizes a conflict in round $i$, it can always start a round that is greater than $i$ in order to solve it. It doesn’t break the second rule of safety, because the value $v$ will be accepted at a greater round.