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CHAPTER 8. CONCLUSION 143 These revisions of quorum intersection, phase completion and value selection come together in section 7.3, when we remove the requirement that epochs are unique to proposals. The technique, referred to as epochs by recovery, generalises over the Fast Paxos algorithm by weakening its quorum intersection requirements. Furthermore, it applies our quorum-based value selection method to allow proposers to complete phase one with fewer promises and provides greater flexibility over the value proposed. We also proposed various alternatives to epoch allocation by recovery, such as epochs from an allocator (§7.1) or epochs by value (§7.2). These can be used instead of or together with the existing epoch allocation methods. 8.3 Implications of contributions Over the course of this thesis, we have proposed a generalised algorithm for solving dis- tributed consensus, a powerful primitive for architecting distributed systems. In §1.4, we proposed the following two research questions: Are the limitations of Paxos inherent to the problem of consensus or specific to the approach taken by the Paxos algorithm? Is the Paxos algorithm the optimal solution to distributed consensus? We believe that we have improved over the Paxos algorithm and demonstrated that some of its limitations are specific to its approach. We now discuss this further, divided into four domains, greater flexibility, new progress guarantees, improved performance and better clarity. 8.3.1 Greater flexibility The algorithm we have proposed is no ‘silver bullet’ solution. Instead, it is a flexible family of approaches for constructing a broad spectrum of consensus algorithms, suitable for many deployment environments, optimised for different workloads and offering new tradeoffs in performance and reliability. The breadth of algorithms proposed aims to reflect the diverse landscape of today’s distributed systems. The algorithms we have proposed in this thesis introduce asymmetry to Paxos, offering multiple pathways for proposers to reach termination, varying depending on the state of the system. We began by using our weakened quorum intersection requirements to introduce the notion of varying quorums by epochs. For example, in §4.2, we presented All aboard Paxos, which provided two routes for proposers (co-located with acceptors) to terminate: • termination in one round trip to all acceptors using epochs 0 to k; or • termination in two rounds trips to a majority of acceptors using epochs from k + 1.PDF Image | Distributed consensus
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