Reading 'A Generalised Solution to Distributed Consensus'
21 Feb 2019
This paper generalizes Paxos and Fast Paxos into a model of write-once registers and provides three consensus algorithms for this model.
What caught my eye in this paper is this quote from the first page
This paper aims to improve performance by offering a generalised solution allowing engineers the flexibility to choose their own trade-offs according to the needs of their particular application and deployment environment.
This jibes with the goal of reflective consistency. Developers using reflective consistency can choose their own tradeoffs on the correctness/time curve. They can sacrifice the consistency of their system to improve its reactiveness.
Instead of the correctness/time curve, each of the alternative algorithms proposed in this paper fall somewhere on a curve balancing tradeoffs between Paxos and Fast Paxos.
Generalized Distributed Consensus
The authors reframe distributed consensus into the processing of write-once registers. They separate processes into servers, storing values, and clients, getting and putting values. They state that solving consensus means ensuring all of the following:
Non-triviality - All output values must have been the input value of a client.
Agreement - All clients that output a value must output the same value.
Progress - All clients must eventually output a value if the system is reliable and synchronous for a sufficient period.
Their general solution organizes write-once registers into the following taxonomy
- Quorums: non-empty sets of servers.
- Decided register values: when a quorum has identical, non-nil values for any register.
- Register sets: the sets containing a copy of a single register from each server.
They then go on to define a general algorithm to establish consensus. I could not keep up with the discussion of the algorithm, but it seemed well-grounded. In the end they offer proofs for their algorithms but passingly mention that one goal of this work is to create a general enough model for distributed consensus so that the correctness of its algorithms become obvious. Sounds tough to me, but what a noble goal!
Modern distributed consensus often means Raft. I’m curious if this could generalize raft in addition to Paxos and Fast Paxos.
It’d be interesting to see a benchmark comparing the different example consensus algorithms proposed in section 6.