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README.md

raft

Working on a single process simulation of raft for the purposes of building a generally usable full raft impementation in Clojure. This algorithm is not complete or ready to be used, the section Implementation Status below tracks current progress.

Goals

The goals of consensus are outlined here: https://www.wikiwand.com/en/Consensus_(computer_science)

The Raft Consensus Algorithm

Raft is a consensus algorithm that is designed to be easy to understand. It's equivalent to Paxos in fault-tolerance and performance. The difference is that it's decomposed into relatively independent subproblems, and it cleanly addresses all major pieces needed for practical systems.

Raft's Semantics

To establish consensus we will ensure a consistent state machine across a cluster of servers.

We will elect leader for the cluster, by a timeout-to-candidate transition. If a node has not received an update from a leader in the timeout period, it changes state from follower to candidate. With this transition, the node broadcasts a RequestVote message to the entire cluster, incrementing its own election term.

Nodes respond to a RequestVote by granting a vote if they have not yet voted in the latest term. RequestVote messages received after a vote has been granted are responded to with a false.

Once a node has received a majority of votes from the cluster, it transitions from candidate to leader. The leader then periodically broadcasts AppendEntries messages to all nodes in the cluster.

Implementation status

  • Leader Election

  • Log Replication

  • Committed entries

  • Log Compaction

Usage

Currently developing with cider, so running the code involved loading the project up and running a fn in core.

Run main and the nodes will start with the state bound in network.

Project Goals

I started this project out because I wanted to learn how raft works. Along the way, I learned that one of the reasons leader election is so difficult to implement is because of how difficult it is to simulate. I believe that the messaging layer of raft can be abstracted out such that one could run in a single process, many proccesses, or over a network, so that a consensus-based store can exist in those forms. I'm not really sure of the why yet, I'm just enjoying the challenge of designing it.

Community goals

This isn't really useful yet so I doubt any kind of communal support organization would exist around it yet. However, if this project were to become useful to others, I would add some technical goals and acceptable design tradeoffs to help organize contributors.

License

Copyright © 2016 Mikaela Patella

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.