Raft

This is a Ruby implementation of the Raft algorithm.

Raft is a distributed consensus algorithm designed to be easy to understand. The algorithm is the work of Diego Ongaro and John Ousterhout at Stanford University. The implementation here is based upon this paper:

https://ramcloud.stanford.edu/wiki/download/attachments/11370504/raft.pdf

Many thanks to the authors for their hard work!

Technical Design

This gem provides a Raft::Node class that handles log replication across a cluster of peer nodes. Design decisions about the RPC protocol, concurrency mechanism, error handling and data persistence are left to the client.

For convenience and testing, an example implementation is provided based on Goliath and EventMachine, with in-memory data persistence. Contributions of further examples are very welcome!

Usage

Install the gem:

gem install raft

In your code, add a require:

require 'raft'

If you'd like to use the example Goliath implementation, add:

require 'raft/goliath'

Raft replicates commands across a cluster of nodes. Each node in the cluster is aware of every other node in the cluster. Let's create a new cluster and define its configuration:

@cluster = Raft::Cluster.new('alpha', 'beta', 'gamma')

@config = Raft::Config.new(
  rpc_provider,       # see Raft::RpcProvider
  async_provider,     # see Raft::AsyncProvider
  election_timeout,   # in seconds
  election_splay,     # in seconds
  update_interval,    # in seconds
  heartbeat_interval) # in seconds

Now we can create Raft nodes for each node defined in the cluster:

@nodes = @cluster.node_ids.map do |node_id|
  Raft::Node.new(node_id, @config, @cluster)
end

Since the concurrency mechanism is left to the client, you must call Raft::Node#update regularly to allow the node to participate in the cluster:

# Threaded example:
@update_threads = @nodes.map do |node|
 Thread.new do
   while true
     node.update
     sleep(node.config.update_interval)
   end
 end
end

# Evented example
@update_timers =  @nodes.map do |node|
  EventMachine.add_periodic_timer(node.config.update_interval) do
    EM.synchrony do
      node.update
    end
  end
end

We can send commands (which are strings) to the cluster and they will be appended to the command log, which will be replicated across the cluster.

command = 'example'
request = Raft::CommandRequest.new(command)
node = @nodes.sample
response = node.handle_command(request) # response is a Raft::CommandResponse

Raft::Node#handle_command will not return success until the command has been replicated to a majority of nodes, so that it is considered committed and is safe to execute.

If you would like to execute commands as they are committed, you can assign a commit handler for each node:

@nodes.each do |node|
  node.commit_handler = Proc.new do |command|
    puts "Node #{node.id} executing command #{command}!"
  end
end

Issues and Feedback

If you encounter problems with this gem, please feel free to raise an issue in the GitHub issue tracker.

Suggestions are also welcome. For general (non-actionable) feedback, try Twitter.

Contributing

Fork this repository and make a pull request!

License

Raft is released under the MIT license.