Simple atomic locks and Master Election with RethinkDB and Celluloid

master.rb

require 'rethinkdb'
require 'celluloid'

include RethinkDB::Shortcuts

LOCK_TIMEOUT = 15
UPDATE_TIMEOUT = 5

r.connect.repl

r.branch(r.table_list.contains('locks'),
        r.expr(created: 1),
        r.table_create('locks')).run


class MasterElectionExample

  include Celluloid
  attr_reader :is_master

  def initialize
    @is_master = false
    request_master_election
    @timer = every(UPDATE_TIMEOUT) do
      if @is_master
        puts 'i am the master, updating lock'
        update_lock 'master'
      else
        request_master_election
      end
    end
  end

  def aquire_lock?(lock_name)
    res = r.branch(r.table('locks').get(lock_name),
            r.do(r.table('locks').get(lock_name)) {|lock|
            r.branch(lock[:time] > (r.now - LOCK_TIMEOUT),
              r.expr(unchanged: 1, replaced: 0, inserted: 0),
              r.table('locks').replace(id: lock_name, time: r.now))},
            r.table('locks').insert(id: lock_name, time: r.now)
          ).run
    res['inserted'] == 1 || res['replaced'] == 1
  end

  def update_lock(lock_name)
    r.table('locks').update({id: lock_name, time: r.now}).run
  end

  def request_master_election
    if aquire_lock? 'master'
      @is_master = true
      puts 'i am the master. :)'
    else
      @is_master = false
      puts 'i am not the master. :('
    end
  end

end

class MasterGroup < Celluloid::SupervisionGroup
  supervise MasterElectionExample, as: :actor
end

MasterGroup.run