# File lib/mongo/cluster.rb, line 575
    def add(host, add_options=nil)
      address = Address.new(host, options)
      if !addresses.include?(address)
        server = Server.new(address, self, @monitoring, event_listeners, options.merge(
          monitor: false))
        @update_lock.synchronize { @servers.push(server) }
        if add_options.nil? || add_options[:monitor] != false
          server.start_monitoring
        end
        server
      end
    end