# File lib/nanite/cluster.rb, line 154
    def rr(service, tags=[])
      @last ||= {}
      @last[service] ||= 0
      candidates = nanites_providing(service,tags)
      return [] if candidates.empty?
      @last[service] = 0 if @last[service] >= candidates.size
      candidate = candidates[@last[service]]
      @last[service] += 1
      [candidate]
    end