# File lib/mongo/util/read_preference.rb, line 55
    def select_secondary_pool(candidates, tag_sets, latency)
      tag_sets = [tag_sets] unless tag_sets.is_a?(Array)

      if !tag_sets.empty?
        matches = []
        tag_sets.detect do |tag_set|
          matches = candidates.select do |candidate|
            tag_set.none? { |k,v| candidate.tags[k.to_s] != v } &&
            candidate.ping_time
          end
          !matches.empty?
        end
      else
        matches = candidates
      end

      matches.empty? ? nil : select_near_pool(matches, latency)
    end