# File lib/mongo/util/node.rb, line 153
    def check_set_membership(config)
      if !config.has_key?('hosts')
        message = "Will not connect to #{host_string} because it's not a member " +
          "of a replica set."
        raise ConnectionFailure, message
      elsif config['hosts'].length == 1 && !config['ismaster'] &&
        !config['secondary']
        message = "Attempting to connect to an unhealthy, single-node replica set."
        raise ConnectionFailure, message
      end
    end