def close
      return if sessions.empty?
      sessions.each do |session|
        session.close
        session.delete_observer self
      end
      # TODO: should close database connection
    end
def close
      return if sessions.empty?
      sessions.each do |session|
        session.close
        session.delete_observer self
      end
      # TODO: should close database connection
    end
Closes all sessions.