def heartbeat
      return @heartbeat if @heartbeat
      @heartbeat = Concurrent::TimerTask.new(execution_interval: heartbeat_frequency) do
        Attention.redis.call.setex "instance_#{ @id }", Attention.options[:ttl], JSON.dump(info)
      end
      @heartbeat.execute
      @heartbeat
    end
def heartbeat
      return @heartbeat if @heartbeat
      @heartbeat = Concurrent::TimerTask.new(execution_interval: heartbeat_frequency) do
        Attention.redis.call.setex "instance_#{ @id }", Attention.options[:ttl], JSON.dump(info)
      end
      @heartbeat.execute
      @heartbeat
    end
Uses a {Concurrent::TimerTask} to periodically tell Redis that this
 server is still online
 @!visibility public
 @api private