public function onUpdate() {
    
    if (file_exists($this->savedTimestampPath)) {
      $readTimestamp = file_get_contents($this->savedTimestampPath);
      if ($readTimestamp != $this->savedTimestamp) {
        foreach ($this->clients as $sendto) {
          $sendto->send($readTimestamp);
        }
        $this->savedTimestamp = $readTimestamp;
      }
    }
    
  }