public function touch($key, $expiration = 0) {
    $result = $this->get($key, null, $cas_token);
    if ($result) {
      $result = $this->cas($cas_token, $key, $result, $expiration);
    }
    $this->result_code = $result ? self::RES_SUCCESS : self::RES_FAILURE;
    return $result;
  }