private function _update_next($id): bool
  {
    $id_alias = $this->db->select_one('bbn_observers', 'id_alias', ['id' => $id]);
    return $this->db->query(<<<MYSQL
      UPDATE bbn_observers
      SET next = NOW() + INTERVAL frequency SECOND
      WHERE id = ?
MYSQL
      ,
      hex2bin($id_alias ?: $id)) ? true : false;
  }