/**
  * the singleton pattern
  *
  * @return Tinebase_User_Abstract
  */
 public static function getInstance()
 {
     if (self::$_instance === NULL) {
         $backendType = self::getConfiguredBackend();
         if (Tinebase_Core::isLogLevel(Zend_Log::DEBUG)) {
             Tinebase_Core::getLogger()->debug(__METHOD__ . '::' . __LINE__ . ' accounts backend: ' . $backendType);
         }
         self::$_instance = self::factory($backendType);
     }
     return self::$_instance;
 }