public function dbInit()
    {
      $mongo_index = ( (phpversion('mongo') >= '1.5.0') ? ('createIndex') : ('ensureIndex') );
      $this->log("maint: {$mongo_index} on ".$this->getConfig('collection'));
      $this->sessions->$mongo_index(array(
                                          'last_accessed' => 1
                                          ));
      $this->log("maint: {$mongo_index} on ".$this->getConfig('lockcollection'));
      $this->locks->$mongo_index(array(
                                       'created' => 1
                                       ));
    }