code: func (az *Cloud) GetActiveZones() (sets.String, error) {
  	if az.nodeInformerSynced == nil {
  		return nil, fmt.Errorf("Azure cloud provider doesn't have informers set")
  	}
  
  	az.nodeCachesLock.Lock()
  	defer az.nodeCachesLock.Unlock()
  	if !az.nodeInformerSynced() {
  		return nil, fmt.Errorf("node informer is not synced when trying to GetActiveZones")
  	}
  
  	zones := sets.NewString()
  	for zone, nodes := range az.nodeZones {
  		if len(nodes) > 0 {
  			zones.Insert(zone)
  		}
  	}
  	return zones, nil
  }
docs: // GetActiveZones returns all the zones in which k8s nodes are currently running.