docs: def handle(self, *args, **options):
          """ execute sync command """
          # store verbosity level in instance attribute for later use
          self.verbosity = int(options.get('verbosity'))
          # blank line
          self.stdout.write('\r\n')
          # retrieve layers
          layers = self.retrieve_layers(*args, **options)
  
          if len(layers) < 1:
              self.stdout.write('no layers to process\n\r')
              return
          else:
              self.verbose('going to process %d layers...' % len(layers))
  
          # loop over
          for layer in layers:
              # retrieve interop class if available
              try:
                  synchronizer_path = layer.external.synchronizer_path
              except (ObjectDoesNotExist, AttributeError):
                  self.stdout.write('External Layer %s does not have a synchronizer class specified\n\r' % layer.name)
                  continue
  
              # if no synchronizer_path jump to next layer
              if synchronizer_path == 'None':
                  self.stdout.write('External Layer %s does not have a synchronizer class specified\n\r' % layer.name)
                  continue
  
              if layer.external.config is None:
                  self.stdout.write('Layer %s does not have a config yet\n\r' % layer.name)
                  continue
  
              # retrieve class
              synchronizer = import_by_path(synchronizer_path)
              self.stdout.write('imported module %s\r\n' % synchronizer.__name__)
  
              # try running
              try:
                  instance = synchronizer(layer, verbosity=self.verbosity)
                  self.stdout.write('Processing layer "%s"\r\n' % layer.slug)
                  messages = instance.sync()
              except ImproperlyConfigured as e:
                  self.stdout.write('Validation error: %s\r\n' % e)
                  continue
              except Exception as e:
                  self.stdout.write('Got Exception: %s\r\n' % e)
                  exception(e)
                  continue
  
              for message in messages:
                  self.stdout.write('%s\n\r' % message)
  
          self.stdout.write('\r\n')