code: func ValidateNode(node *api.Node) field.ErrorList {
  	allErrs := ValidateObjectMeta(&node.ObjectMeta, false, ValidateNodeName, field.NewPath("metadata"))
  
  	// Only validate spec. All status fields are optional and can be updated later.
  
  	// external ID is required.
  	if len(node.Spec.ExternalID) == 0 {
  		allErrs = append(allErrs, field.Required(field.NewPath("spec", "externalID"), ""))
  	}
  
  	// TODO(rjnagal): Ignore PodCIDR till its completely implemented.
  	return allErrs
  }
docs: // ValidateNode tests if required fields in the node are set.