rest.Storage interface contains only one method. This page show how could it utilize golang reflection to do the job.
Why rest.Storage interface have only one method
While reading Kubernetes source code, I found one question really bothered me for several days. The common interface of rest storage is defined in /vendor/k8s.io/apiserver/pkg/registry/rest/rest.go:
type Storage interface {
// New returns an empty object that can be used with Create and Update after request data has been put into it.
// This object must be a pointer type for use with Codec.DecodeInto([]byte, runtime.Object)
New() runtime.Object
}
You can see there’s only one method New defined here. So how could the other actions, like Get or List etc., be completed?
After several day’s search on the internet, I finally found the some code in /vendor/k8s.io/apiserver/pkg/endpoints/installer.go:
func (a *APIInstaller) registerResourceHandlers(path string, storage rest.Storage, ws *restful.WebService) (*metav1.APIResource, *storageversion.ResourceInfo, error) {
.....
// what verbs are supported by the storage, used to know what verbs we support per path
creater, isCreater := storage.(rest.Creater)
namedCreater, isNamedCreater := storage.(rest.NamedCreater)
lister, isLister := storage.(rest.Lister)
getter, isGetter := storage.(rest.Getter)
getterWithOptions, isGetterWithOptions := storage.(rest.GetterWithOptions)
gracefulDeleter, isGracefulDeleter := storage.(rest.GracefulDeleter)
collectionDeleter, isCollectionDeleter := storage.(rest.CollectionDeleter)
updater, isUpdater := storage.(rest.Updater)
patcher, isPatcher := storage.(rest.Patcher)
watcher, isWatcher := storage.(rest.Watcher)
connecter, isConnecter := storage.(rest.Connecter)
storageMeta, isMetadata := storage.(rest.StorageMetadata)
storageVersionProvider, isStorageVersionProvider := storage.(rest.StorageVersionProvider)
see here, registerResourceHandlers utilizes golang type conversion to check the storage’s abilities. So the New method is only a sign of rest.storage interface. The real abilities could be defined in resource storage struct as needed. The methods defined in each specific resource storage type represents its ability in the same time.