Update notification event Target fields

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

registry.go

@@ -95,10 +95,12 @@ type Layer interface {
 	io.ReadSeeker
 	io.Closer
 
-	// Digest returns the unique digest of the blob, which is the tarsum for
-	// layers.
+	// Digest returns the unique digest of the blob.
 	Digest() digest.Digest
 
+	// Length returns the length in bytes of the blob.
+	Length() int64
+
 	// CreatedAt returns the time this layer was created.
 	CreatedAt() time.Time
 }
@@ -134,3 +136,24 @@ type SignatureService interface {
 	// Put stores the signature for the provided digest.
 	Put(dgst digest.Digest, signatures ...[]byte) error
 }
+
+// Descriptor describes targeted content. Used in conjunction with a blob
+// store, a descriptor can be used to fetch, store and target any kind of
+// blob. The struct also describes the wire protocol format. Fields should
+// only be added but never changed.
+type Descriptor struct {
+	// MediaType describe the type of the content. All text based formats are
+	// encoded as utf-8.
+	MediaType string `json:"mediaType,omitempty"`
+
+	// Length in bytes of content.
+	Length int64 `json:"length,omitempty"`
+
+	// Digest uniquely identifies the content. A byte stream can be verified
+	// against against this digest.
+	Digest digest.Digest `json:"digest,omitempty"`
+
+	// NOTE: Before adding a field here, please ensure that all
+	// other options have been exhausted. Much of the type relationships
+	// depend on the simplicity of this type.
+}