Start on a basic registry_model interface and change a single module to use it. This will allow us to completely abstract out how we deal with registry-related tables and ensure that transitioning to the new OCI-like model will be easier to do.

data/registry_model/interface.py

@@ -0,0 +1,21 @@
+from abc import ABCMeta, abstractmethod
+from six import add_metaclass
+
+@add_metaclass(ABCMeta)
+class RegistryDataInterface(object):
+  """ Interface for code to work with the registry data model. The registry data model consists
+      of all tables that store registry-specific information, such as Manifests, Blobs, Images,
+      and Labels.
+  """
+
+  @abstractmethod
+  def find_matching_tag(self, repository_ref, tag_names):
+    """ Finds an alive tag in the repository matching one of the given tag names and returns it
+        or None if none.
+    """
+
+  @abstractmethod
+  def get_most_recent_tag(self, repository_ref):
+    """ Returns the most recently pushed alive tag in the repository, if any. If none, returns
+        None.
+    """