diff --git a/treemanifest/__init__.py b/treemanifest/__init__.py
--- a/treemanifest/__init__.py
+++ b/treemanifest/__init__.py
@@ -1584,7 +1584,11 @@
 
     yield wirepack.closepart()
 
-class remotetreedatastore(object):
+class generatingdatastore(object):
+    """Abstract base class representing stores which generate trees on the
+    fly and write them to the shared store. Thereafter, the stores replay the
+    lookup operation on the shared store expecting it to succeed."""
+
     def __init__(self, repo):
         self._repo = repo
         self._shared = None
@@ -1593,6 +1597,30 @@
         self._shared = shared
 
     def _gettrees(self, name, node):
+        raise NotImplemented()
+
+    def get(self, name, node):
+        self._gettrees(name, node)
+        return self._shared.get(name, node)
+
+    def getdeltachain(self, name, node):
+        self._gettrees(name, node)
+        return self._shared.getdeltachain(name, node)
+
+    def add(self, name, node, data):
+        raise RuntimeError("cannot add to a generating store")
+
+    def getmissing(self, keys):
+        return keys
+
+    def markledger(self, ledger, options=None):
+        pass
+
+    def getmetrics(self):
+        return {}
+
+class remotetreedatastore(generatingdatastore):
+    def _gettrees(self, name, node):
         # Only look at the server if not root or is public
         basemfnodes = []
         if name == '':
@@ -1612,26 +1640,6 @@
         prefetchtrees(self._repo, name, [node], basemfnodes, [])
         self._shared.markforrefresh()
 
-    def get(self, name, node):
-        self._gettrees(name, node)
-        return self._shared.get(name, node)
-
-    def getdeltachain(self, name, node):
-        self._gettrees(name, node)
-        return self._shared.getdeltachain(name, node)
-
-    def add(self, name, node, data):
-        raise RuntimeError("cannot add to a remote store")
-
-    def getmissing(self, keys):
-        return keys
-
-    def markledger(self, ledger, options=None):
-        pass
-
-    def getmetrics(self):
-        return {}
-
 def serverrepack(repo, incremental=False, options=None):
     packpath = repo.vfs.join('cache/packs/%s' % PACK_CATEGORY)