diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index 7155bfdc6..8656455f6 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -644,6 +644,10 @@ class RoomMemberStore(SQLBaseStore):
 
 
 class _JoinedHostsCache(object):
+    """Cache for joined hosts in a room that is optimised to handle updates
+    via state deltas.
+    """
+
     def __init__(self, store, room_id):
         self.store = store
         self.room_id = room_id
@@ -658,6 +662,11 @@ class _JoinedHostsCache(object):
 
     @defer.inlineCallbacks
     def get_destinations(self, state_entry):
+        """Get set of destinations for a state entry
+
+        Args:
+            state_entry(synapse.state._StateCacheEntry)
+        """
         if state_entry.state_group == self.state_group:
             defer.returnValue(frozenset(self.hosts_to_joined_users))
 
diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 01474ff5f..c3eecbe82 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -99,6 +99,12 @@ class StateStore(SQLBaseStore):
         )
 
     def get_state_group_delta(self, state_group):
+        """Given a state group try to return a previous group and a delta between
+        the old and the new.
+
+        Returns:
+            (prev_group, delta_ids), where both may be None.
+        """
         def _get_state_group_delta_txn(txn):
             prev_group = self._simple_select_one_onecol_txn(
                 txn,