1 files changed, 18 insertions, 2 deletions
diff --git a/synapse/storage/stream.py b/synapse/storage/stream.py
index b03fc67f71..8045e17fd7 100644
--- a/synapse/storage/stream.py
+++ b/synapse/storage/stream.py
@@ -364,9 +364,25 @@ class StreamStore(SQLBaseStore):
         )
 
     @defer.inlineCallbacks
-    def get_room_events_max_id(self):
+    def get_room_events_max_id(self, direction='f'):
         token = yield self._stream_id_gen.get_max_token(self)
-        defer.returnValue("s%d" % (token,))
+        if direction != 'b':
+            defer.returnValue("s%d" % (token,))
+        else:
+            topo = yield self.runInteraction(
+                "_get_max_topological_txn", self._get_max_topological_txn
+            )
+            defer.returnValue("t%d-%d" % (topo, token))
+
+    def _get_max_topological_txn(self, txn):
+        txn.execute(
+            "SELECT MAX(topological_ordering) FROM events"
+            " WHERE outlier = ?",
+            (False,)
+        )
+
+        rows = txn.fetchall()
+        return rows[0][0] if rows else 0
 
     @defer.inlineCallbacks
     def _get_min_token(self):