diff --git a/synapse/replication/slave/storage/events.py b/synapse/replication/slave/storage/events.py
index 7034f48b50a13d96e30d2087b8de7c0dbb54e5b2..94ebbffc1b7509e0452a5b0a02e7444770815869 100644
--- a/synapse/replication/slave/storage/events.py
+++ b/synapse/replication/slave/storage/events.py
@@ -153,7 +153,7 @@ class SlavedEventStore(BaseSlavedStore):
     get_room_events_stream_for_rooms = (
         DataStore.get_room_events_stream_for_rooms.__func__
     )
-    is_host_joined = DataStore.is_host_joined.__func__
+    is_host_joined = RoomMemberStore.__dict__["is_host_joined"]
     get_stream_token_for_event = DataStore.get_stream_token_for_event.__func__
 
     _set_before_and_after = staticmethod(DataStore._set_before_and_after)
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index c80d181fc778cca8b92cd07db240edcb8572706c..72ce84b0b8e7f254fcdba4bdc81b43e503b4895f 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -776,6 +776,11 @@ class EventsStore(SQLBaseStore):
                         txn, self.get_rooms_for_user, (member,)
                     )
 
+                for host in set(get_domain_from_id(u) for u in members_changed):
+                    self._invalidate_cache_and_stream(
+                        txn, self.is_host_joined, (room_id, host)
+                    )
+
                 self._invalidate_cache_and_stream(
                     txn, self.get_users_in_room, (room_id,)
                 )
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index e38bbd22a3b163da633708877cb12a138bf18ceb..457ca288d07d3cb1a8004abfd8460c27839b20f4 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -501,7 +501,7 @@ class RoomMemberStore(SQLBaseStore):
 
         defer.returnValue(users_in_room)
 
-    @defer.inlineCallbacks
+    @cachedInlineCallbacks(max_entries=10000)
     def is_host_joined(self, room_id, host):
         if '%' in host or '_' in host:
             raise Exception("Invalid host name")