diff --git a/synapse/util/caches/room_change_cache.py b/synapse/util/caches/room_change_cache.py
new file mode 100644
index 0000000000..3a873c9c30
--- /dev/null
+++ b/synapse/util/caches/room_change_cache.py
@@ -0,0 +1,86 @@
+# -*- coding: utf-8 -*-
+# Copyright 2016 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from synapse.util.caches import cache_counter, caches_by_name
+
+
+from blist import sorteddict
+import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+class RoomStreamChangeCache(object):
+ """Keeps track of the stream_id of the latest change in rooms.
+
+ Given a list of rooms and stream key, it will give a subset of rooms that
+ may have changed since that key. If the key is too old then the cache
+ will simply return all rooms.
+ """
+ def __init__(self, name, current_key, size_of_cache=10000):
+ self._size_of_cache = size_of_cache
+ self._room_to_key = {}
+ self._cache = sorteddict()
+ self._earliest_known_key = current_key
+ self.name = name
+ caches_by_name[self.name] = self._cache
+
+ def get_room_has_changed(self, room_id, key):
+ if key <= self._earliest_known_key:
+ return True
+
+ room_key = self._room_to_key.get(room_id, None)
+ if room_key is None:
+ return True
+
+ if key < room_key:
+ return True
+
+ return False
+
+ def get_rooms_changed(self, store, room_ids, key):
+ """Returns subset of room ids that have had new things since the
+ given key. If the key is too old it will just return the given list.
+ """
+ if key > self._earliest_known_key:
+ keys = self._cache.keys()
+ i = keys.bisect_right(key)
+
+ result = set(
+ self._cache[k] for k in keys[i:]
+ ).intersection(room_ids)
+
+ cache_counter.inc_hits(self.name)
+ else:
+ result = room_ids
+ cache_counter.inc_misses(self.name)
+
+ return result
+
+ def room_has_changed(self, store, room_id, key):
+ """Informs the cache that the room has been changed at the given key.
+ """
+ if key > self._earliest_known_key:
+ old_key = self._room_to_key.get(room_id, None)
+ if old_key:
+ key = max(key, old_key)
+ self._cache.pop(old_key, None)
+ self._cache[key] = room_id
+
+ while len(self._cache) > self._size_of_cache:
+ k, r = self._cache.popitem()
+ self._earliest_key = max(k, self._earliest_key)
+ self._room_to_key.pop(r, None)
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