diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py
new file mode 100644
index 0000000000..c673b1bdfc
--- /dev/null
+++ b/synapse/util/caches/stream_change_cache.py
@@ -0,0 +1,107 @@
+# -*- 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 StreamChangeCache(object):
+ """Keeps track of the stream positions of the latest change in a set of entities.
+
+ Typically the entity will be a room or user id.
+
+ Given a list of entities and a stream position, it will give a subset of
+ entities that may have changed since that position. If position key is too
+ old then the cache will simply return all given entities.
+ """
+ def __init__(self, name, current_stream_pos, max_size=10000):
+ self._max_size = max_size
+ self._entity_to_key = {}
+ self._cache = sorteddict()
+ self._earliest_known_stream_pos = current_stream_pos
+ self.name = name
+ caches_by_name[self.name] = self._cache
+
+ def has_entity_changed(self, entity, stream_pos):
+ """Returns True if the entity may have been updated since stream_pos
+ """
+ assert type(stream_pos) is int
+
+ if stream_pos < self._earliest_known_stream_pos:
+ cache_counter.inc_misses(self.name)
+ return True
+
+ if stream_pos == self._earliest_known_stream_pos:
+ # If the same as the earliest key, assume nothing has changed.
+ cache_counter.inc_hits(self.name)
+ return False
+
+ latest_entity_change_pos = self._entity_to_key.get(entity, None)
+ if latest_entity_change_pos is None:
+ cache_counter.inc_misses(self.name)
+ return True
+
+ if stream_pos < latest_entity_change_pos:
+ cache_counter.inc_misses(self.name)
+ return True
+
+ cache_counter.inc_hits(self.name)
+ return False
+
+ def get_entities_changed(self, entities, stream_pos):
+ """Returns subset of entities that have had new things since the
+ given position. If the position is too old it will just return the given list.
+ """
+ assert type(stream_pos) is int
+
+ if stream_pos >= self._earliest_known_stream_pos:
+ keys = self._cache.keys()
+ i = keys.bisect_right(stream_pos)
+
+ result = set(
+ self._cache[k] for k in keys[i:]
+ ).intersection(entities)
+
+ cache_counter.inc_hits(self.name)
+ else:
+ result = entities
+ cache_counter.inc_misses(self.name)
+
+ return result
+
+ def entity_has_changed(self, entity, stream_pos):
+ """Informs the cache that the entity has been changed at the given
+ position.
+ """
+ assert type(stream_pos) is int
+
+ if stream_pos > self._earliest_known_stream_pos:
+ old_pos = self._entity_to_key.get(entity, None)
+ if old_pos:
+ stream_pos = max(stream_pos, old_pos)
+ self._cache.pop(old_pos, None)
+ self._cache[stream_pos] = entity
+ self._entity_to_key[entity] = stream_pos
+
+ while len(self._cache) > self._max_size:
+ k, r = self._cache.popitem()
+ self._earliest_known_stream_pos = max(k, self._earliest_known_stream_pos)
+ self._entity_to_key.pop(r, None)
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