diff --git a/synapse/util/caches/snapshot_cache.py b/synapse/util/caches/snapshot_cache.py
new file mode 100644
index 0000000000..09f00afbc5
--- /dev/null
+++ b/synapse/util/caches/snapshot_cache.py
@@ -0,0 +1,93 @@
+# -*- coding: utf-8 -*-
+# Copyright 2015 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.async import ObservableDeferred
+
+
+class SnapshotCache(object):
+ """Cache for snapshots like the response of /initialSync.
+ The response of initialSync only has to be a recent snapshot of the
+ server state. It shouldn't matter to clients if it is a few minutes out
+ of date.
+
+ This caches a deferred response. Until the deferred completes it will be
+ returned from the cache. This means that if the client retries the request
+ while the response is still being computed, that original response will be
+ used rather than trying to compute a new response.
+
+ Once the deferred completes it will removed from the cache after 5 minutes.
+ We delay removing it from the cache because a client retrying its request
+ could race with us finishing computing the response.
+
+ Rather than tracking precisely how long something has been in the cache we
+ keep two generations of completed responses. Every 5 minutes discard the
+ old generation, move the new generation to the old generation, and set the
+ new generation to be empty. This means that a result will be in the cache
+ somewhere between 5 and 10 minutes.
+ """
+
+ DURATION_MS = 5 * 60 * 1000 # Cache results for 5 minutes.
+
+ def __init__(self):
+ self.pending_result_cache = {} # Request that haven't finished yet.
+ self.prev_result_cache = {} # The older requests that have finished.
+ self.next_result_cache = {} # The newer requests that have finished.
+ self.time_last_rotated_ms = 0
+
+ def rotate(self, time_now_ms):
+ # Rotate once if the cache duration has passed since the last rotation.
+ if time_now_ms - self.time_last_rotated_ms >= self.DURATION_MS:
+ self.prev_result_cache = self.next_result_cache
+ self.next_result_cache = {}
+ self.time_last_rotated_ms += self.DURATION_MS
+
+ # Rotate again if the cache duration has passed twice since the last
+ # rotation.
+ if time_now_ms - self.time_last_rotated_ms >= self.DURATION_MS:
+ self.prev_result_cache = self.next_result_cache
+ self.next_result_cache = {}
+ self.time_last_rotated_ms = time_now_ms
+
+ def get(self, time_now_ms, key):
+ self.rotate(time_now_ms)
+ # This cache is intended to deduplicate requests, so we expect it to be
+ # missed most of the time. So we just lookup the key in all of the
+ # dictionaries rather than trying to short circuit the lookup if the
+ # key is found.
+ result = self.prev_result_cache.get(key)
+ result = self.next_result_cache.get(key, result)
+ result = self.pending_result_cache.get(key, result)
+ if result is not None:
+ return result.observe()
+ else:
+ return None
+
+ def set(self, time_now_ms, key, deferred):
+ self.rotate(time_now_ms)
+
+ result = ObservableDeferred(deferred)
+
+ self.pending_result_cache[key] = result
+
+ def shuffle_along(r):
+ # When the deferred completes we shuffle it along to the first
+ # generation of the result cache. So that it will eventually
+ # expire from the rotation of that cache.
+ self.next_result_cache[key] = result
+ self.pending_result_cache.pop(key, None)
+
+ result.observe().addBoth(shuffle_along)
+
+ return result.observe()
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