1 files changed, 93 insertions, 0 deletions
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()