1 files changed, 46 insertions, 0 deletions
diff --git a/synapse/util/caches/response_cache.py b/synapse/util/caches/response_cache.py
new file mode 100644
index 0000000000..be310ba320
--- /dev/null
+++ b/synapse/util/caches/response_cache.py
@@ -0,0 +1,46 @@
+# -*- 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.async import ObservableDeferred
+
+
+class ResponseCache(object):
+    """
+    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.
+    """
+
+    def __init__(self):
+        self.pending_result_cache = {}  # Requests that haven't finished yet.
+
+    def get(self, key):
+        result = self.pending_result_cache.get(key)
+        if result is not None:
+            return result.observe()
+        else:
+            return None
+
+    def set(self, key, deferred):
+        result = ObservableDeferred(deferred)
+        self.pending_result_cache[key] = result
+
+        def remove(r):
+            self.pending_result_cache.pop(key, None)
+            return r
+
+        result.addBoth(remove)
+        return result.observe()