diff --git a/synapse/rest/client/transactions.py b/synapse/rest/client/transactions.py
new file mode 100644
index 0000000000..1db972a378
--- /dev/null
+++ b/synapse/rest/client/transactions.py
@@ -0,0 +1,85 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014-2016 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.
+
+"""This module contains logic for storing HTTP PUT transactions. This is used
+to ensure idempotency when performing PUTs using the REST API."""
+import logging
+
+from synapse.api.auth import get_access_token_from_request
+from synapse.util.async import ObservableDeferred
+
+logger = logging.getLogger(__name__)
+
+
+def get_transaction_key(request):
+ """A helper function which returns a transaction key that can be used
+ with TransactionCache for idempotent requests.
+
+ Idempotency is based on the returned key being the same for separate
+ requests to the same endpoint. The key is formed from the HTTP request
+ path and the access_token for the requesting user.
+
+ Args:
+ request (twisted.web.http.Request): The incoming request. Must
+ contain an access_token.
+ Returns:
+ str: A transaction key
+ """
+ token = get_access_token_from_request(request)
+ return request.path + "/" + token
+
+
+class HttpTransactionCache(object):
+
+ def __init__(self):
+ self.transactions = {
+ # $txn_key: ObservableDeferred<(res_code, res_json_body)>
+ }
+
+ def fetch_or_execute_request(self, request, fn, *args, **kwargs):
+ """A helper function for fetch_or_execute which extracts
+ a transaction key from the given request.
+
+ See:
+ fetch_or_execute
+ """
+ return self.fetch_or_execute(
+ get_transaction_key(request), fn, *args, **kwargs
+ )
+
+ def fetch_or_execute(self, txn_key, fn, *args, **kwargs):
+ """Fetches the response for this transaction, or executes the given function
+ to produce a response for this transaction.
+
+ Args:
+ txn_key (str): A key to ensure idempotency should fetch_or_execute be
+ called again at a later point in time.
+ fn (function): A function which returns a tuple of
+ (response_code, response_dict)d
+ *args: Arguments to pass to fn.
+ **kwargs: Keyword arguments to pass to fn.
+ Returns:
+ synapse.util.async.ObservableDeferred which resolves to a tuple
+ of (response_code, response_dict).
+ """
+ try:
+ return self.transactions[txn_key]
+ except KeyError:
+ pass # execute the function instead.
+
+ deferred = fn(*args, **kwargs)
+ observable = ObservableDeferred(deferred)
+ self.transactions[txn_key] = observable
+ return observable
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