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diff --git a/synapse/rest/client/v1/transactions.py b/synapse/rest/client/v1/transactions.py
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+++ b/synapse/rest/client/v1/transactions.py
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+# -*- coding: utf-8 -*-
+# Copyright 2014, 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.
+
+"""This module contains logic for storing HTTP PUT transactions. This is used
+to ensure idempotency when performing PUTs using the REST API."""
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+# FIXME: elsewhere we use FooStore to indicate something in the storage layer...
+class HttpTransactionStore(object):
+
+    def __init__(self):
+        # { key : (txn_id, response) }
+        self.transactions = {}
+
+    def get_response(self, key, txn_id):
+        """Retrieve a response for this request.
+
+        Args:
+            key (str): A transaction-independent key for this request. Usually
+                this is a combination of the path (without the transaction id)
+                and the user's access token.
+            txn_id (str): The transaction ID for this request
+        Returns:
+            A tuple of (HTTP response code, response content) or None.
+        """
+        try:
+            logger.debug("get_response TxnId: %s", txn_id)
+            (last_txn_id, response) = self.transactions[key]
+            if txn_id == last_txn_id:
+                logger.info("get_response: Returning a response for %s", txn_id)
+                return response
+        except KeyError:
+            pass
+        return None
+
+    def store_response(self, key, txn_id, response):
+        """Stores an HTTP response tuple.
+
+        Args:
+            key (str): A transaction-independent key for this request. Usually
+                this is a combination of the path (without the transaction id)
+                and the user's access token.
+            txn_id (str): The transaction ID for this request.
+            response (tuple): A tuple of (HTTP response code, response content)
+        """
+        logger.debug("store_response TxnId: %s", txn_id)
+        self.transactions[key] = (txn_id, response)
+
+    def store_client_transaction(self, request, txn_id, response):
+        """Stores the request/response pair of an HTTP transaction.
+
+        Args:
+            request (twisted.web.http.Request): The twisted HTTP request. This
+            request must have the transaction ID as the last path segment.
+            response (tuple): A tuple of (response code, response dict)
+            txn_id (str): The transaction ID for this request.
+        """
+        self.store_response(self._get_key(request), txn_id, response)
+
+    def get_client_transaction(self, request, txn_id):
+        """Retrieves a stored response if there was one.
+
+        Args:
+            request (twisted.web.http.Request): The twisted HTTP request. This
+            request must have the transaction ID as the last path segment.
+            txn_id (str): The transaction ID for this request.
+        Returns:
+            The response tuple.
+        Raises:
+            KeyError if the transaction was not found.
+        """
+        response = self.get_response(self._get_key(request), txn_id)
+        if response is None:
+            raise KeyError("Transaction not found.")
+        return response
+
+    def _get_key(self, request):
+        token = request.args["access_token"][0]
+        path_without_txn_id = request.path.rsplit("/", 1)[0]
+        return path_without_txn_id + "/" + token