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+=================================
+Merchant Reference Implementation
+=================================
+
+-----------------------
+Architectural Overview
+-----------------------
+
+The merchant reference implementationis divided into two independent
+compontents, the `frontend` and the `backend`.
+
+The `frontend` is the existing shopping portal of the merchant.
+The architecture tries to minimize the amount of modifications necessary
+to the `frontend` as well as the trust that needs to be placed into the
+`frontend` logic. Taler requires the frontend to facilitate two
+JSON-based interactions between the wallet and the `backend`, and
+one of those is trivial.
+
+The `backend` is a standalone C application intended to implement all
+the cryptographic routines required to interact with the Taler wallet
+and a Taler mint.
+
+
+------------------------------
+The Merchant Backend HTTP API
+------------------------------
+
+The following API are made available by the merchant's `backend` to the merchant's `frontend`.
+
+.. http:post:: /contract
+
+ Ask the backend to add some missing (mostly related to cryptography) information to the contract.
+
+ :reqheader Content-Type: application/json
+
+ The `proposition` that is to be sent from the frontend is a `contract` object without the fields
+
+ * `merchant_pub`
+ * `mints`
+ * `H_wire`
+
+ The `backend` then completes this information based on its configuration.
+
+ **Success Response**
+
+ :status 200 OK: The backend has successfully created the contract.
+ :resheader Content-Type: application/json
+
+ The `frontend` should pass this response verbatim to the wallet.
+
+ **Failure Responses: Bad contract**
+
+ :status 400 Bad Request: Request not understood. The JSON was invalid. Possibly due to some error in formatting the JSON by the `frontend`.
+
+.. http:post:: /pay
+
+ Asks the `backend` to execute the transaction with the mint and deposit the coins.
+
+ :reqheader Content-Type: application/json
+
+ The `frontend` passes the :ref:`deposit permission <deposit-permission>` received from the wallet, by adding the fields `max_fee`, `amount` (see :ref:`contract`) and optionally adding a field named `edate`, indicating a deadline by which he would expect to receive the bank transfer for this deal
+
+ **Success Response: OK**
+
+ :status 200 OK: The mint accepted all of the coins. The `frontend` should now fullfill the contract. This response has no meaningful body, the frontend needs to generate the fullfillment page.
+
+ **Failure Responses: Bad mint**
+
+ :status 400 Precondition failed: The given mint is not acceptable for this merchant, as it is not in the list of accepted mints and not audited by an approved auditor.
+
+
+ **Failure Responses: Mint trouble**
+
+ The `backend` will return verbatim the error codes received from the mint's :ref:`deposit <deposit>` API. If the wallet made a mistake, like by double-spending for example, the `frontend` should pass the reply verbatim to the browser/wallet. This should be the expected case, as the `frontend` cannot really make mistakes; the only reasonable exception is if the `backend` is unavailable, in which case the customer might appreciate some reassurance that the merchant is working on getting his systems back online.