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Design Doc 007: Specification of the Payment Flow
#################################################

Summary
=======

This design document describes how the payment flow works in the browser, and how
features like session IDs, re-purchase detection and refunds interact.

Requirements
============

* The payment flow must both support wallets that are integrated in the browser,
  as well as external wallets (mobile phone, command line)
* The initiator of the payment can be a Website or another channel,
  such as an e-mail or a messaging service.
* For paid digital works, there should be a reasonable technical barrier to
  sharing the information with unauthorized users
* A simple API should be offered to shops
* Sharing of links or re-visiting of bookmarks should result in well-defined
  behavior instead of random, ugly error messages.

Proposed Solution
=================



Session-bound payment flow for Web resources
--------------------------------------------

In this payment flow, the user initiates the payment by navigating to a
paywalled Web resource.  Let *resource-URL* be the URL of the paywalled resource.

Storefront
^^^^^^^^^^

When *resource-URL* is requested, the storefront runs the following steps:

1. Extract the *resource name* from the *resource-URL*.
2. Extract the *order-ID* and *session-ID* (or null) from the request's validated cookie (for example, by using signed cookies).
3. If *session-ID* and *order-ID* are valid and the storefront's
   *session-payment-cache* contains the tuple (*order-ID*, *resource-name*, *session-ID*),
   return to the client the resource associated with *resource name*.  **Terminate.**
4. If *session-ID* and *order-ID* are invalid, assign a fresh session ID and create a new order for *resource name* by doing a ``POST /private/orders`` to
   the merchant backend.  Set both in the cookie to be sent with the response.
5. Check the status of the payment for *order-ID* under *session-ID* by doing a ``GET /private/orders/{order-ID}?session_id={session-ID}``.
   This results in the *order-status*, *refund-amount* and the *client-order-status-URL*.
6. If the *order-status* is paid and *refund-amount* is non-zero,
   return to the client a page with an explanation that the payment has been refunded.
   If the client has not (fully) obtained the granted refunds yet, show a link to the public order page
   of the backend to allow the client to obtain the refund.  **Terminate.**
7. If the *order-status* is paid, store the tuple (*order-ID*, *resource-name*, *session-ID*) in *session-payment-cache*
   and return to the client the resource associated with *resource name*.  **Terminate.**
8. Otherwise, the *order-status* is unpaid.  Redirect the client to *client-order-status-URL*. **Terminate.**

.. note::

   The use of a *session-page-cache* is optional, but recommended for performance. When a refund is given, the corresponding tuple must be removed from the *session-page-cache*.

Backend Private Order Status
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The merchant backend runs the following steps to generate the
*client-order-status-URL* when processing a request for ``GET
/private/orders/{order-ID}?session_id={session-ID}&timeout_ms={timeout}``:

1. Let *session-ID* be the session ID of the request or null if not given (note: **not** the last paid session ID)
2. If *order-ID* does not identify an existing order, return a 40 Not Found response.  **Terminate**.
3. If *order-ID* identifies an order that is *unclaimed* and has claim token *claim-token*, return the URL

   ::

     {backendBaseUrl}/orders/{order-ID}?token={claim-token}&session_id={session-ID}

   (if no claim-token was generated, omit that parameter from the above URI). **Terminate.**

4. Here *order-ID* identifies an order that is *claimed*.  If the order is *unpaid*, wait until timeout or payment.

5. If the order remains unpaid or was paid for a different *session-ID*, obtain the contract terms hash *contract-hash* and return the URL

   ::

     {backendBaseUrl}/orders/{order-ID}?h_contract={contract-hash}&session_id={session-ID}

  together with the status *unpaid*. (If *session-ID* is null, it does not matter for which session the contract was paid.) **Terminate.**

6. Here *order-ID* must now identify an order that is *paid* or *refunded*. Obtain the contract terms hash *contract-hash* and return the URL

   ::

     {backendBaseUrl}/orders/{order-ID}?h_contract={contract-hash}&session_id={session-ID}

  together with the status *paid* or *refunded* (and if applicable, with details about the applied refunds). **Terminate.**



Backend Client Order Status Page
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The merchant backend runs the following steps to generate the HTML page for
``GET /orders/{order-ID}?session_id={session-ID}&token={claim-token}&h_contract={contract-hash}``:

1. If *order-ID* does not identify an existing order, render a 404 Not Found response.  **Terminate.**
2. If *order-ID* identifies a paid order (where the *session-ID* matches the one from the payment), run these steps:

   1. If the *contract-hash* request parameter does not match the contract terms hash of the order,
      return a 403 Forbidden response. **Terminate.**

   2. If the order has granted refunds that have not been obtained by the wallet yet, prompt the URI

      ::

        taler{proto_suffix}://refund/{/merchant_prefix*}/{order-id}/{session-id}

      The generated Web site should long-poll until all refunds have been obtained,
      then redirect to the *fulfillment-URL* of the order once the refunds have been
      obtained.  **Terminate.**
      ----- FIXME: IIRC our long-polling API does only allow waiting for the granted refund amount, not for the *obtained* refund amount. => API change?

   3. Here the order has been paid and possibly refunded.
      Redirect to the *fulfillment-URL* of the order.
      **Terminate.**


3. If *order-ID* identifies an *unclaimed* order, run these steps:

   1. If the order is *unclaimed* and the *claim-token* request parameter does not
      match the claim token of the order, return a 403 Forbidden response. **Terminate**.

   2. Prompt the URI

      ::

        taler{proto_suffix}://pay/{/merchant_prefix*}/{order-id}/{session-ID}?c={claim-token}

      The generated Web site should long-poll to check for the payment happening.
      It should then redirect to the *fulfillment-URL* of the order once
      payment has been proven under *session-ID*, or possibly redirect to the
      *already-paid-order-ID*. Which of these happens depends on the (long-polled) JSON replies.
      **Terminate.**

4. If *order-ID* identifies an *claimed* and *unpaid* order, run these steps:

   1. If the order is *claimed* and the *contract-hash* request parameter does not
      match the contract hash of the order, return a 403 Forbidden response. **Terminate**.

   2. If there is a non-null *already-paid-order-ID* for *session-ID* stored under the current order,
      redirect to the *fulfillment-URL* of *already-paid-order-ID*. **Terminate**.

   3. Prompt the URI

      ::

        taler{proto_suffix}://pay/{/merchant_prefix*}/{order-id}/{session-ID}

      The generated Web site should long-poll to check for the payment happening.
      It should then redirect to the *fulfillment-URL* of the order once
      payment has been proven under *session-ID*, or possibly redirect to the
      *already-paid-order-ID*. Which of these happens depends on the (long-polled) JSON replies.
      **Terminate.**


Discussion / Q&A
================

Notes
-----

* The *timeout_ms* argument is expected to be ignored when generating HTML.
  Long-polling simply makes no sense if a browser accesses the site directly.


Covered Scenarios
-----------------

* **Re-purchase detection**. Let's say a detached wallet has already successfully paid for a resource URL.
  A browser navigates to the resource URL.  The storefront will generate a new order and assign a session ID.
  Upon scanning the QR code, the wallet will detect that it already has puchased the resource (checked via the fulfillment URL).
  It will then prove the payment of the **old** order ID under the **new** session ID.


* **Bookmarks of Lost Purchases / Social Sharing of Fulfillment URLs**

  FIXME: explain how we covered this by moving order ID into session cookie!
  Let's say I bought some article a few months ago and I lost my wallet. I still have the augmented fulfillment URL
  for the article bookmarked.  When I re-visit the URL, I will be prompted via QR code, but I can *never* prove
  that I already paid, because I lost my wallet!

  In this case, it might make sense to include some "make new purchase" link on the client order status page.
  It's not clear if this is a common/important scenario though.

  But we might want to make clear on the client order status page that it's showing a QR code for something
  that was already paid.

  The same concern applies when sending the fulfillment URL of a paid paywalled Web resource to somebody else.



Problematic Scenarios
---------------------

The Back Button
^^^^^^^^^^^^^^^

The following steps lead to unintuitive navigation:
1. Purchase a paywalled URL for the first time via a detached wallet
2. Marvel at the fulfillment page
3. Press the back button

This will display an error message, as the authentication via the claim token on the
``/orders/{order-ID}`` page is not valid anymore.

We could consider still allowing authentication with the claim token in this case.