`
or a credit card paywall if they don't have the Taler wallet.
Each article thus links to a `offer URL`, whose layout is shown below.
`https://shop.demo.taler.net/essay/Appendix_A:_A_Note_on_Software`
Once the server side logic receives a request for a offer URL, it needs to
instruct the wallet to retrieve a Taler contract. This action can be taken
either with or **without** the use of JavaScript, see the next section.
-----------------------
Triggering the contract
-----------------------
It is important to note that the contract is not returned simply
as the offer URL's response, but rather the frontend `instructs`
the browser on how to retrieve the contract. That is needed for
the right handling of the cases where the wallet is not installed.
.. note::
The code samples shown below are intentionally "incomplete", as often
one function contains logic for multiple actions. Thus in order to not
mix concepts form different actions under one section, parts of code not
related to the section being documented have been left out.
**With JavaScript**
In this case, the objective is to call the function ``taler.offerContractFrom()`` into the user browser, which will then retrieve the
contract. In order to do that, we return a HTML page, whose
template is in ``talerfrontends/blog/templates/purchase.html``,
that imports ``taler-wallet-lib.js``, so that the function
``taler.offerContractFrom()`` can be invoked into the user's
browser.
The server side handler for a offer URL needs to render ``purchase.html`` by passing
the right parameters to ``taler.offerContractFrom()``.
The rendering is done by the ``article`` function at ``talerfrontends/blog/blog.py``,
and is done by Flask's ``render_template()``, see below.
.. sourcecode:: python
# 'name' is the article name, and is set to the right value
# by Flask
# The 'data' parameter is used to send images along
# the articles, however its use is beyond the scope of
# this survey.
def article(name, data=None):
...
...
return render_template('templates/purchase.html',
article_name=name,
no_contract=1,
contract_url=quote(contract_url),
data_attribute="data-taler-contractoffer=%s" % contract_url)
After the rendering, (part of) ``purchase.html`` will look like shown below.
.. sourcecode:: html
...
...
...
...
Processing payment with GNU Taler, please wait
...
The script ``purchase.js`` is now in charge of implementing the behaviour we seek.
It needs to register two handlers: one called whenever the wallet is detected in the
browser, the other if the user has no wallet installed.
That is done with:
.. sourcecode:: javascript
taler.onPresent(handleWalletPresent);
taler.onAbsent(handleWalletAbsent);
Note that the ``taler`` object is exported by ``taler-wallet-lib.js``, and contains all
is needed to communicate with the wallet.
``handleWalletAbsent`` doesn't need to do much: it has to only hide the "please wait"
message and uncover the credit card pay form. See below.
.. sourcecode:: javascript
function handleWalletAbsent() {
// Hide "please wait" message
document.getElementById("talerwait").style.display = "none";
// Uncover credit card pay form
document.body.style.display = "";
}
On the other hand, ``handleWalletPresent`` needs to firstly hide the credit card
pay form and show the "please wait" message. After that, it needs to fetch the
contract URL from the responsible ``meta`` tag, and finally invoke ``taler.offerContractFrom()`` using it. See below both parts.
.. sourcecode:: javascript
function handleWalletPresent() {
// Hide credit card paywall
document.getElementById("ccfakeform").style.display = "none";
// Show "please wait" message
document.getElementById("talerwait").style.display = "";
...
...
// Fetch contract URL from 'meta' tag.
let contract_url = document.querySelectorAll("[name=contract_url]")[0];
// If this call is successful, it will obtain the contract,
// hand it to the wallet, so the wallet can eventually
// show it to the user.
taler.offerContractFrom(decodeURIComponent(contract_url.getAttribute("value")));
...
}
.. note::
In order to get our code validated by W3C validators, we can't have inline
JavaScript in our pages, we are forced to import any used script instead.
**Without JavaScript**
This case is handled by the function ``article`` defined in
``talerfrontends/blog/blog.py``. Its objective is to set the "402 Payment
Required" HTTP status code, and the HTTP header ``X-Taler-Contract-Url``
to the actual contract's URL for this purchase.
Upon returning such a response, the wallet will automatically fetch the
contract from the URL indicated by ``X-Taler-Contract-Url``, and show it
to the user.
Below is shown how the function ``article`` prepares and returns such a
response.
.. sourcecode:: python
# 'name' is the article name, and is set to the right value
# by Flask
# The 'data' parameter is used to send images along
# the articles, however its use is beyond the scope of
# this survey.
def article(name, data=None):
...
...
# Create response.
response = make_response(render_template('templates/fallback.html'), 402)
# Set "X-Taler-Contract-Url" header to the contract's URL.
response.headers["X-Taler-Contract-Url"] = contract_url
return response
The ``make_response`` function is exported by Flask, so it's beyond the scope
of this document to explain it; however, it returns a "response object" having
the "402 Payment Required" as HTTP status code, and the
HTML file ``talerfrontends/blog/templates/fallback.html`` as the body.
``fallback.html`` contains the credit card pay form, so that if the wallet is
not installed, the browser would keep that page shown.
``contract_url`` is defined in the earlier steps of the same function; however,
in this example it looks like:
`https://shop.demo.taler.net/essay/generate-contract?article_name=Appendix_A:_A_Note_on_Software`.
The frontend will also have to provide the contract. That is done
by the handler ``generate_contract``, defined in
``talerfrontends/blog/blog.py``. See below.
.. sourcecode:: python
def generate_contract():
now = int(time.time())
tid = random.randint(1, 2**50)
article_name = expect_parameter("article_name")
contract = make_contract(article_name=article_name, tid=tid, timestamp=now)
contract_resp = sign_contract(contract)
logger.info("generated contract: %s" % str(contract_resp))
return jsonify(**contract_resp)
Its task is to feed the ``make_contract`` subroutine with all the
values it needs to generate a contract. Those values are: the timestamp
for the contract, the transaction ID, and the article name; respectively,
``now``, ``tid``, and ``article_name``.
After ``make_contract`` returns, the variable ``contract`` will hold a
`dict` type that complies with a contract :ref:`proposition `.
We then call ``sign_contract`` feeding it with the proposition, so that
it can forward it to the backend and return it signed. Finally we return
the signed proposition, complying with the :ref:`Offer ` object.
For simplicity, any article costs the same price, so the frontend
doesn't need to map articles to prices.
Both ``make_contract`` and ``sign_contract`` are defined in
``talerfrontends/blog/helpers.py``.
At this point, the user can accept the contract, which triggers the wallet
to visit the fulfillment page. The main logic for a fulfillment page handler
is to (1) return the claimed product, if it has been paid, or (2) instruct the
wallet to send the payment.
-----------------
Fulfillment logic
-----------------
The state accounts for a product being paid or not, so the fulfillment handler
will firstly check that:
.. sourcecode:: python
# 'name' is the article name, and is set to the right value
# by Flask
# The 'data' parameter is used to send images along
# the articles, however its use is beyond the scope of
# this survey.
def article(name, data=None):
# Get list of payed articles from the state
payed_articles = session.get("payed_articles", [])
if name in payed_articles:
...
# The articles has been paid, so return it to the
# customer.
return send_file(get_article_file(article))
...
In case the article has not been paid yet, the fulfillment handler needs
to `reconstruct` the contract, in order to get a precise reference about the
purchase in being served.
All the information needed to reconstruct the contract is contained in the
fulfillment URL parameters. See below the URL layout:
`https://shop.demo.taler.net/essay/Appendix_A:_A_Note_on_Software?uuid=×tamp=tid=`
The way the contract is reconstructed is exactly the same as it was generated
in the previous steps: we need to call ``make_contract`` to get the original
:ref:`proposition ` and then ``sign_contract``. Recall that aside
from allowing the backend to add missing fields to the proposition, ``sign_contract``
returns the contract hashcode also, that we should compare with the ``uuid``
parameter provided by the wallet.
In our blog, all the fulfillment logic is implemented in the function ``article``,
defined in ``talerfrontends/blog/blog.py``. It is important to note that this
function is `the same` function that runs the offer URL; in fact, as long as your
URL design allows it, it is not mandatory to split up things. In our example, the
offer URL differs from the fulfillment URL respect to the number (and type) of
parameters, so the ``article`` function can easily decide whether it has to handle
a "offer" or a "fulfillment" case. See below how the function detects the right
case and reconstructs the contract.
.. sourcecode:: python
# 'name' is the article name, and is set to the right value
# by Flask
# The 'data' parameter is used to send images along
# the articles, however its use is beyond the scope of
# this survey.
def article(name, data=None):
...
hc = request.args.get("uuid")
tid_str = request.args.get("tid")
timestamp_str = request.args.get("timestamp")
if hc is None or tid_str is None or timestamp_str is None:
# Offer URL case.
contract_url = make_url("/generate-contract", ("article_name",name))
... # Go on operating the offer URL and return
# Fulfillment URL case from here on.
try:
tid = int(tid_str)
except ValueError:
raise MalformedParameterError("tid")
try:
timestamp = int(timestamp_str)
except ValueError:
raise MalformedParameterError("timestamp")
restored_contract = make_contract(article_name=name, tid=tid, timestamp=timestamp)
contract_resp = sign_contract(restored_contract)
# Return error if uuid mismatch with the hashcode coming from the backend
if contract_resp["H_contract"] != hc:
e = jsonify(error="contract mismatch", was=hc, expected=contract_resp["H_contract"])
return e, 400
# We save the article's name in the state since after
# receiving the payment this value will point to the
# article to be delivered to the customer. Note how the
# contract's hashcode is used to index the state.
session[hc] = si = session.get(hc, {})
si['article_name'] = name
After a successful contract reconstruction, the handler needs to instruct
the wallet to actually send the payment. There are as usual two ways this
can be accomplished: with and without JavaScript.
**With JavaScript**
..
Mention that the template is the same we used for a offer URL!
We return a HTML page, whose template is in
``talerfrontends/blog/templates/purchase.html``, that imports ``taler-wallet-lib.js``,
so that the function ``taler.executePayment()`` can be invoked into the user's
browser.
The fulfillment handler needs to render ``purchase.html`` so that the right
parameters get passed to ``taler.executePayment()``.
See below how the function ``article`` does the rendering.
.. sourcecode:: python
# 'name' is the article name, and is set to the right value
# by Flask
# The 'data' parameter is used to send images along
# the articles, however its use is beyond the scope of
# this survey.
def article(name, data=None):
...
...
return render_template('templates/purchase.html',
hc=hc,
pay_url=quote(pay_url),
offering_url=quote(offering_url),
article_name=name,
no_contract=0,
data_attribute="data-taler-executecontract=%s,%s,%s" % (hc, pay_url, offering_url))
After the rendering, (part of) ``purchase.html`` will look like shown below.
.. sourcecode:: html
...
...
...
...
Processing payment with GNU Taler, please wait
...
The script ``purchase.js`` is now in charge of calling ``taler.executePayment()``.
It will try to register two handlers: one called whenever the wallet is detected in the
browser, the other if the user has no wallet installed.
That is done with:
.. sourcecode:: javascript
taler.onPresent(handleWalletPresent);
taler.onAbsent(handleWalletAbsent);
.. note::
So far, the template and the imported script (``purchase.js``)
are exactly the same as the offer URL case, since we use them
for both cases. See below how the script distinguishes "offer"
from "fulfillment" case.
Note that the ``taler`` object is exported by ``taler-wallet-lib.js``, and contains all
is needed to communicate with the wallet.
``handleWalletAbsent`` doesn't need to do much: it has to only hide the "please wait"
message and uncover the credit card pay form. See below.
.. sourcecode:: javascript
function handleWalletAbsent() {
// Hide "please wait" message
document.getElementById("talerwait").style.display = "none";
// Uncover credit card pay form
document.body.style.display = "";
}
On the other hand, ``handleWalletPresent`` needs to firstly hide the credit card
pay form and show the "please wait" message. After that, it needs to fetch the
needed parameters from the responsible ``meta`` tags, and finally invoke
``taler.offerContractFrom()`` using those parameters. See below its whole definition.
Note, that since we are in the fulfillment case, the credit card pay form is `almost`
useless, as it is highly unlikely that the wallet is not installed.
.. sourcecode:: javascript
function handleWalletPresent() {
// Hide the credit card pay form
document.getElementById("ccfakeform").style.display = "none";
// Show "please wait" message
document.getElementById("talerwait").style.display = "";
// The `no_contract` value is provided by the function `article` via a
// 'meta' tag in the template. When this value equals 1, then we are in the
// "offer" URL case, otherwise we are in the "fulfillment" URL case.
let no_contract = document.querySelectorAll("[name=no_contract]")[0];
if (Number(no_contract.getAttribute("value"))) {
// "Offer" case
let contract_url = document.querySelectorAll("[name=contract_url]")[0];
taler.offerContractFrom(decodeURIComponent(contract_url.getAttribute("value")));
}
else {
// "Fulfillment" case.
let hc = document.querySelectorAll("[name=hc]")[0];
let pay_url = document.querySelectorAll("[name=pay_url]")[0];
let offering_url = document.querySelectorAll("[name=offering_url]")[0];
taler.executePayment(hc.getAttribute("value"),
decodeURIComponent(pay_url.getAttribute("value")),
decodeURIComponent(offering_url.getAttribute("value")));
}
}
Once the browser executes ``taler.executePayment(...)``, the wallet will send the coins
to ``pay_url``. Once the payment succeeds, the wallet will again visit the
fulfillment URL, this time getting the article thanks to the "payed" status set by
the ``pay_url`` handler.
**Without JavaScript**
This case is handled by the function ``article`` defined in
``talerfrontends/blog/blog.py``. Its objective is to set the "402 Payment
Required" HTTP status code, along with the HTTP headers ``X-Taler-Contract-Hash``,
``X-Taler-Pay-Url``, and ``X-Taler-Offer-Url``.
..
FIXME:
Are those three parameters anywhere, at least 'kindof' introduced?
Upon returning such a response, the wallet will automatically send the
payment to the URL indicated in ``X-Taler-Pay-Url``.
The excerpt below shows how the function ``article`` prepares and returns such a
response.
.. sourcecode:: python
# 'name' is the article name, and is set to the right value
# by Flask
# The 'data' parameter is used to send images along
# the articles, however its use is beyond the scope of
# this survey.
def article(name, data=None):
...
# 'make_response' is exported by Flask. It returns a
# "response object" with customizable status code, HTTP
# headers and body
response = make_response(render_template('templates/fallback.html'), 402)
response.headers["X-Taler-Contract-Hash"] = hc
response.headers["X-Taler-Pay-Url"] = pay_url
response.headers["X-Taler-Offer-Url"] = offering_url
return response
The template ``fallback.html`` contains the credit card pay form, which will be
used in the rare case where the wallet would not be detected in a fulfillment
session. Once the payment succeeds, the wallet will again visits the
fulfillment URL, this time getting the article thanks to the "payed" status set by
the ``pay_url`` handler.
---------
Pay logic
---------
The pay handler for the blog is implemented by the function
``pay`` at ``talerfrontends/blog/blog.py``. Its main duty is
to receive the :ref:`deposit permission `
from the wallet, forward it to the backend, and return the outcome
to the wallet. See below the main steps of its implementation.
.. sourcecode:: python
def pay():
# Get the uploaded deposit permission
deposit_permission = request.get_json()
if deposit_permission is None:
e = jsonify(error="no json in body")
return e, 400
# Pick the contract's hashcode from deposit permission
hc = deposit_permission.get("H_contract")
# Return error if no hashcode was found
if hc is None:
e = jsonify(error="malformed deposit permission", hint="H_contract missing")
return e, 400
# Get a handle to the state for this contract, using the
# hashcode from deposit permission as the index
si = session.get(hc)
# If no session was found for this contract, then either it
# expired or one of the hashcodes (the one we got from
# reconstructing the contract in the fulfillment handler,
# and the one we just picked from the deposit permission)
# is bogus. Note how using the contract's hashcode as index
# makes harder for the wallet to use different hashcodes
# in different steps of the protocol.
if si is None:
e = jsonify(error="no session for contract")
return e, 400
# Forward the deposit permission to the backend
r = requests.post(urljoin(BACKEND_URL, 'pay'), json=deposit_permission)
# Return error if the backend returned a HTTP status code
# other than 200 OK
if 200 != r.status_code:
raise BackendError(r.status_code, r.text)
# The payment went through..
...
# Resume the article name
article = si["article_name"]
# Set the article's state as "payed". This is realized by
# appending it to a *list* of articles the customer is currently
# allowed to read.
payed_articles = session["payed_articles"] = session.get("payed_articles", [])
if article not in payed_articles:
payed_articles.append(article)
...
# Return success
return r.text, 200