docs: add ADR for decoupling LTI launches from the XBlock

This commit adds an ADR describing our strategy for decoupling LTI launches from the XBlock and from the edX platform.

docs/decisions/0007-lti-launch-decoupling.rst

+0007 Decouple LTI Launches from XBlock and edX Platform
+#######################################################
+
+Status
+******
+**Draft (=> Accepted)**
+
+Context
+*******
+
+**Definitions**
+
+In this document, the terms "launch context" and "launch information" are used. Launch context describes the
+technological environment or application in which an LTI launch occurs. For example, the launch context may be an
+XBlock, a course, a Django application, an independently deployable application (IDA), etc. Launch information describes
+information included in or necessary for an LTI launch. Launch information is relative to the launch context. For
+example, a user identifier or a user role is launch information. Launch information may refer to data that are used as
+query or form parameters of a request, as claims in a JSON web token (JWT), or as data needed by the library to
+function.
+
+**Problems**
+
+Open edX would like to support the following LTI launch use cases:
+
+* content launches from an XBlock launch context
+* content launches from multiple XBlocks, reusing LTIConfigurations between them (i.e. course launch context)
+* content launches from outside an XBlock launch context in the platform (e.g. LTI course tabs, LTI 1.3 embedded
+  discussions)
+* content launches from an independently deployable application (i.e. IDA or Django application context)
+
+This library implements both LTI 1.1 and LTI 1.3 launches.
+
+**LTI 1.1**
+
+Currently, LTI 1.1 launches are already decoupled from the XBlock and the edx-platform. The `lti_embed
+<https://github.com/openedx/xblock-lti-consumer/blob/master/lti_consumer/lti_1p1/contrib/django.py#L13>`_ function
+accepts a series of arguments that define necessary launch information and returns an HTML template with Javascript that
+initiates an LTI 1.1 launch. No additional work is required to decouple LTI 1.1 launches, but this document will
+describe changes necessary to bring LTI 1.1 launches in alignment with the below strategy for LTI 1.3 launches.
+
+**LTI 1.3**
+
+LTI 1.3 launches rely on the ``location`` field of the ``LtiConfiguration`` model. The ``location`` field is
+a ``UsageKeyField`` and represents the location of the XBlock associated with the ``LtiConfiguration`` instance. This
+association was made because, originally, the XBlock was the source of truth for LTI configuration information and was
+the only launch context in which LTI launches occurred.
+
+The dependence on the ``location`` field prevents LTI 1.3 launches from being possible outside the launch context of an
+XBlock. This prevents Open edX from using LTI more widely across the platform and across the ecosystem, because its use
+is restricted to specific launch contexts in which ``location`` is meaningful.
+
+For example, because ``LTIConfiguration`` instances are associated with a particular ``location``, it is not possible
+for a ``LtiConfiguration`` to be reused across different XBlocks (i.e. a course launch context), even if the
+``LtiConfiguration`` would be identical amongst the different XBlocks.
+
+In an LTI 1.3 launch, the ``location`` field is used to determine the
+``https://purl.imsglobal.org/spec/lti/claim/roles`` and ``https://purl.imsglobal.org/spec/lti/claim/context`` claims of
+the LTI 1.3 launch. In an LTI 1.3 launch, the ``https://purl.imsglobal.org/spec/lti/claim/roles`` claim is required, and
+the ``https://purl.imsglobal.org/spec/lti/claim/context`` claim is optional. This couples the LTI 1.3 launches to the
+XBlock through the ``location`` field.
+
+Furthermore, LTI 1.3 launches are also dependent on functions imported from edx-platform. These functions are used `to
+get the requesting user's role
+<https://github.com/openedx/xblock-lti-consumer/blob/master/lti_consumer/plugin/views.py#L182>`_, `to get the requesting
+user's external ID <https://github.com/openedx/xblock-lti-consumer/blob/master/lti_consumer/plugin/views.py#L183>`_, `to
+get the course key from the location field
+<https://github.com/openedx/xblock-lti-consumer/blob/master/lti_consumer/plugin/views.py#L181>`_, etc. This prevents LTI
+1.3 launches from running outside of the platform.
+
+https://github.com/openedx/xblock-lti-consumer/pull/254 decoupled the LTI 1.3 launch handler endpoint
+``launch_gate_endpoint``, the LTI 1.3 keyset endpoint ``public_keyset_endpoint``, and the LTI 1.3 access token endpoint
+``access_token_endpoint`` from the XBlock by moving these endpoints into the Django plugin. This was the first step in
+decoupling the XBlock from the LTI 1.3 launch. This decision describes how to fully decouple the LTI 1.3 launch from the
+XBlock and how to modify the LTI 1.1 launch to follow this pattern.
+
+Decision
+********
+
+**LTI 1.3**
+
+* For backwards compatability, we will not remove references to the ``location`` field from the
+  ``public_keyset_endpoint`` and ``access_token_endpoint`` endpoint. This is because LTI 1.3 integrations may exist in
+  the platform and in other installations that use URLs that contain the
+  ``usage_key`` in them.
+* We will change the URLs that are displayed in Studio for the ``public_keyset_endpoint`` and
+  ``access_token_endpoint` endpoints. This will prevent additional LTI 1.3 configurations that use the ``usage_key``. This
+  makes us more able to deprecate URLs that use ``usage_key`` and to remove references to ``location``.
+* The responsibility of determining the launch information included in an LTI launch (e.g. roles and context claims)
+  will be that of the launch context in which the LTI 1.3 launch occurs (e.g. XBlock, IDA).
+* For LTI 1.3 launches, launch information will be encoded in an instance of a launch data class called
+  Lti1p3LaunchData.
+* The library will expose a Python API function that will return the URL that initiates the LTI 1.3 launch - the third
+  party initiated login URL. This Python API function may be `get_lti_1p3_content_url
+  <https://github.com/openedx/xblock-lti-consumer/blob/master/lti_consumer/api.py#L186>`_.
+* Each launch context will supply the launch information to the LTI 1.3 launch by including an instance of
+  Lti1p3LaunchData as an argument to the aforementioned function. Each launch context will call this function to fetch
+  the URL to trigger an LTI 1.3 launch.
+* In order for this launch information to be available throughout the LTI 1.3 launch, the library will cache the the
+  launch information provided by the launch context in the third-party initiated login request and include the cache_key
+  in the ``lti_message_hint`` query or form parameter. This allows the library to read the launch_data out of the cache
+  when necessary by using the ``lti_message_hint``.
+* The third-party initiated login request ``login_hint`` query or form parameter will be used to include the user
+  identifier. The ``login_hint`` will not be used to identify the ``LtiConfiguration`` instance. Information necessary
+  to identify the ``LtiConfiguration`` instance will be included in the launch information instead.
+* We will not describe how to decouple LTI Advantage Services in this document. This may be done at a later time.
+
+**LTI 1.1**
+
+* For LTI 1.1 launches, launch information will be encoded in an instance of a launch data class called
+  Lti1p1LaunchData.
+* In order to have a similar interface to LTI 1.3, we will modify the lti_embed function to use an instance of the
+  Lti1p1LaunchData class.
+* We will modify the ``LtiConsumerXBlock`` to use the `lti_embed
+  <https://github.com/openedx/xblock-lti-consumer/blob/master/lti_consumer/lti_1p1/contrib/django.py#L13>`_ function to
+  initiate the LTI 1.1 launch.
+
+Consequences
+************
+
+* Basic LTI 1.3 launches will be decoupled from the XBlock and from the edX platform. This will allow Open edX to better
+  leverage LTI across the platform and across the ecosystem.
+* LTI Advantage Services will not be decoupled from the XBlock and from the edX platform. This means that LTI Advantage
+  Services will not necessarily be available in all launch contexts. This fits our current needs, but it may not fit
+  future needs. This challenge will need to be addressed in a future decision.
+
+  * This means that the ``location`` field on the ``LtiConfiguration`` will need to remain for the time being, because
+    it is currently used in our implementation of LTI Advantage Services.
+
+* Individual launch contexts will be responsible for sending launch information to the library. This may pose a
+  challenge if the launch information is stored in edX platform but the launch context is outside the edX platform (e.g.
+  user role). A launch context may choose to determine or define this launch information on its own.
+* This library will remain Open edX LTI Certified, because all three Advantage Services are supported on the
+  ``CONFIG_ON_XBLOCK`` ``config_store`` setting.
+
+Rejected Alternatives
+******************
+
+* Launch contexts define their own views, which import and wrap LTI 1.3 launch views. Launch contexts define launch
+  information that they then pass to the LTI 1.3 launch views.
+
+  * Importing and wrapping LTI 1.3 launch views requires a lot of boilerplate code within each launch context's code.
+  * For an LTI 1.3 launch, each view would need to be imported and wrapped to supply launch information to each step of
+    the LTI 1.3 launch.
+
+* The ``login_hint`` encodes information about the launch context in which the LTI 1.3 launch is occurring, and the
+  library determines the launch information based on the launch context.
+
+  * This requires the library to be aware of various launch contexts and how to determine launch information for each
+    launch context. This may make extension of the library to other launch contexts more challenging. Contexts would not
+    be able to define their own launch information.
+  * Launch contexts can simply fetch the URL that initiates the LTI 1.3 launch and trigger the LTI 1.3 launch without
+    supplying information to the library.
+
+* The ``lti_message_hint`` encodes the launch information necessary for an LTI 1.3 launch as a JWT.
+
+  * Some browsers enforce limits on the length of URLs for GET requests, and because LTI Tools may make GET requests to
+    views defined in this library that support ``lti_message_hint`` as a query or form parameter, using the 
+    ``lti_message_hint`` parameter may not work properly in all browsers.
+
+References
+**********
+
+* `Github Issue 273: Remove XBlock location dependencies from LTI 1.3 launches
+  <https://github.com/openedx/xblock-lti-consumer/issues/273>`_
+* `Github Pull Request 254: [BB-5559] Decouple LTI 1.3 from LTI Consumer XBlock functionality
+  <https://github.com/openedx/xblock-lti-consumer/pull/254>`_
+* `Github Pull Request 288: feat: decouple LTI 1.3 launch from the XBlockLtiConsumer
+  <https://github.com/openedx/xblock-lti-consumer/pull/288>`_
+* `Github Pull Request 294: API gets config from launch data via config_id
+  <https://github.com/openedx/xblock-lti-consumer/pull/294>`_
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