Merge pull request #23829 from edx/feanil/settings_adr

An initial draft of a decision around how we manage settings files.

docs/decisions/0004-managing-django-settings.rst

+Status
+======
+
+Accepted
+
+
+Context
+=======
+
+Settings are confusing right now and there are way too many layers. We have
+multiple python settings file that "inherit" from each other. In some cases some
+layers of the inheritance tree also pull settings from config files leading to
+more confusion.
+
+
+Decision
+========
+
+Rather than having multiple python settings files that override things
+differently we will move to just a few python files with most of the settings
+variance living in YAML config files that are managed by environment operators.
+The structure of these files try to match `OEP-45` as bast as we can. Because
+the edx-platform houses both the ``LMS`` and ``Studio`` applications, our
+implementation will differ slightly from the guidance provided by `OEP-45`.
+
+LMS and CMS settings will continue to live under ``lms/envs/...`` and ``cms/envs/...``.
+
+Underneath those folders, we will follow the layout suggested by `OEP-45`.
+
+
+To recap that here:
+
+``__init__.py`` will handle the loading of defaults, environment overrides, and ensuring that required settings are set.
+
+``defaults.py`` will house and document all default settings.
+
+``required.py`` will document and validate that required settings have been defined.
+
+For edx-platform there may be cases where settings are additive, in this case
+the managing of those additive settings will be managed within
+``__init__.py``
+
+For example, FEATURES, or JWT_AUTH are both settings where we
+currently pull parts of the data from the config and inject it into an existing
+data structure. While it is possible that in the future we would make these be
+more explicit, we don't want to take on that work now as it may increase
+complexity.
+
+
+.. _OEP-45: https://github.com/edx/open-edx-proposals/pull/143/files
+
+Consequences
+============
+
+Rather than having both python and yaml override files for our dev and test
+environments, we will move towards having all settings defined in a yaml file
+and for all environments to use __init__.py to load their settings.
+
+The following files should be obviated by this change:
+
+* bok_choy_docker.py
+* bok_choy.py
+* devstack_docker.py
+* devstack_optimized.py
+* devstack.py
+* devstack_with_worker.py
+* load_test.py
+* openstack.py
+* static.py
+* test.py
+* test_static_optimized.py
+
+If there exist default YAML files for any of the above environments they should
+be updated to absorb any overrides that lived in these python settings files.
+Any environments that use these files should be updated.
+
+Alternatives Considered
+=======================
+
+Don't use YAML files, just python modules
+-----------------------------------------
+
+Instead of having YAML and then translating that to python, we can just have
+python settings files for all variants we want.
+
+Pros:
+
+* Settings can still be in a single place per environment.
+* You get the full power of python when building out settings.
+* Settings values can be complex python objects.
+
+Cons:
+* Because settings have secrets, we would have to keep our settings file out of
+  the public repository.
+* This wouldn't solve the problem where we would still try to "inherit" from other settings files and make it harder to read the current value of any given setting.
+
+This alternative gives us a lot more power but it's power that we don't actually need.  Building limitiations into what settings can be helps us keep them simple and understandable.