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+Better handling of docs URLs
+============================
+
+``proxito`` is the component of our code base in charge of serving documentation
+to users and handling any other URLs from the user documentation domain.
+
+The current implementation has some problems that are discussed in this document,
+and an alternative implementation is proposed to solve those problems.
+
+Goals
+-----
+
+* Simplifying our parsing logic for URLs
+* Removing reserved paths and ambiguities from URLs
+* Allow serving docs from a different prefix and subproject prefix.
+
+Non-goals
+---------
+
+* Allowing fully arbitrary URL generation for projects,
+  like changing the order of the elements or removing them.
+
+Current implementation
+----------------------
+
+The current implementation is based on Django URLs
+trying to match a pattern that looks like a single project, a versioned project,
+or a subproject, this means that a couple of URLs are *reserved*,
+and won't resolve to the correct file if it exists
+(https://github.com/readthedocs/readthedocs.org/issues/8399, https://github.com/readthedocs/readthedocs.org/issues/2292),
+this usually happens with single version projects.
+
+And to support custom URLs we are hacking into Django's urlconf
+to override it at runtime,
+this doesn't allow us to implement custom URLs for subprojects easily
+(https://github.com/readthedocs/readthedocs.org/pull/8327).
+
+Alternative implementation
+--------------------------
+
+Instead of trying to map a URL to a view,
+we first analyze the root project (given from the subdomain),
+and based on that we map each part of the URL to the *current* project and version.
+
+This will allow us to re-use this code in our unresolver
+without the need to override the Django's urlconf at runtime,
+or guessing a project only by the structure of its URL.
+
+Terminology:
+
+Root project
+  The project from where the documentation
+  is served (usually the parent project of a subproject or translation).
+Current project
+  The project that owns the current file being served
+  (a subproject, a translation, etc).
+Requested file
+  The final path to the file that we need to serve from the current project.
+
+Look up process
+---------------
+
+Proxito will process all documentation requests from a single *docs serve* view,
+exluding ``/_`` URLs.
+
+This view then will process the current URL using the root project as follows:
+
+- Check if the root project has translations
+  (the project itself is a translation if isn't a single version project),
+  and the first part is a language code and the second is a version.
+
+  - If the lang code doesn't match, we continue.
+  - If the lang code matches, but the version doesn't, we return 404.
+
+- Check if it has subprojects and the first part of the URL matches the subprojects prefix (``projects``),
+  and if the second part of the URL matches a subproject alias.
+
+  - If the subproject prefix or the alias don't match, we continue.
+  - If they match, we try to match the rest of the URL for translations/versions and single versions
+    (i.e, we don't search for subprojects) and we use the subproject as the new *root project*.
+
+- Check if the project is a single version.
+  Here we just try to serve the rest of the URL as the file.
+
+- Check if the first part of the URL is ``page``,
+  then this is a ``page`` redirect.
+  Note that this is done after we have discarded the project being a single version
+  project, since it doesn't makes sense to use that redirect with single version projects,
+  and it could collide with the project having a ``page/`` directory.
+
+- 404 if none of the above rules match.
+
+Custom URLs
+~~~~~~~~~~~
+
+We are using custom URLs mainly to serve the documentation
+from a different directory:
+
+- deeplearning/nemo/user-guide/docs/$language/$version/$filename
+- deeplearning/nemo/user-guide/docs/$language/$version/$filename
+- deeplearning/frameworks/nvtx-plugins/user-guide/docs/$language/$version/$filename
+
+We always keep the lang/version/filename order,
+do we need/want to support changing this order?
+Doesn't seem useful to do so.
+
+So, what we need is have a way to specify a prefix only.
+We would have a prefix used for translations and another one used for subprojects.
+These prefixes will be set in the root project.
+
+The look up order would be as follow:
+
+- If the root project has a custom prefix, and the current URL matches that prefix,
+  remove the prefix and follow the translations and single version look up process.
+  We exclude subprojects from it, i.e, we don't check for ``{prefix}/projects``.
+- If the root project has subprojects and a custom subprojects prefix (``projects`` by default),
+  and if the current URL matches that prefix,
+  and the next part of the URL matches a subproject alias,
+  continue with the subproject look up process.
+
+Examples
+--------
+
+The next examples are organized in the following way:
+
+- First there is a list of the projects involved,
+  with their available versions.
+- The first project would be the root project.
+- The other projects will be related to the root project
+  (their relationship is given by their name).
+- Next we will have a table of the requests,
+  and their result.
+
+Project with versions and translations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Projects:
+
+- project (latest, 1.0)
+- project-es (latest, 1.0)
+
+Requests:
+
+.. list-table::
+   :header-rows: 1
+
+   * - Request
+     - Requested file
+     - Current project
+     - Note
+   * - /en/latest/manual/index.html
+     - /latest/manual/index.html
+     - project
+     -
+   * - /en/1.0/manual/index.html
+     - /1.0/manual/index.html
+     - project
+     -
+   * - /en/1.0/404
+     - 404
+     - project
+     - The file doesn't exist
+   * - /en/2.0/manual/index.html
+     - 404
+     - project
+     - The version doesn't exist
+   * - /es/latest/manual/index.html
+     - /latest/manual/index.html
+     - project-es
+     -
+   * - /es/1.0/manual/index.html
+     - /1.0/manual/index.html
+     - project-es
+     -
+   * - /es/1.0/404
+     - 404
+     - project-es
+     - The translation exist, but not the file
+   * - /es/2.0/manual/index.html
+     - 404
+     - project-es
+     - The translation exist, but not the version
+   * - /pt/latest/manual/index.html
+     - 404
+     - project
+     - The translation doesn't exist
+
+Project with subprojects and translations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Projects:
+
+- project (latest, 1.0)
+- project-es (latest, 1.0)
+- subproject (latest, 1.0)
+- subproject-es (latest, 1.0)
+
+.. list-table::
+   :header-rows: 1
+
+   * - Request
+     - Requested file
+     - Current project
+     - Note
+   * - /projects/subproject/en/latest/manual/index.html
+     - /latest/manual/index.html
+     - subproject
+     -
+   * - /projects/subproject/en/latest/404
+     - 404
+     - subproject
+     - The subproject exists, but not the file
+   * - /projects/subproject/en/2.x/manual/index.html
+     - 404
+     - subproject
+     - The subproject exists, but not the version
+   * - /projects/subproject/es/latest/manual/index.html
+     - /latest/manual/index.html
+     - subproject-es
+     -
+   * - /projects/subproject/br/latest/manual/index.html
+     - 404
+     - subproject
+     - The subproject exists, but not the translation
+   * - /projects/nothing/en/latest/manual/index.html
+     - 404
+     - project
+     - The subproject doesn't exist
+   * - /manual/index.html
+     - 404
+     - project
+     -
+
+Single version project with subprojects
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Projects:
+
+- project (latest)
+- subproject (latest, 1.0)
+- subproject-es (latest, 1.0)
+
+.. list-table::
+   :header-rows: 1
+
+   * - Request
+     - Requested file
+     - Current project
+     - Note
+   * - /projects/subproject/en/latest/manual/index.html
+     - /latest/manual/index.html
+     - subproject
+     -
+   * - /projects/subproject/en/latest/404
+     - 404
+     - subproject
+     - The subproject exists, but the file doesn't
+   * - /projects/subproject/en/2.x/manual/index.html
+     - 404
+     - subproject
+     - The subproject exists, but the version doesn't
+   * - /projects/subproject/es/latest/manual/index.html
+     - /latest/manual/index.html
+     - subproject-es
+     -
+   * - /projects/subproject/br/latest/manual/index.html
+     - 404
+     - subproject
+     - The subproject exists, but the translation doesn't
+   * - /projects/nothing/en/latest/manual/index.html
+     - 404
+     - project
+     - The subproject doesn't exist
+   * - /manual/index.html
+     - /latest/manual/index.html
+     - project
+     -
+   * - /404
+     - 404
+     - project
+     - The file doesn't exist
+   * - /projects/index.html
+     - /latest/projects/index.html
+     - project
+     - The project has a ``projects`` directory!
+   * - /en/index.html
+     - /latest/en/index.html
+     - project
+     - The project has an ``en`` directory!
+
+Project with single version subprojects
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Projects:
+
+- project (latest, 1.0)
+- project-es (latest, 1.0)
+- subproject (latest)
+
+.. list-table::
+   :header-rows: 1
+
+   * - Request
+     - Requested file
+     - Current project
+     - Note
+   * - /projects/subproject/manual/index.html
+     - /latest/manual/index.html
+     - subproject
+     -
+   * - /projects/subproject/en/latest/manual/index.html
+     - 404
+     - subproject
+     - The subproject is single version
+   * - /projects/subproject/404
+     - 404
+     - subproject
+     - The subproject exists, but the file doesn't
+   * - /projects/subproject/br/latest/manual/index.html
+     - /latest/br/latest/manual/index.html
+     - subproject
+     - The subproject has a ``br`` directory!
+   * - /projects/nothing/manual/index.html
+     - 404
+     - project
+     - The subproject doesn't exist
+   * - /en/latest/manual/index.html
+     - /latest/manual/index.html
+     - project
+     -
+   * - /404
+     - 404
+     - project
+     -
+
+Project with custom prefix
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- project (latest, 1.0)
+- subproject (latest, 1.0)
+
+``project`` has the ``prefix`` prefix, and ``sub`` subproject prefix.
+
+.. list-table::
+   :header-rows: 1
+
+   * - Request
+     - Requested file
+     - Current project
+     - Note
+   * - /en/latest/manual/index.html
+     - 404
+     - project
+     - The prefix doesn't match
+   * - /prefix/en/latest/manual/index.html
+     - /latest/manual/index.html
+     - project
+     -
+   * - /projects/subproject/en/latest/manual/index.html
+     - 404
+     - project
+     - The subproject prefix doesn't match
+   * - /sub/subproject/en/latest/manual/index.html
+     - /latest/manual/index.html
+     - subproject
+     -
+   * - /sub/nothing/en/latest/manual/index.html
+     - 404
+     - project
+     - The subproject doesn't exist
+
+Project with custom subproject prefix (empty)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- project (latest, 1.0)
+- subproject (latest, 1.0)
+
+``project`` has the ``/`` subproject prefix,
+this allow us to serve subprojects without using a prefix.
+
+.. list-table::
+   :header-rows: 1
+
+   * - Request
+     - Requested file
+     - Current project
+     - Note
+   * - /en/latest/manual/index.html
+     - /latest/manual/index.html
+     - project
+     -
+   * - /projects/subproject/en/latest/manual/index.html
+     - 404
+     - project
+     - The subproject prefix doesn't match
+   * - /subproject/en/latest/manual/index.html
+     - /latest/manual/index.html
+     - subproject
+     -
+   * - /nothing/en/latest/manual/index.html
+     - /latest/manual/index.html
+     - project
+     - The subproject/file doesn't exist
+
+Implementation example
+----------------------
+
+This is a simplified version of the implementation,
+there are some small optimizations and validations that will be in the
+final implementation.
+
+In the final implementation we will be using regular expressions to extract
+the parts from the URL.
+
+.. code-block:: python
+
+   from readthedocs.projects.models import Project
+
+   LANGUAGES = {"es", "en"}
+
+   def pop_parts(path, n):
+       if path[0] == '/':
+          path  = path[1:]
+       parts = path.split('/', maxsplit=n)
+       start, end = parts[:n], parts[n:]
+       end = end[0] if end else ''
+       return start, end
+
+
+   def resolve(canonical_project: Project, path: str, check_subprojects=True):
+       prefix = '/'
+       if canonical_project.prefix:
+           prefix = canonical_project.prefix
+       subproject_prefix = "/projects"
+       if canonical_project.subproject_prefix:
+           subproject_prefix = canonical_project.subproject_prefix
+
+       # Multiversion project.
+       if path.startswith(prefix):
+           new_path = path.removeprefix(prefix)
+           parts, new_path = pop_parts(new_path, 2)
+           language, version_slug = parts
+           if not canonical_project.single_version and language in LANGUAGES:
+               if canonical_project.language == language:
+                   project = canonical_project
+           else:
+               project = canonical_project.translations.filter(language=language).first()
+               if project:
+                   version = project.versions.filter(slug=version_slug).first()
+                   if version:
+                       return project, version, new_path
+                   return project, None, None
+
+       # Subprojects.
+       if check_subprojects and path.startswith(subproject_prefix):
+           new_path = path.removeprefix(subproject_prefix)
+           parts, new_path = pop_parts(new_path, 1)
+           project_slug = parts[0]
+           project = canonical_project.subprojects.filter(alias=project_slug).first()
+           if project:
+               return resolve(
+                   canonical_project=project,
+                   path=new_path,
+                   check_subprojects=False,
+               )
+
+       # Single project.
+       if path.startswith(prefix):
+           new_path = path.removeprefix(prefix)
+           if canonical_project.single_version:
+               version = canonical_project.versions.filter(
+                   slug=canonical_project.default_version
+               ).first()
+               if version:
+                   return canonical_project, version, new_path
+               return canonical_project, None, None
+
+       return None, None, None
+
+
+   def view(canonical_project, path):
+       current_project, version, file = resolve(
+           canonical_project=canonical_project,
+           path=path,
+       )
+       if current_project and version:
+           return serve(current_project, version, file)
+
+       if current_project:
+           return serve_404(current_project)
+
+       return serve_404(canonical_project)
+
+
+   def serve_404(project, version=None):
+       pass
+
+
+   def serve(project, version, file):
+       pass
+
+
+Performance
+~~~~~~~~~~~
+
+Performance is mainly driven by the number of database lookups.
+There is an additional impact performing a regex lookup.
+
+- A single version project:
+
+  - ``/index.html``: 1, the version.
+  - ``/projects/guides/index.html``: 2, the version and one additional lookup for a path that looks like a subproject.
+
+- A multi version project:
+
+  - ``/en/latest/index.html``: 1, the version.
+  - ``/es/latest/index.html``: 2, the translation and the version.
+  - ``/br/latest/index.html``: 1, the translation (it doesn't exist).
+
+- A project with single version subprojects:
+
+  - ``/projects/subproject/index.html``: 2, the subproject and its version.
+
+- A project with multi version subprojects:
+
+  - ``/projects/subproject/en/latest/index.html``: 2, the subproject and its version.
+  - ``/projects/subproject/es/latest/index.html``: 3, the subproject, the translation, and its version.
+  - ``/projects/subproject/br/latest/index.html``: 2, the subproject and the translation (it doesn't exist).
+
+As seen, the number of database lookups are the minimal
+required to get the current project and version,
+this is a minimum of 1, and maximum of 3.
+
+Questions
+---------
+
+- When using custom URLs,
+  should we support changing the URLs
+  that aren't related to doc serving?
+
+  These are:
+
+  - Health check
+  - Proxied APIs
+  - robots and sitemap
+  - The ``page`` redirect
+
+  This can be useful for people that proxy us from another path.
+
+- Should we use the urlconf from the subproject when processing it?
+  This is an URL like ``/projects/subproject/custom/prefix/en/latest/index.html``.
+
+  I don't think that's useful, but it should be easy to support if needed.
+
+- Should we support the page redirect when using a custom subproject prefix?
+  This is ``/{prefix}/subproject/page/index.html``.