Redirects (design doc): improving existing functionality

docs/dev/design/redirects.rst

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+Improving redirects
+===================
+
+Redirects are a core feature of Read the Docs,
+they allow users to keep old URLs working when they rename or move a page.
+
+The current implementation lacks some features and has some undefined/undocumented behaviors.
+
+.. contents::
+   :local:
+   :depth: 3
+
+Goals
+-----
+
+- Improve the user experience when creating redirects.
+- Improve the current implementation without big breaking changes.
+
+Non-goals
+---------
+
+- Replicate every feature of other services without
+  having a clear use case for them.
+- Improve the performance of redirects.
+  This can be discussed in an issue or pull request.
+  Performance should be considered when implementing new improvements.
+- Allow importing redirects.
+  We should push users to use our API instead.
+- Allow specifying redirects in the RTD config file.
+  We have had several discussions around this,
+  but we haven't reached a consensus.
+
+Current implementation
+----------------------
+
+We have five types of redirects:
+
+Prefix redirect:
+   Allows to redirect all the URLs that start with a prefix to a new URL
+   using the default version and language of the project.
+   For example: a prefix redirect with the value ``/prefix/``
+   will redirect ``/prefix/foo/bar`` to ``/en/latest/foo/bar``.
+
+   They are basically the same as an exact redirect with a wildcard at the end.
+   They are a shortcut for a redirect like:
+
+   From:
+     ``/prefix/$rest``
+   To:
+     ``/en/latest/``
+
+   Or maybe we could use a prefix redirect to replace the exact redirect with a wildcard?
+
+Page redirect:
+   Allows to redirect a single page to a new URL using the current version and language.
+   For example: a page redirect with the value ``/old/page.html``
+   will redirect ``/en/latest/old/page.html`` to ``/en/latest/new/page.html``.
+
+   Cross domain redirects are not allowed in page redirects.
+   They apply to all versions,
+   if you want it to apply only to a specific version you can use an exact redirect.
+
+   A whole directory can't be redirected with a page redirect,
+   an exact redirect with a wildcard at the end needs to be used instead.
+
+   A page redirect on a single version project is the same as an exact redirect.
+
+Exact redirect:
+   Allows to redirect an exact URL to a new URL,
+   it allows a wildcard at the end to redirect.
+   For example: an exact redirect with the value ``/en/latest/page.html``
+   will redirect ``/en/latest/page.html`` to the new URL.
+
+   If an exact redirect with the value ``/en/latest/dir/$rest``
+   is created, it will redirect all paths that start with ``/en/latest/dir/``,
+   the rest of the path will be added to the new URL automatically.
+
+   - Cross domain redirects are allowed in exact redirects.
+   - They apply to all versions.
+   - A wildcard is allowed at the end of the URL.
+   - If a wildcard is used, the rest of the path will be added to the new URL automatically.
+
+Sphinx HTMLDir to HTML:
+   Allows to redirect clean-URLs to HTML URLs.
+   Useful in case a project changed the style of their URLs.
+
+   They apply to all projects, not just Sphinx projects.
+
+Sphinx HTML to HTMLDir:
+   Allows to redirect HTML URLs to clean-URLs.
+   Useful in case a project changed the style of their URLs.
+
+   They apply to all projects, not just Sphinx projects.
+
+How other services implement redirects
+--------------------------------------
+
+- Gitbook implementation is very basic,
+  they only allow page redirects.
+
+  https://docs.gitbook.com/integrations/git-sync/content-configuration#redirects
+
+- Cloudflare pages allow to capture placeholders and one wildcard (in any part of the URL).
+  They also allow you to set the status code of the redirect,
+  and redirects can be specific in a ``_redirects`` file.
+
+  https://developers.cloudflare.com/pages/platform/redirects/
+
+  They have a limit of 2100 redirects.
+  In case of multiple matches, the topmost redirect will be used.
+
+- Netlify allows to capture placeholders and a wildcard (only allowed at the end).
+  They also allow you to set the status code of the redirect,
+  and redirects can be specific in a ``_redirects`` file.
+
+  - Forced redirects
+  - Match query arguments
+  - Match by country/language and cookies
+  - Per-domain and protocol redirects
+  - In case of multiple matches, the topmost redirect will be used.
+
+  https://docs.netlify.com/routing/redirects/
+
+Improvements
+------------
+
+General improvements
+~~~~~~~~~~~~~~~~~~~~
+
+The following improvements will be applied to all types of redirects.
+
+- Allow choosing the status code of the redirect.
+  We already have a field for this, but it's not exposed to users.
+- Allow to explicitly define the order of redirects.
+  This will be similar to the automation rules feature,
+  where users can reorder the rules so the most specific ones are first.
+  We currently rely on the implicit order of the redirects (updated_at).
+- Allow to disable redirects.
+  It's useful when testing redirects, or when debugging a problem.
+  Instead of having to re-create the redirect,
+  we can just disable it and re-enable it later.
+- Allow to add a short description.
+  It's useful to document why the redirect was created.
+
+Don't run redirects on domains from pull request previews
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+We currently run redirects on domains from pull request previews,
+this is a problem when moving a whole project to a new domain.
+
+We don't the need to run redirects on external domains, they
+should be treated as temporary domains.
+
+Normalize paths with trailing slashes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Currently, if users want to redirect a path with a trailing slash and without it,
+they need to create two separate redirects (``/page/`` and ``/page``).
+
+We can simplify this by normalizing the path before matching it.
+
+For example:
+
+From:
+  ``/page/``
+To:
+  ``/new/page``
+
+The from path will be normalized to ``/page``,
+and the filename to match will also be normalized before matching it.
+This is similar to what Netlify does:
+https://docs.netlify.com/routing/redirects/redirect-options/#trailing-slash.
+
+Page and exact redirects without a wildcard at the end will be normalized,
+all other redirects need to be matched as is.
+
+This makes it impossible to match a path with a trailing slash.
+
+Explicit ``$rest`` placeholder
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Explicitly place the ``$rest`` placeholder in the target URL,
+instead of adding it automatically.
+
+Some times users want to redirect to a different path,
+we have been adding a query parameter in the target URL to
+prevent the old path from being added in the final path.
+For example ``/new/path/?_=``.
+
+Instead of adding the path automatically,
+users have to add the ``$rest`` placeholder in the target URL.
+For example:
+
+From:
+  ``/old/path/$rest``
+To:
+  ``/new/path/$rest``
+
+From:
+  ``/old/path/$rest``
+To:
+  ``/new/path/?page=$rest&foo=bar``
+
+Improving page redirects
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+- Allow to redirect to external domains.
+  This can be useful to apply a redirect of a well known path
+  in all versions to another domain.
+
+  For example, ``/security/`` to a their security policy page in another domain.
+
+  This new feature isn't strictly needed,
+  but it will be useful to simplify the explanation of the feature
+  (one less restriction to explain).
+
+- Allow a wildcard at the end of the from path.
+  This will allow users to migrate a whole directory to a new path
+  without having to create an exact redirect for each version.
+
+  Similar to exact redirects, users need to add the ``$rest`` placeholder explicitly.
+  This means that that page redirects are the same as exact redirects,
+  with the only difference that they apply to all versions.
+
+Improving Sphinx redirects
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+These redirects are useful, but we should rename them to something more general,
+since they apply to all types of projects, not just Sphinx projects.
+
+Proposed names:
+
+- HTML URL to clean URL redirect (``file.html`` to ``file/``)
+- Clean URL to HTML URL redirect (``file/`` to ``file.html``)
+
+Other ideas to improve redirects
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- Run forced redirects before built-in redirects.
+  We currently run built-in redirects before forced redirects,
+  this is a problem when moving a whole project to a new domain.
+  For example, a forced redirect like ``/$rest``,
+  won't work for the root URL of the project,
+  since ``/`` will first redirect to ``/en/latest/``.
+
+  But shouldn't be a real problem, since users will still need to
+  handle the ``/en/latest/file/`` paths.
+
+- Run redirects on the edge.
+  Cloudflare allow us to create redirects on the edge,
+  but they have some limitations around the number of
+  redirect rules that can be created.
+
+  And they will be useful for forced exact redirects only,
+  since we can't match a redirect based on the response of the origin server.
+
+- Merge prefix redirects with exact redirects.
+  Prefix redirects are the same as exact redirects with a wildcard at the end.
+
+- Merge all redirects into a single type.
+  This may simplify the implementation,
+  but it will make it harder to explain the feature to users.
+  And to replace some redirects we need to implement some new features.
+
+- Placeholders.
+  I haven't seen users requesting this feature.
+  We can consider adding it in the future.
+  Maybe we can expose the current language and version as placeholders.
+
+- Replace ``$rest`` with ``*`` in the from_url.
+  This will be more consistent with other services,
+  but it will require users to re-learn the feature.
+
+- Per-protocol redirects.
+  We should push users to always use HTTPS.
+
+- Allow a prefix wildcard.
+  We currently only allow a suffix wildcard,
+  adding support for a prefix wildcard should be easy.
+  But do users need this feature?
+
+- Per-domain redirects.
+  The main problem that originated this request was that we were applying redirects on external domains,
+  if we stop doing that, there is no need for this feature.
+  We can also try to improve how our built-in redirects work
+  (specially our canonical domain redirect).
+
+Allow matching query arguments
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+We can do this in two ways:
+
+- At the DB level with some restrictions.
+  If done at the DB level,
+  we would need to have a different field
+  with just the path, and other with the query arguments normalized and sorted.
+
+  For example, if we have a redirect with the value ``/foo?blue=1&yellow=2&red=3``,
+  if would be normalized in the DB as ``/foo`` and ``blue=1&red=3&yellow=2``.
+  This implies that the URL to be matched must have the exact same query arguments,
+  it can't have more or less.
+
+  I believe the implementation described here is the same being used by Netlify,
+  since they have that same restriction.
+
+      If the URL contains other parameters in addition to or instead of id, the request doesn't match that rule.
+
+      https://docs.netlify.com/routing/redirects/redirect-options/#query-parameters
+
+- At the Python level.
+  If done at the DB level,
+  we would need to have a different field
+  with just the path, and other with query arguments.
+
+  The matching of the path would be done at the DB level,
+  and the matching of the query arguments would be done at the Python level.
+  Here we can be more flexible, allowing any query arguments in the matched URL.
+
+  We had some performance problems in the past,
+  but I believe it was mainly due to the use of regex instead of using string operations.
+  And matching the path is still done at the DB level.
+  We could limit the number of redirects that can be created with query arguments,
+  or the number of redirects in general.
+
+We hava had only one user requesting this feature,
+so this is not a priority.
+
+Migration
+---------
+
+Most of the proposed improvements are backwards compatible,
+and just need a data migration to normalize existing redirects.
+
+For the exception of adding the ``$rest`` placeholder in the target URL explicitly,
+that needs users to re-learn how this feature works, i.e, they may be expecting
+to have the path added automatically in the target URL.
+
+We can create a small blog post explaining the changes.