Webmaster doc

docs/webmaster-modules.md

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+# Webmaster modules
+
+Administrators on webmaster libraries has the ability to upload and
+enable extra modules. These can be both standard Drupal contrib
+modules, or modules developed by/for the library.
+
+Developing bespoke modules doesn't not differ from developing modules
+in general, as installing and configuring modules works as expected.
+
+However, it not uncommon for the developer to wish to provide some
+configuration "outside" the modules own configuration, adding taxonomy
+vocabularies, adding fields to nodes or even changing basic
+configuration variables, which is somewhat challenging as DPL CMS
+controls the configuration.
+
+So to avoid every module doing it differently, this guide exists. And
+if enough modules finds the need, the technique described here could
+be implemented in a `dpl_webmaster` module that provide it as a
+service.
+
+## Webmaster module configuration handling
+
+First off, a warning: when using this technique you take the
+responsibility of the configuration added/overridden in this way. If
+you for instance override the node form display configuration for a
+node type, it's your responsibility to keep it up to date with changes
+in DPL CMS core.
+
+Adding new configuration for modules not in DPL CMS is more safe, but
+you should look out for dependencies. If a configuration depends on a
+particular node type or field being available for instance.
+
+In general, you should get more familiar with the configuration YAML
+you're putting into your module, than you're used to from general
+Drupal configuration management, and know exactly why you need each
+one.
+
+### Overview
+
+Configuration handling in webmaster modules consists of three parts:
+
+1. The configuration itself, in Drupals configuration export YAML
+   format, in a `config/sync` directory in the module.
+2. An event subscriber that overlays the modules configuration when
+   Drupal imports the configuration.
+3. A `hook_install` and possibly `hook_update_N` functions that
+   trigger configuration import when the module is installed and
+   updated.
+
+We'll describe the parts in detail in the following walk-through.
+
+### Walk-through
+
+#### Initial configuration
+
+1. Start with a fresh DPL CMS site with the code base from git, that
+   has up to date configuration (`task dev:cli -- drush cim` should do
+   nothing).
+2. Make the required configuration changes in the Drupal
+   administration interface.
+3. Run `task dev:cli -- drush cex -y` to export the configuration.
+4. Now `git status` (or your preferred Git tool) tells you which files
+   has been changed, these are the files you need.
+5. Copy the changed configuration files to `config/sync` in your
+   module and revert the changes to the files in the root
+   `config/sync` folder.
+
+#### Event subscriber
+
+In order to get DPL CMS to actually use the configuration you just
+saved, we'll need to make it visible to Drupal. This can be done by
+implementing an event subscriber that overlays the configuration on
+`ConfigEvents::STORAGE_TRANSFORM_IMPORT`.
+
+An implementation can be found in
+[kdb_brugbyen](https://github.com/kdb/kdb_brugbyen/blob/main/src/EventSubscriber/OverlayConfigEventSubscriber.php).
+You can simply copy that and fix the two references to the module (the
+namespace and the configuration path).
+
+#### Install/update hook
+
+The above will add in the module configuration when the configuration
+is imported, but installing or updating a module does not trigger a
+configuration import, so you'll need to do it yourself.
+
+Outside of DPL CMS, triggering a configuration import from
+install/update hooks is not recommended, but inside DPL CMS we're in a
+controlled environment where library sites should always be in sync
+with the provided configuration. So triggering an import to overlay
+module configuration can be considered safe.
+
+Importing configuration is both simple and horribly convoluted. The
+`ConfigImporter` class does all the heavy lifting, but it's not a
+service and requires digging out thirteen different services in order
+to be instantiated. Again, you can just copy the code from
+[kdb_brugbyen](https://github.com/kdb/kdb_brugbyen/blob/main/kdb_brugbyen.install).
+
+### Maintenance
+
+Keeping the module configuration up to date might pose some
+challenges.
+
+#### Module configuration changes
+
+Changing the configuration is often simple, just do the modifications,
+export and copy the changed files. You'll need to add an update hook
+that triggers a configuration import in order for the changes to take
+effect when the module is updated.
+
+#### DPL CMS configuration changes
+
+Merging in changes from DPL CMS could be problematic, as applying the
+configuration changes from the module that's based on the previous
+version of DPL CMS might throw a `ConfigImporterException` and make
+the deployment fail.
+
+The hard failure modes of this sounds bad, but the deployment policy
+of first rolling out new DPL CMS releases to the libraries
+`moduletest` environments should catch these issues before they hit
+production.
+
+When this happens, you can either rebuild the configuration in the
+administration interface and export it fresh, or by careful inspection
+of diffs apply the needed changes by hand to the YAML files.
+
+Even if importing the configuration doesn't show any errors, the fact
+that the module in effect replaces part of the DPL CMS configuration
+might cause upstream changes to get lost, so you should occasionally
+check for this. This can be done by doing a configuration export and
+inspecting the diff of the configuration changes for changes that's
+unrelated to the modules changes.
+
+Deploying the fixed version requires some extra care. Obviously, you
+cannot update the module after updating DPL CMS, as the upgrade will
+fail when trying to apply the old changes from the module to the new
+DPL CMS configuration. So you'll have to update the module first, but
+you should not trigger an configuration import when doing so, as that
+would likely fail just as bad when trying to overlay the modules new
+configuration on the old DPL CMS configuration (so no `hook_update_N`
+for this version of the module).
+
+Test it out first on `moduletest`.
+
+1. Ask the hosting team to get `moduletest` reverted to the previous
+DPL CMS version and database and files synced with production.
+2. Update the module.
+3. Ask the hosting team to upgrade the `moduletest` environment.
+
+If it works, you can update the module in production and ask for
+upgrading production. Make the library aware that, in the timespan
+between updating the module and updating DPL CMS, they cannot do
+anything that triggers a configuration import. In practice this should
+translate to "don't upload any webmaster modules", as that should be
+about the only thing apart from a DPL CMS update that triggers an
+import.