The Drupal 8 configuration management works best when importing and exporting the whole set of the sites configuration. However, sometimes developers like to opt out of the robustness of CM and have a super-set of configuration active on their development machine. The canonical example for this is to have the devel module enabled or having a few block placements or views in the development environment and then not export them into the set of configuration to be deployed, yet still being able to share the development configuration with colleagues.

This module allows to define sets of configuration that will get exported to separate directories when exporting, and get merged together when importing. It is possible to define in settings.php which of these sets should be active and considered for the export and import.

Let us assume that you configured your sync directory as follows:

$settings['config_sync_directory'] = '../config/sync';

Create a split with the folder ../config/my-split-folder and create that directory. Now add a module that is currently active that you wish not to export, say devel. Next export all the configuration (with drush cex). This should have removed devel from core.extensions and moved the devel configuration to the split folder. Next you can disable the split in the UI and enable it with a config override.

// In settings.php assuming your split was called 'my_split'
$config['config_split.config_split.my_split']['status'] = TRUE;

Now export the configuration again and you will see the split being deactivated But it is still active on your development site due to the override.

Now deploy the configuration and devel will be un-installed. On another developers machine just import the configuration, add the override, clear the cache, and import again to have devel enabled on that environment.

You should only edit active splits as inactive splits will not take effect when exporting the configuration.

NOTE: Do NOT put configuration directories inside of each other. In particular the split folder MUST NOT be inside of the sync directory. Recommended is a sibling, or in other words a folder that shares the same parent as the sync directory or in a folder with other split folders which is next to the sync folder.

Examples:

../config/
├── dev
├── sync
└── test

../config/
├── sync
└── splits
    ├── dev
    └── live

The module depends on Config Filter for the integration with the import/export pipeline of the Drupal UI and drush. The configuration is read from the main directory and also from split directories under the hood. Presenting Drupal with the unified configuration of the sync directory and the extra configuration defined in splits. Importing and exporting works the same way as before, except some configuration is read from and written to different directories. Importing configuration still removes configuration not present in the files. Thus, the robustness and predictability of the configuration management remains.

You may need to single-import the split configuration if it changes apart from which extensions are split off. And remember to clear the caches when overriding the splits configuration.

The important part to remember is to use Drupal 8s configuration management the way it was intended to be used. This module does not interfere with the active configuration but instead filters on the import/export pipeline. If you use this module you should have a staging environment where you can let the configuration management do its job and verify that everything is good for deployment.