This tutorial covers common use cases for configuration splits as a strategy for configuration management in Drupal 8.

Specifically it covers:

Default application configuration
Environment specific configuration (e.g., local, data, test, prod, etc.)
Site-specific configuration (when multisite is used)
"Feature" specific configuration (e.g. a distinct blog feature that is shared across multiple sites). Not to be confused with the features module.
Miscellaneous troubleshooting information

Scenario Background

For the sake of this tutorial, let's assume that we have a "kitchen sink" site that requires all of these types of splits. It is a multisite application and it will be hosted in multiple environments, which must share some default configuration between all sites. It also must allow some features (like a blog) to be enabled on some sites and not others.

Application requirements

The following modules are required to implement the covered strategies:

You should also have drush installed on your local machine.

If you are using BLT, these tools are already available to your application.

Default configuration

Let's start out by exporting the default configuration for the application. This is the configuration that will be imported for your application by default, even if no splits are defined or active. It will be shared by all sites using this application.

For the sake of this tutorial, let's focus on one particular configuration setting: system.performance. This controls caching and aggregation settings for Drupal core.

Navigate to /admin/config/development/performance and enable caching and aggregation.
drush en config_split -y
drush config-export -y
drush cr

This will populate ../config/default with all configuration for the site. ../config/default/system.performance.yml should now exist and contain the following configuration (this is a partial representation):

    cache:
      page:
        max_age: 3600
    css:
      preprocess: true
      gzip: true
    js:
      preprocess: true
      gzip: true

Test overriding and reverting

You can test the process of importing configuration by:

Navigating to /admin/config/development/performance
Disabling caching and aggregation.
Executing drush config-import.

You should then find that caching and aggregation have been re-enabled, congruent with the previously exported configuration.

This is the simplest use case for the configuration management system.

Environment split

By default, we want caching and CSS and JavaScript aggregation enabled on all of our environments. However, we would like the local environment to be an exception to this. Caching and aggregation should be disabled on local machines to expedite the development process.

To accomplish this, we will create a "local" configuration split.

mkdir -p ../config/envs/local

/admin/config/development/configuration/config-split/add:

 status: false
 label: Local
 folder: ../config/envs/local
 blacklist: {  }
 graylist:
   - system.performance
 graylist_dependents: true
 graylist_skip_equal: true
 weight: 0

drush config-export -y. This will export the configuration definition for the split itself, which is stored in config/default/config_split.config_split.local.yml.
drush cr. Doing this will allow configuration split to recognize that the local split is active. We rely on BLT to designate this split as active on local machines via a settings.php include.
Navigate to /admin/config/development/performance and disable caching and aggregation.
drush config-export -y. Because the local split is active, this will export the local split system.performance settings to ../config/envs/local/system.performance.yml. It should contain the following configuration:
```
 cache:
 page:
   max_age: 0
 css:
 preprocess: false
 gzip: false
 js:
 preprocess: false
 gzip: false
```

A few notes regarding the settings

The folder is relative to the drupal docroot.
We set active to zero because we don't want configuration management to manage whether this split is active. Instead, we will rely on BLT to enable this split, when appropriate, via a settings.php include. If you are using BLT, this should already be loaded for you as a consequence of including blt.settings.php in your settings.php file. However, you may override this logic by setting $split in settings.php prior to including blt.settings.php.
You may see that even on your local environment, after running drush config-import, the local configuration split has a status of "active (overwritten)". This is normal and does not indicate a problem. The mere fact that it is active is an override of the exported active: 0 setting in the split itself. It does not necessarily indicate that the configuration which the split controls is actually overridden.

Feature split

Consider that we are creating a multisite Drupal application. We would like Sites A and B to have blogs, and Site C to have no blog. The blog feature itself should be managed via configuration management.

To accomplish this, we will create a "blog" configuration split. That split will be active on Sites A and B but not on Site C.

Creating a feature split

Create blog content type.
mkdir -p ../config/features/blog

/admin/config/development/configuration/config-split/add:

 status: false
 label: Blog
 folder: ../config/features/blog
 blacklist:
   - core.base_field_override.node.blog_entry.promote
   - core.entity_form_display.node.blog_entry.default
   - core.entity_view_display.node.blog_entry.default
   - core.entity_view_display.node.blog_entry.teaser
   - field.field.node.blog_entry.body
   - node.type.blog_entry
   - system.action.user_add_role_action.blog_entry_creator
   - system.action.user_add_role_action.blog_entry_reviewer
   - system.action.user_remove_role_action.blog_entry_creator
   - system.action.user_remove_role_action.blog_entry_reviewer
   - user.role.blog_entry_creator
   - user.role.blog_entry_reviewer
 graylist: {  }
 graylist_dependents: true
 graylist_skip_equal: true
 weight: 0

Visit /admin/config/development/configuration/ignore and add the following line to "Configuration entity names to ignore":
```
 config_split.config_split.blog:status
```
This will instruct the configuration management system to ignore the status of the blog configuration split. This will permit you to export a default status of false for the blog split, and still manually enable that split on selected sites without causing the split to be flagged as overwritten.
drush config-export -y. The configuration for the blog split itself should now exist in ../config/default/config_split.config_split.blog.yml.
drush cr

Enabling a feature split

Let's assume that you would like to enable the blog split for multisite Site 2. To enable the blog feature:

Visit /admin/config/development/configuration/config-split and enable the Blog split.
Import configuration for site2 via drush config-import --uri=site2.

Issues with this approach

The status of the feature split is not managed via configuration management. Therefore, you must enable the split by using the user interface even on a production environment.
It can be very difficult to identify all of the configuration that a given feature should encompass.
It is not entirely possible to disentangle a single feature from all related configuration. For instance, we may segment the node configuration and fields for the "Blog" feature. However, config/default/search_api.index.content.yml and config/default/views.view.search.yml still contain references to blog_entry in their exported configuration. This is not necessarily problematic, but it feels a little messy.

Multisite split

Consider that we would like site to to have different cache lifetimes then the default configuration specifies. We have the following two directories:

docroot/sites/default
docroot/sites/site2

Creating a site split

Execute mkdir -p ../config/site2 to ensure that we have the following configuration directories:

config/default
config/site2

Create a split for site2:

 status: false
 label: Site 2
 folder: ../config/site2
 blacklist: {  }
 graylist: {  }
 graylist_dependents: true
 graylist_skip_equal: true
 weight: 0

Executing commands against multisites

When executing a drusd command against a multisite, include the uri option. For instance, drush --uri=site2.
When executing a BLT command against a multisite, include the site config value. For instance, blt setup --define site=site2. BLT also allows you to create site-specific configuration, see BLT multisite documentation for more information.

Misc

Exporting to a split that is not active

When developing locally, we often need to export to a split other than local. For instance, we may want to change some of the configuration in the dev split. There are a few ways to do this:

Manually modify the configuration files. This is the simplest and most straightforward, but it can be very time-consuming.
Use the drush config-split-export [split] command to export to a specific split. For instance, to export the current configuration on your local machine to the dev split, he would execute drush config-split-export dev

Conflicting configuration

Higher weight takes precedence

Where possible, you should avoid exporting the same configuration (with different values) to multiple splits. However, this is sometimes desirable. When two splits define the same configuration be split with the higher weight will take precedence. This is somewhat counterintuitive, as the common Drupal convention is for elements with lesser weights to take precedence.

Multiple splits blacklist the same configuration

If you would like to export configuration that is blacklisted in more than one split, then you will need to use the drush config-split-export [split] commands and specify the split to which you would like configuration to be exported.

Terminology

blacklist

Blacklisted splits are blacklisted from config/default. If a given split is active, and that Split defines a configuration setting in its blacklist, that configuration setting will not be exported to config/default when drush config-export is executed.

Exported to split
Not exported to default configuration

graylist

Graylist splits allow a given configuration setting to be exported to both the default configuration and also to a split's configuration (overriding default when active).

Exported to split
Also exported to default configuration

Development settings

To disable the plug-in discovery cache, add the following to your local.settings.php file: $settings['cache']['bins']['discovery'] = 'cache.backend.null';

This will obviate the need to clear caches in order to register a status change in a configuration split.

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