Shared configuration for migrations

Share configuration for migrations has previously been handled via a migrate_group config entity provided by migrate_plus.

With the addition of shared configuration in Migrate tools, the use of groups is no longer needed.

Previously:

# migrate_plus.migration_group.default.yml
langcode: en
status: true
dependencies: {  }
id: default
label: Default
description: ''
source_type: ''
module: null
shared_configuration: 
  source:
    key: drupal_7

An entry was placed in the migration like:

# d7_node.yml
id: d7_node
label: Nodes
migration_tags:
  - Drupal 7
  - Content
migration_group: default #<== Addition of migration_group shared configuration into a migration
source:
  plugin: d7_node
process:
destination:

The source key: is shared to all migrations registered with the default migrate group and it let all migrations connect to the source database with a db key of drupal_7.

New approach

The limitation was that only a single group could be used. With the new approach, none, one or multiple shared configurations can be used.

1. Create a shared configuration file in your module: my_migrate_module.migrate_shared_configuration.yml
   

   # my_migrate_module.migrate_shared_configuration.yml
   default_source_configuration:
     source:
       key: drupal_7
   default_destination_configuration:
     destination:
       key: default
   

2. Insert references to the shared configuration in your migration:
   

   # d7_node.yml
   id: d7_node
   label: Nodes
   migration_tags:
     - Drupal 7
     - Content
   # Add one shared configuration into a migration
   include: default_source_configuration
   # Also valid are multiple:
   include:
     - default_source_configuration
     - default_destination_configuration
   source:
     plugin: d7_node
   process:
   destination: