Migrations and migration groups
For the migration classes you implement to be available for reviewing status, running imports, etc., instances of them must be registered. There are two methods of getting your migrations and migration groups registered:
hook_migrate_api()
This is the recommended means of registering your migrations and migration groups if they're static - that is, if you don't need to pick-and-choose at runtime what migrations to register or what arguments to set for them.
It's recommended that you explicitly assign your migrations to a migration group (if you don't, they will be in the 'default' group). Groups can be registered in the API array by including a 'groups' key, whose value is an array keyed by the group machine name and the value an array of arguments for the group. The 'title' argument is special - this is the name of the group as it will be presented in the UI (if you omit the title, the machine name will be displayed instead). Any other arguments are saved as part of the group, and made available to any migrations in the group when they are instantiated.
The 'migrations' key should be an array, keyed by the migration machine name, with the value an array of arguments. 'class_name' is the one required argument (although 'group_name' is recommended) - any others are class-specific arguments.
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