How to execute a migration?
Drupal 8 Migrations are plugins. So you can create instance and execute like this:

&lt;?php

use Drupal\migrate\MigrateExecutable;
use Drupal\migrate\MigrateMessage;


$migration_id = 'MIGRATION_ID';
$migration = \Drupal::service('plugin.manager.migration')-&gt;createInstance($migration_id);
$executable = new MigrateExecutable($migration, new MigrateMessage());
$executable-&gt;import();