How To: reindex customers after writing directly to `customer_entity` table

In magento2.x development sometimes speed is crucial. Therefore directly accessing database and writing into tables can save lots of time.

Assuming we need to update a record for a customer eg for setting a default group id:

 /**
  * @var $resource \Magento\Framework\DB\Adapter\Pdo\Mysql\Interceptor
  */
$default_customer_group_id=1
$customerIds = [1,2,111,444];

$resource->update('customer_entity'
['group_id'=>$default_customer_group_id],
  ['entity_id in (?)'=>$customerIds,]
);

The code above will update the customers But it won't update customer indexes in elasticsearch (or opensearch) therefore we have to update by ourselves:

/**
 * @var $indexerFactory Magento\Framework\Indexer\IndexerInterfaceFactory
 */
 $indexer = $indexerFactory->create();
 $indexer->load('customer_grid');
 $indexer->reinderList($customerIds);

But How On earth I will get an instance of IndexerInterfaceFactory tbw?

It can be either using ObjectManager:

$objectManager = ObjectManager::getInstance();
$indexerFactory = $objectManager->create(\Magento\Framework\Indexer\IndexerInterfaceFactory::class);

Or via dependency injection upon your constructor:

use \Magento\Framework\Indexer\IndexerInterfaceFactory;

// class Definition here

private $indexer;

public function __construct(
        // Rest of services here
        IndexerInterfaceFactory $indexerFactory
    ) {

        $this->indexer=$indexerFactory->create();
        $this->indexer->load('customer_grid');
    }

Magento 2 uses for any class Dependency Injection. Furthermore it auto-injects any class to the constructor as well.