FieldStorageDefinition.php

<?php

namespace Drupal\entity_test;

use Drupal\Core\Field\BaseFieldDefinition;

/**
 * A custom field storage definition class.
 *
 * For convenience we extend from BaseFieldDefinition although this should not
 * implement FieldDefinitionInterface.
 *
 * @todo Provide and make use of a proper FieldStorageDefinition class instead:
 *   https://www.drupal.org/node/2280639.
 */
class FieldStorageDefinition extends BaseFieldDefinition {
    
    /**
     * {@inheritdoc}
     */
    public function isBaseField() {
        return FALSE;
    }

}