drupal/core/modules/mysql/mysql.install

<?php

/**
 * @file
 * Install, update and uninstall functions for the mysql module.
 */

use Drupal\Core\Database\Database;

/**
 * Implements hook_requirements().
 */
function mysql_requirements($phase) {
  $requirements = [];

  if ($phase === 'runtime') {
    // Test with MySql databases.
    if (Database::isActiveConnection()) {
      $connection = Database::getConnection();
      // Only show requirements when MySQL is the default database connection.
      if (!($connection->driver() === 'mysql' && $connection->getProvider() === 'mysql')) {
        return [];
      }

      $query = 'SELECT @@SESSION.tx_isolation';
      // The database variable "tx_isolation" has been removed in MySQL v8.0.3 and
      // has been replaced by "transaction_isolation".
      // @see https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_tx_isolation
      // @see https://dev.mysql.com/doc/refman/8.0/en/added-deprecated-removed.html
      if (!$connection->isMariaDb() && version_compare($connection->version(), '8.0.2-AnyName', '>')) {
        $query = 'SELECT @@SESSION.transaction_isolation';
      }

      $isolation_level = $connection->query($query)->fetchField();

      $requirements['mysql_transaction_level'] = [
        'title' => t('Database Isolation Level'),
        'severity' => $isolation_level === 'READ-COMMITTED' ? REQUIREMENT_OK : REQUIREMENT_WARNING,
        'value' => $isolation_level,
        'description' => t('For the best performance and to minimize locking issues, the READ-COMMITTED transaction isolation level is recommended. See the <a href=":performance_doc">setting MySQL transaction isolation level</a> page for more information.', [
          ':performance_doc' => 'https://www.drupal.org/docs/system-requirements/setting-the-mysql-transaction-isolation-level',
        ]),
      ];
    }
  }

  return $requirements;
}