One of the benefits of using Upgrade Status is it will tell you about environment compatibility alongside extension compatibility. It will note if your PHP or Apache or database versions are out of date. Of particular note are Drupal 9’s MySQL/Percona and MariaDB version requirements. The MySQL/Percona/MariaDB driver that’s included in Drupal 9 core requires MySQL/Percona 5.7+ or MariaDB 10.3.7+. The intention with raising the bar from Drupal 8’s requirement of MySQL/Percona 5.6 and MariaDB 10.0 was to utilise some of the newer features in these database versions. There was also the risk of a dependency starting to require the new versions, given the end of life nature of the older database versions at this point. Neither happened yet but we did not know that ahead of time of course.
MySQL 5.6 and MariaDB 10.0 database driver for Drupal 9 to the rescue! If you are on a long term supported operating system and receive security coverage for your database, you might not need to update to MySQL/Percona 5.7 or MariaDB 10.3 immediately after all. Only a few contributed projects utilise the new capabilities, for example the JSON/JSONB field module. If you are certain that none of your modules require the newer versions, keep in mind that core itself can actually run fine with older database versions still. Follow the instructions on the project page to install this driver for Drupal 9. I would still suggest you plan an upgrade of your database, but now it can be decoupled from your Drupal major upgrade.
Looking ahead, some of the Drupal 10 platform requirements are already defined, and MySQL/Percona/MariaDB requirements will not be raised further from Drupal 9’s minimum versions. However there are no guarantees that the new features will not be actually utilised then.
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