The following steps can be used to perform a rolling upgrade between minor versions of MariaDB (for example from MariaDB 10.3.12 to MariaDB 10.3.13) when Galera Cluster is being used. In a rolling upgrade, each node is upgraded individually, so the cluster is always operational. There is no downtime from the application's perspective.
Before you upgrade, it would be best to take a backup of your database. This is always a good idea to do before an upgrade. We would recommend Mariabackup.
For each node, perform the following steps:
my.cnf. This includes removing any system variables or options that are no longer supported. mysql_upgrade with the --skip-write-binlog option. mysql_upgrade does two things: mysql database are fully compatible with the new version. When this process is done for one node, move onto the next node.
Note that when upgrading the Galera wsrep provider, sometimes the Galera protocol version can change. The Galera wsrep provider should not start using the new protocol version until all cluster nodes have been upgraded to the new version, so this is not generally an issue during a rolling upgrade. However, this can cause issues if you restart a non-upgraded node in a cluster where the rest of the nodes have been upgraded.
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https://mariadb.com/kb/en/upgrading-between-minor-versions-with-galera-cluster/