In this blog I will show you how you can setup a basic MariaDB Galera Cluster on Debian 11 (bullseye).
If you wish to have a more general overview first you can have a look here: https://mariadb.com/kb/en/what-is-mariadb-galera-cluster/
For this we have 3 servers: galera-cluster-1a/b/c. The following steps will first be executed on my first node, galera-cluster-1a still these steps have to executed on the other nodes as well to build the whole cluster.

Step 1: Installation

First the servers will be prepared for the installation. This includes different packages to be installed and ports to be opened.

galera-cluster-1a:~$ sudo apt update
galera-cluster-1a:~$ sudo apt-get install ufw
galera-cluster-1a:~$ sudo ufw enable
galera-cluster-1a:~$ sudo ufw allow 3306
galera-cluster-1a:~$ sudo ufw allow 4444
galera-cluster-1a:~$ sudo ufw allow 4567
galera-cluster-1a:~$ sudo ufw allow 4568
galera-cluster-1a:~$ sudo ufw allow 22

Installing MariaDB 11.3.2 and other packages

galera-cluster-1a:~$ sudo apt-get install libsnmp-perl -y
galera-cluster-1a:~$ sudo curl -LsS -O https://downloads.mariadb.com/MariaDB/mariadb_repo_setup
galera-cluster-1a:~$ sudo bash mariadb_repo_setup --os-type=debian --os-version=bullseye --mariadb-server-version=11.3.2
galera-cluster-1a:~$ sudo wget http://ftp.us.debian.org/debian/pool/main/r/readline5/libreadline5_5.2+dfsg-3+b13_amd64.deb
galera-cluster-1a:~$ sudo dpkg -i libreadline5_5.2+dfsg-3+b13_amd64.deb
galera-cluster-1a:~$ sudo apt-get update
galera-cluster-1a:~$ sudo apt-get install mariadb-server mariadb-client -y
galera-cluster-1a:~$ sudo systemctl enable mariadb
sudo apt-get install percona-toolkit -y

Now you can login to MariaDB and check the version

galera-cluster-1a:~$ mariadb
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 32
Server version: 11.3.2-MariaDB-1:11.3.2+maria~deb11 mariadb.org binary distribution
Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

For the final touch for the basic installation of MariaDB we are going to secure it. Answering yes to all the changes it proposes.

galera-cluster-1a:~$ sudo mariadb-secure-installation

Step 2: Configuring MariaDB server

We recommend creating a dedicated backup user for mariabackup like this:

galera-cluster-1a:~$ mariadb
MariaDB [(none)]> create user 'mariabackup'@'%' identified by 'XXX';
Query OK, 0 rows affected (0.001 sec)
MariaDB [(none)]> GRANT RELOAD, PROCESS, LOCK TABLES, REPLICATION CLIENT ON *.* TO 'mariabackup'@'%';
Query OK, 0 rows affected (0.001 sec)
MariaDB [(none)]> create user 'mariabackup'@'localhost' identified by 'XXX';
Query OK, 0 rows affected (0.001 sec)
MariaDB [(none)]> GRANT RELOAD, PROCESS, LOCK TABLES, REPLICATION CLIENT ON *.* TO 'mariabackup'@'localhost';
Query OK, 0 rows affected (0.001 sec)
MariaDB [(none)]> flush privileges;
MariaDB [(none)]> exit

Creating a dedicated directory for logs.

sudo mkdir /var/log/mariadb
sudo chown mysql:mysql /var/log/mariadb

In the /etc/mysql/mariadb.conf.d/50-server.cnf file we add the following options at the end to the [mariadb-11.3] section.

# This group is only read by MariaDB-11.3 servers.
# If you use the same .cnf file for MariaDB of different versions,
# use this group for options that older servers don't understand
[mariadb-11.3]
log_warnings=2
log_error=/var/log/mysql/mariadb.err
general_log = 1
general_log_file = /var/log/mysql/mariadb.log

[mysqldump]
user=dump
password=XXX
max_allowed_packet = 512M

[mariabackup]
user=mariabackup
password=XXX
databases-exclude=lost+found

If you want to use mariabackup for backups and replication you will have to create the user and add the last section [mariabackup]

Step 3: Configuring Galera Cluster

Change the galera config file to the following. It is also possible to use the IPs instead of hostnames.

galera-cluster-1a:~$ cat /etc/mysql/mariadb.conf.d/60-galera.cnf
[galera]
# Mandatory settings
wsrep_on = ON
wsrep_cluster_name = MariaDB Galera Cluster
wsrep_cluster_address = gcomm://galera-cluster-1a,galera-cluster-1b,galera-cluster-1c
wsrep_provider = /usr/lib/galera/libgalera_smm.so
binlog_format = row
default_storage_engine = InnoDB
innodb_autoinc_lock_mode = 2
# Allow server to accept connections on all interfaces.
bind-address = 0.0.0.0

# Optional settings
wsrep_slave_threads = 1
innodb_flush_log_at_trx_commit = 0
wsrep_sst_auth = mariabackup:manager
wsrep_sst_method=mariabackup
wsrep_node_address=galera-cluster-1a
wsrep_sst_receive_address= galera-cluster-1a

Step 4: Bootstrapping Galera Cluster

Now everything is ready and we can start the MariaDB Server in cluster mode.

galera-cluster-1a:~$  sudo systemctl stop mariadb.service
galera-cluster-1a:~$  sudo galera_new_cluster

Lets check if the one node cluster has successfully started. The start position on the very first start should look like the following

galera-cluster-1a:~$ ps -f -u mysql
UID        PID  PPID  C STIME TTY          TIME CMD
mysql     2081     1  0 14:37 ?        00:00:00 /usr/sbin/mariadbd --wsrep-new-cluster --wsrep_start_position= 00000000-0000-0000-0000-000000000000:-1

Login to your instance and check the cluster size

galera-cluster-1a:~$ mariadb

MariaDB [(none)]> show global status like 'wsrep_cluster_size';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| wsrep_cluster_size | 1     |
+--------------------+-------+
MariaDB [(none)]> quit

Step 5: Setting up the other servers

Repeat steps 1-3 on the other two nodes but you do not have to bootstrap these nodes.
Simply restart the mariadb.service at the end for them to sync with the cluster. 

galera-cluster-1b:~$ sudo systemctl restart mariadb.service
galera-cluster-1c:~$ sudo systemctl restart mariadb.service

Afterwards you can check the cluster size again.

galera-cluster-1a:~$ mariadb
MariaDB [(none)]> show global status like 'wsrep_cluster_size';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| wsrep_cluster_size | 3     |
+--------------------+-------+

Check out this blog about using mariabackup to repair your replication: https://www.dbi-services.com/blog/mariadb-repair-replication-through-db-restore-mariabackup/

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Daniel Burgert