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In this guide we will add two nodes node 4 and 5 to an existing Galera Cluster consisting of 3 nodes as shown here.

And end up with a cluster consisting of 5 nodes as shown here.

We will be adding two more nodes (node 4 and 5) simultaneously and a little preparation is needed on the existing Galera cluster.

On every node on the existing cluster add the new ip addresses of the new nodes we are adding.

vi /etc/mysql/conf.d/galera.cnf

Add the ips so the lhe line looks like this.

wsrep_cluster_address = "gcomm://node1_ip,node2_ip,node3_ip,node4_ip,node5_ip"

And then restart mariadb on all nodes on the existing cluster in order for the changes to take effect.

On the new nodes (node 4 and 5) that we are adding run the following using sudo or root.

apt update
apt install -y mariadb-server mariadb-client
systemctl start mariadb.service
systemctl enable mariadb.service
mysql_secure_installation

Apply the following when running mysql_secure_installation

Enter current password for root (enter for none): press enter
Switch to unix_socket authentication [Y/n]        press n
Change the root password? [Y/n]                   press y
New password:                                     enter new root password
Re-enter new password:                            enter new root password again
Remove anonymous users?                           press y
Disallow root login remotely?                     press y
Remove test database and access to it?            press y
Reload privilege tables now?                      press y

On node 4

vi /etc/mysql/conf.d/galera.cnf

Add the following lines.

[mysqld]
binlog_format = ROW
default-storage-engine = innodb
innodb_autoinc_lock_mode = 2
bind-address = 0.0.0.0

# Galera Provider Configuration
wsrep_on = ON
wsrep_provider=/usr/lib/galera/libgalera_smm.so

# Galera Cluster Configuration
wsrep_cluster_name = "galera_cluster"
wsrep_cluster_address = "gcomm://node1_ip,node2_ip,node3_ip,node4_ip,node5_ip"

# Galera Synchronization Configuration
wsrep_sst_method = rsync

# Galera Node Configuration
wsrep_node_address = "node4_ip"
wsrep_node_name = "node4"

wsrep_sst_donor = "node3"

On node 5

vi /etc/mysql/conf.d/galera.cnf

Add the following lines.

[mysqld]
binlog_format = ROW
default-storage-engine = innodb
innodb_autoinc_lock_mode = 2
bind-address = 0.0.0.0

# Galera Provider Configuration
wsrep_on = ON
wsrep_provider=/usr/lib/galera/libgalera_smm.so

# Galera Cluster Configuration
wsrep_cluster_name = "galera_cluster"
wsrep_cluster_address = "gcomm://node1_ip,node2_ip,node3_ip,node4_ip,node5_ip"

# Galera Synchronization Configuration
wsrep_sst_method = rsync

# Galera Node Configuration
wsrep_node_address = "node5_ip"
wsrep_node_name = "node5"

wsrep_sst_donor = "node3"

Important as you can see we are using a specific donor “node3” you need to use the name of the donor node and not the ip address. If we were not using a specific donor node the Galera cluster would select one as random as donor node. Donor nodes are beeing set to “Read Only” so it makes sense to specify a specifik donor node.

Once the configuration is in place restart mariadb on node 4 and node 5. They should start replicating using node 3 as donor.

Note it is also possible to use more than 1 donor node this can be done like this.

wsrep_sst_donor = "node3,node2"

Optional Once everything is up and running you can delete the “wsrep_sst_donor” line in /etc/mysql/conf.d/galera.cnf

Verify the cluster size

mysql -u root -p

SHOW STATUS LIKE 'wsrep_cluster_size';

+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| wsrep_cluster_size | 5     |
+--------------------+-------+