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Set up Galera arbitrator

The size of a cluster increases when a node joins the cluster and decreases when a node leaves. A cluster reacts to replication problems with inconsistency voting. The size of the cluster determines the required votes to achieve a quorum. If a node no longer responds and is disconnected from the cluster the remaining nodes vote. The majority of the nodes that vote are considered to be in the cluster.

The arbitrator is important if you have an even number of nodes remaining in the cluster. The arbitrator keeps the number of nodes as an odd number, which avoids the split-brain situation.

A Galera Arbitrator is a lightweight member of a Percona XtraDB Cluster. This member can vote but does not do any replication and is not included in flow control calculations. The Galera Arbitrator is a separate daemon called garbd. You can start this daemon separately from the cluster and run this daemon either as a service or from the shell. You cannot configure this daemon using the my.cnf file.

Note

For more information on how to set up a cluster you can read in the Configuring Percona XtraDB Cluster on Ubuntu or Configuring Percona XtraDB Cluster on CentOS manuals.

Installation

Galera Arbitrator does not need a dedicated server and can be installed on a machine running other applications. The server must have good network connectivity.

Galera Arbitrator can be installed from Percona’s repository on Debian/Ubuntu distributions with the following command:

root@ubuntu:~# apt install percona-xtradb-cluster-garbd

Galera Arbitrator can be installed from Percona’s repository on RedHat or derivative distributions with the following command:

[root@centos ~]# yum install percona-xtradb-cluster-garbd

Start garbd and configuration

Note

On Percona XtraDB Cluster 8.0, SSL is enabled by default. To run the Galera Arbitrator, you must copy the SSL certificates and configure garbd to use the certificates.

It is necessary to specify the cipher. In this example, it is AES128-SHA256. If you do not specify the cipher, an error occurs with a “Terminate called after throwing an instance of ‘gnu::NotSet’” message.

For more information, see socket.ssl_cipher

When starting from the shell, you can set the parameters from the command line or edit the configuration file. This is an example of starting from the command line:

$ garbd --group=my_ubuntu_cluster \
--address="gcomm://192.168.70.61:4567, 192.168.70.62:4567, 192.168.70.63:4567" \
--option="socket.ssl=YES; socket.ssl_key=/etc/ssl/mysql/server-key.pem; \
socket.ssl_cert=/etc/ssl/mysql/server-cert.pem; \
socket.ssl_ca=/etc/ssl/mysql/ca.pem; \
socket.ssl_cipher=AES128-SHA256"

To avoid entering the options each time you start garbd, edit the options in the configuration file. To configure Galera Arbitrator on Ubuntu/Debian, edit the /etc/default/garb file. On RedHat or derivative distributions, the configuration can be found in /etc/sysconfig/garb file.

The configuration file should look like this after the installation and before you have added your parameters:

# Copyright (C) 2013-2015 Codership Oy
# This config file is to be sourced by garb service script.

# REMOVE THIS AFTER CONFIGURATION

# A comma-separated list of node addresses (address[:port]) in the cluster
# GALERA_NODES=""

# Galera cluster name, should be the same as on the rest of the nodes.
# GALERA_GROUP=""

# Optional Galera internal options string (e.g. SSL settings)
# see http://galeracluster.com/documentation-webpages/galeraparameters.html
# GALERA_OPTIONS=""

# Log file for garbd. Optional, by default logs to syslog
# Deprecated for CentOS7, use journalctl to query the log for garbd
# LOG_FILE=""

Add the parameter information about the cluster. For this document, we use the cluster information from Configuring Percona XtraDB Cluster on Ubuntu.

Note

Please note that you need to remove the # REMOVE THIS AFTER CONFIGURATION line before you can start the service.

# This config file is to be sourced by garb service script.

# A comma-separated list of node addresses (address[:port]) in the cluster
GALERA_NODES="192.168.70.61:4567, 192.168.70.62:4567, 192.168.70.63:4567"

# Galera cluster name, should be the same as on the rest of the nodes.
GALERA_GROUP="my_ubuntu_cluster"

# Optional Galera internal options string (e.g. SSL settings)
# see http://galeracluster.com/documentation-webpages/galeraparameters.html
# GALERA_OPTIONS="socket.ssl_cert=/etc/ssl/mysql/server-key.pem;socket./etc/ssl/mysql/server-key.pem"

# Log file for garbd. Optional, by default logs to syslog
# Deprecated for CentOS7, use journalctl to query the log for garbd
# LOG_FILE="/var/log/garbd.log"

You can now start the Galera Arbitrator daemon (garbd) by running:

root@server:~# service garbd start
Expected output 
[ ok ] Starting /usr/bin/garbd: :.

Note

On systems that run systemd as the default system and service manager, use systemctl instead of service to invoke the command. Currently, both are supported.

root@server:~# systemctl start garb

root@server:~# service garb start
Expected output 
[ ok ] Starting /usr/bin/garbd: :.

Additionally, you can check the arbitrator status by running:

root@server:~# service garbd status
Expected output 
[ ok ] garb is running.

root@server:~# service garb status
Expected output 
[ ok ] garb is running.

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Last update: 2023-07-21