4.12. Galera Arbitrator¶
If the expense of adding, for example, a third datacenter is too high, you can use Galera Arbitrator. Galera Arbitrator is a member of the cluster which participates in voting, but not in actual replication.
Warning
Galera Arbitrator does not participate in actual replication, but it receives the same data as the nodes. Use a secure network connection for the arbitrator.
Galera Arbitrator servers two purposes:
- It helps to avoid split-brain situations by acting as an odd node in a cluster that is spread only across two nodes.
- It can request a consistent application state snapshot.
Galera Arbitrator is depicted in the figure below:
Galera Arbitrator
In the figure above, if one of the data centers fails or loses WAN connection, the node that sees the arbitrator (and therefore sees clients) will continue operation.
Note
garbd must see all replication traffic although it does not store it anywhere. Placing the arbitrator in a location with poor connectivity to the rest of the cluster may lead to poor cluster performance.
Arbitrator failure does not affect cluster operation and a new instance can be reattached to the cluster at any time. There can be several arbitrators in the cluster.
4.12.1. Configuring Galera Arbitrator¶
As a Galera Cluster cluster member, Galera Arbitrator accepts all Galera Cluster parameters except those prefixed by replicator..
See also
Chapters Galera Parameters and Starting Galera Arbitrator.
4.12.2. Starting Galera Arbitrator¶
Galera Arbitrator it is a separate daemon called garbd. You can start is manually as follows:
# /etc/init.d/garb start
You can also add configuration options to the command.
You can also automate running Galera Arbitrator by using an /etc/default/garb init script, such as the one below:
# Copyright (C) 2013 Codership Oy
# This config file is to be sourced by garb service script.
# A space-separated list of node addresses (address[:port]) in the cluster
GALERA_NODES="192.168.1.1:4567 192.168.1.2:4567"
# Galera cluster name, should be the same as on the rest of the nodes.
GALERA_GROUP="example_wsrep_cluster"
# Optional Galera internal options string (e.g. SSL settings)
# see http://www.codership.com/wiki/doku.php?id=galera_parameters
GALERA_OPTIONS="socket.ssl_cert = /etc/galera/cert/cert.pem; socket.ssl_key = /$
# Log file for garbd. Optional, by default logs to syslog
LOG_FILE="/var/log/garbd.log"
