Mysql – Is Galera Arbitrator really required

galeraMySQL

I have three nodes(as recommended, in different switches) forming a Galera Cluster. Is there any real requirement of 'Garbd'? I don't see much reasons for using this apart from avoiding split-brain or taking backups?

Another doubt regarding garbd is that suppose I have 3 nodes in the cluster, and adding a garbd node will make the cluster size of 4, which is an even-numbered quorum. Will this not cause a split-brain situation?

Best Answer

If you have minimum of three active nodes in your cluster, you don't need to add garbd.

You should use a garbd with the following conditions:

When you have an even number of nodes and you can't add new nodes (because of cost or ...) to avoid split-brain situations.
when you want to backup data (SST method) to a server.

More information: http://galeracluster.com/documentation-webpages/arbitrator.html

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Galera (for MySQL, Percona XtraDB Cluster or MariaDB Cluster, they are essentially the same but from different vendors and base mysql version) can work perfectly with 2 machines, and it is a very common setup for substituting standard MySQL replication.

Requiring 3 nodes is not a requirement of Galera, but of any cluster that cares about data consistency over availability. In other words, it is needed in order to provide availability and data integrity in the case of only one node suffering from network partition (causing a "split brain" on the cluster). We need an odd number of nodes and 1 node is not a cluster :-). You probably knew that, but I wanted to clarify this for any people reading this answer.

Given that the only reason to have 3 nodes is for availability in case of a node failure/network failure, the recommended way to setup a 2 node cluster with network split protection is to setup a Galera Arbitrator which is essentially an emulation of a MySQL node, but without storing any local data. You can install that anywhere, although you must be careful because if still may influence the performance of the cluster if it has network latency problems.

Running several instances on the same physical node is useless (docker is not needed, by the way, you can do that by just changing the local configuration), as if you absolutely can only run things on 2 machines, you better disable the automatic shutdown on one of the nodes and manage the failover manually. The 3 limitation is just for data consistency, there is nothing physical or on the protocol requiring a certain number of nodes.

You can read more information about it on the official documentation.

MySQL Cluster – Management Node Redundancy

If you have split exactly in half brain situation MySQL Cluster resolves it designating one node as master. If you look into ndb_mgm output below you will notice asterisk at the end of node 10 line. It marks master.

It means that if your cluster will be split in two equal parts the half which contains master will continue running. Nodes from the other half will be shut down.

ndb_mgm> show
Connected to Management Server at: 10.1.1.10:1186
Cluster Configuration
---------------------
[ndbd(NDB)]     2 node(s)
id=10   @10.1.1.20  (mysql-5.6.24 ndb-7.3.9, Nodegroup: 0, *)
id=11   @10.1.1.21  (mysql-5.6.24 ndb-7.3.9, Nodegroup: 0)

Best Answer

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