The reason why DB clustering need 3 nodes

Brief

Consider regarding database clustering consists of 3 nodes basically.

Database clustering which needs 3 nodes at least

• Redis
• MariaDB Galera Cluster
• etcd
• Consul
• influxdb

Regarding distributed system, there is CAP theory.

• Consistency
• Availability
• Partition Tolerance

In these features, Partition Tolerance is the biggest factor which defines the default number of database clustering.

If there were 3 nodes and split them, selected majority and continues to work.

If there were 4 nodes and split them, can not select majority and continue to work.

In the view of Partition Tolerance, database clustering needs 3 nodes at least. If increased nodes, the number of nodes would be odd number naturally.

Remark

There are database clustering which do not need.

• Apache cassandra
• Basho riak
• Aerospike