Quorum is the term used to describe the state of a fully formed cluster with an elected master. In order to achieve quorum, certain conditions need to be met. Most importantly, 51% or more of the cluster nodes must be online and intercommunicating in order for quorum to be achieved.

There is an exception to the 51% rule if you use Clusters with Sites. This allows quorum to be achieved in certain circumstances with just 50% of the cluster nodes online and intercommunicating. See the section About Active/Active Clustering with Sites for details.

In this example, we examine a three-realm cluster, distributed across three physical locations (such as a primary location and two disaster recovery locations). The 51% quorum requirement means there must always be a minimum of two realms active for the cluster to be online and operational:

Note: Dotted lines represent interrupted communication owing to server or network outages.

Quorum, in conjunction with our deployment guidelines, prevents split brain (the existence of multiple masters) scenarios from occurring. By requiring at least 51% of realms to be online and intercommunicating before an election of a new master realm can take place, it is impossible for two sets of online but not intercommunicating realms to both elect a new master.

To simplify the reliable achievement of quorum, we recommend a cluster be created with an odd number of member realms, preferably in at least three separate locations. In the above three-realm/three-location example, should any one location become unavailable, sufficient realms remain available to achieve quorum and, if necessary, elect a new master (see Election of a new Master).