Clustering is a technology used on two different physical servers for data recovery if one of the physical server fails.One server takes the responsibility of the another server in case of failure. There would be 4 nameservers for your domains. 2 nameservers will point to primary server and 2 nameservers will point to secondary server. Your websites wont face any sort of downtime incase primary server goes down as the domains will continue to resolve from secondary server using the
other 2 nameservers.
Clustering refers to a group of more than two servers which are also called as nodes. Nodes represents as a single virtual private server to a network.By default A records of domains on secondary server will point to primary server but when the main server goes down then the monitoring script on secondary server will change A records of domains and make them point to secondary server. When a client connects to a clustered dedicated hosting server it is treated as a single server (not more than one).The responsibilities are taken by another server in cluster when one of the nodes fails. In this case end user notices nothing about the failure.
Clustering is not the 100% backup system for the application and this is the most overlooked aspect of it. The main advantage in clustering is “no downtime” and “100 % recovery”. Ability to recover from failed server hardware and failed software is the main benefit of clustering. Clustering could not do anything in case of potential disasters, such as power outages or acts of God.