Clustering several computers can improve uptime and application availability.
Computer uptime is critical in the business world and the e-commerce world, as users and clients expect services to be available 24 hours a day, seven days a week. This demand for continual uptime can be partially met by using a technique known as "clustering," and is typically associated with the server world. Although it does not cure every server uptime problem, it provides several advantages that make server administrators' lives easier.
Clustering
Clustering involves connecting two or more computers so that any one of the clustered servers can satisfy a request from a user. The clustered configuration is transparent to the user, except that he sees higher availability levels for the applications that run on the server cluster.
Maintenance
Clustering makes maintenance easier in that you can vary one server offline without taking down the application. The other server or servers remain available while you change out a hard drive or some other malfunctioning piece of equipment. Clustering makes this type of maintenance a non-issue for the user, whereas a standalone server would be down for the duration of the maintenance.
Failover
Typically the clustering is accomplished via software so that each server in the cluster checks the "heartbeat" of the others in the group to ensure the health of each. This is called "failover" capability, and is instrumental in maintaining high server availability. A failover should go unnoticed by users of the applications housed on the server.
Backups
Backups are a critical part of server administration and management, and are typically performed on a nightly, weekly and monthly basis. Clustering allows you to vary one server offline for backups while allowing the other servers to continuing servicing the user base. In turn, each server is backed up while the others take the load and continue processing.
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