What Is a Cluster?

A cluster is a group of two or more computers functioning together to provide a common set of applications or services with a single apparent identity to clients. The computers are physically connected by hardware in the form of either a network or shared storage. The clustering software provides a common interface externally while managing the resources and load internally.

Windows Clustering provides the following benefits:

• High availability When a clustered application or service fails or a computer in the cluster fails, the cluster responds by restarting the application or service on another member of the cluster or by distributing the load from the failed server to the rest of the cluster.
• Scalability For cluster-aware applications, adding more machines to the cluster adds additional capabilities.
• Manageability Administrators can move applications, services, and data from computer to computer within the cluster, allowing them to manually balance loads and to offload machines scheduled for maintenance.

Network Load Balancing Clusters

Network Load Balancing (NLB)—known as the Windows Load Balancing Service in Microsoft Windows NT 4—gives TCP/IP-based services and applications high availability and scalability by combining up to 32 servers running Windows 2000 Advanced Server in a single cluster. Client requests for applications and services provided by the cluster are distributed across the available servers in the cluster in a way that is transparent to the client.

If a server fails or is taken offline, the cluster is automatically reconfigured and the client connections redistributed across the remaining servers. If additional servers are added to the cluster, they are automatically recognized and the load is reconfigured and distributed.

Server Clusters

Server clusters distribute the workload among the servers in a cluster, with each server running its own workload. Like other types of clusters, server clusters are scalable and highly available. In the event of a failure, applications and services that can be restarted, such as print queues and file services, are restarted transparently. Ownership of shared resources passes to the remaining servers. When the failed server becomes available again, the workload is automatically rebalanced.

Server clusters use shared disk drives to provide transparent access. They require specially certified hardware, but when properly set up and configured, they provide for extremely high availability with no single point of failure. Windows 2000 Advanced Server supports only two-node server clusters. Windows 2000 Datacenter Server supports up to four-node server clusters. Windows 2000 Server does not natively support clustering.