Cluster Server Concepts and Terminology

As defined by Microsoft® Cluster Server, a cluster is a loosely coupled collection of two or more independent Microsoft® Windows NT® Server systems. Network clients and administrators view this collection as a single system.

The computer systems that make up the collection are known as nodes. To ensure availability, the nodes communicate across redundant network paths. Cluster resources, the entities that Cluster Server manages, are hosted by the cluster nodes. Cluster Server can manage as a resource any entity that can be taken offline, brought online, and moved to another node if a failure occurs.

A cluster projects its own unique identity distinct from the identities of its nodes. This identity is used by network clients to access the applications and services running on the cluster nodes and by administrators to manage the cluster. Maintaining a unique identity allows Cluster Server to transparently move applications and related resources between nodes, to replace a malfunctioning node, and to add a new node. Network clients continue to see a single computer system.

A cluster is similar to a general distributed system, except that it provides these additional capabilities:

Every node has full connectivity and communication with the other nodes in the cluster through the following:
- One or more shared SCSI buses for storage. Each shared SCSI bus attaches one or more disks that hold data used to manage the cluster. Cluster Server provides a dual-access storage model whereby multiple systems in the cluster can access the same storage.
- A private network, or interconnect, that carries only internal cluster communication. Private networks are recommended, but not required.
- One or more public networks. Public networks serve as backup for a private network and can be used both for internal cluster communication and to host client applications. Network adapters, known to the cluster as network interfaces, attach nodes to networks.
Every node in the cluster is aware when another system joins or leaves the cluster.
Every node in the cluster is aware of the resources that are running locally as well as the resources that are running on the other cluster nodes.

Networks, network interfaces, nodes, and resources are four of six components that applications and network administrators work with when they program for or use Cluster Server. There are also cluster groups and resource types. Each of the Cluster Server components is associated with:

A set of operations that is made available through the Cluster API, the Resource API, the Cluster Utility functions, or the Cluster Administrator Extension API.
Data that is stored in the cluster database and accessed through specific functions in the Cluster API.

The following table briefly describes the Cluster Server components:

Cluster Server object	Description
Network	A network across which a cluster node can communicate.
Network interface	A network adapter that is installed in a cluster node.
Node	A computer system that is a member of a cluster.
Resource	An entity that is hosted by a node, managed by Cluster Server, and can be started, stopped, and moved to another node.
Resource type	A category of resources, such as physical disks.
Group	A collection of resources that run together on a node and are failed over as a single unit.