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What Is a Server Cluster?

Updated: March 28, 2003

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

What Is a Server Cluster?

In this section

A server cluster is a group of independent servers running Windows Server 2003, Enterprise Edition, or Windows Server 2003, Datacenter Edition, and working together as a single system to provide high availability of services for clients. When a failure occurs on one computer in a cluster, resources are redirected and the workload is redistributed to another computer in the cluster. You can use server clusters to ensure that users have constant access to important server-based resources.

Server clusters are designed for applications that have long-running in-memory state or frequently updated data. Typical uses for server clusters include file servers, print servers, database servers, and messaging servers.

Introduction to Server Clusters

A cluster consists of two or more computers working together to provide a higher level of availability, reliability, and scalability than can be obtained by using a single computer. Microsoft cluster technologies guard against three specific types of failure:

  • Application and service failures, which affect application software and essential services.

  • System and hardware failures, which affect hardware components such as CPUs, drives, memory, network adapters, and power supplies.

  • Site failures in multisite organizations, which can be caused by natural disasters, power outages, or connectivity outages.

The ability to handle failure allows server clusters to meet requirements for high availability, which is the ability to provide users with access to a service for a high percentage of time while reducing unscheduled outages.

In a server cluster, each server owns and manages its local devices and has a copy of the operating system and the applications or services that the cluster is managing. Devices common to the cluster, such as disks in common disk arrays and the connection media for accessing those disks, are owned and managed by only one server at a time. For most server clusters, the application data is stored on disks in one of the common disk arrays, and this data is accessible only to the server that currently owns the corresponding application or service.

Server clusters are designed so that the servers in the cluster work together to protect data, keep applications and services running after failure on one of the servers, and maintain consistency of the cluster configuration over time.

Dependencies on Other Technologies

Server clusters require network technologies that use IP-based protocols and depend on the following basic elements of network infrastructure:

  • The Active Directory directory service (although server clusters can run on Windows NT, which does not use Active Directory).

  • A name resolution service, that is, Windows Internet Name Service (WINS), the Domain Name System (DNS), or both. You can also use IP broadcast name resolution. However, because IP broadcast name resolution increases network traffic, and is ineffective in routed networks, within this Technical Reference we assume that you are using WINS or DNS.

In addition, for IP addressing for clients, Dynamic Host Configuration Protocol (DHCP) is often used.

Note

  • The Cluster service does not support the use of IP addresses assigned from a DHCP server for the cluster administration address (which is an IP address resource associated with the cluster name) or any other IP address resources. If possible, we recommend that you use static IP addresses for all cluster systems.

Types of Server Clusters

There are three types of server clusters, based on how the cluster systems, called nodes, are connected to the devices that store the cluster configuration and state data. This data must be stored in a way that allows each active node to obtain the data even if one or more nodes are down. The data is stored on a resource called the quorum resource. The data on the quorum resource includes a set of cluster configuration information plus records (sometimes called checkpoints) of the most recent changes made to that configuration. A node coming online after an outage can use the quorum resource as the definitive source for recent changes in the configuration.

The sections that follow describe the three different types of server clusters:

  • Single quorum device cluster, also called a standard quorum cluster

  • Majority node set cluster

  • Local quorum cluster, also called a single node cluster

Single Quorum Device Cluster

The most widely used cluster type is the single quorum device cluster, also called the standard quorum cluster. In this type of cluster there are multiple nodes with one or more cluster disk arrays, also called the cluster storage, and a connection device, that is, a bus. Each disk in the array is owned and managed by only one server at a time. The disk array also contains the quorum resource. The following figure illustrates a single quorum device cluster with one cluster disk array.

Single Quorum Device Cluster

4-Node Server Cluster Using a Single Quorum Device

Because single quorum device clusters are the most widely used cluster, this Technical Reference focuses on this type of cluster.

Majority Node Set Cluster

Windows Server 2003 supports another type of cluster, the majority node set cluster. In a majority node set cluster, each node maintains its own copy of the cluster configuration data. The quorum resource keeps configuration data consistent across the nodes. For this reason, majority node set clusters can be used for geographically dispersed clusters. Another advantage of majority node set clusters is that a quorum disk can be taken offline for maintenance and the cluster as a whole will continue to operate.

The major difference between majority node set clusters and single quorum device clusters is that single quorum device clusters can operate with just one node, but majority node set clusters need to have a majority of the cluster nodes available for the server cluster to operate. The following figure illustrates a majority node set cluster. For the cluster in the figure to continue to operate, two of the three cluster nodes (a majority) must be available.

Majority Node Set Cluster

Majority Node Set Cluster

This Technical Reference focuses on the single quorum device cluster.

Local Quorum Cluster

A local quorum cluster, also called a single node cluster, has a single node and is often used for testing. The following figure illustrates a local quorum cluster.

Local Quorum Cluster

Local Quorum Cluster

This Technical Reference focuses on the single quorum device cluster, which is explained earlier in this section.

Related Information

The following sources provide information that is relevant to this section.

  • “Planning Server Deployments” in the Windows Server 2003 Deployment Kit on the Microsoft Web site for information about planning for high availability and scalability, choosing among the different types of clustering technology, and designing and deploying server clusters.

  • Network Load Balancing Technical Reference

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