System-Level Fault Tolerance (Clustering/Network Load Balancing) - Choosing the Right Clustering Technology
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For these fault-tolerant clustering technologies to be most effective, administrators must carefully choose which technology and configuration best fits their application or network service needs. NLB is best suited to provide connectivity to TCP/IP-based services such as Terminal Services, Web sites, VPN services, and streaming media services. This provides scalability, and the amount of redundancy it provides depends on the number of systems in the NLB set. The Windows Server 2003 Cluster Service provides server failover functionality for mission-critical applications such as enterprise messaging, databases, and file and print services.
Although Microsoft does not support using both NLB and MSCS on the same server, multi-tiered applications can take advantage of both technologies by using NLB to load-balance front-end application servers and using MSCS to provide failover capabilities to back-end databases that contain data too large to replicate during the day.
Microsoft Cluster Service
Microsoft Cluster Service (MSCS) is a clustering technology that provides system-level fault tolerance by using a process called failover. Cluster Service is used best to provide access to resources such as file shares, print queues, email or database services, and back-end applications. Applications and network services defined and managed by the cluster, along with cluster hardware including shared disk storage and network cards, are called cluster resources. Cluster Service monitors these resources to ensure proper operation.
When a problem is encountered with a cluster resource, Cluster Service attempts to fix the problem before failing it completely. The cluster node running the failing resource attempts to restart the resource on the same node first. If the resource cannot be restarted, the cluster will fail the resource, take the cluster group offline, and move it to another available node, where it can then be restarted.
Several conditions can cause a cluster group to fail over. Failover can occur when an active node in the cluster loses power or network connectivity or suffers a hardware failure. Also, when a cluster resource cannot remain available on an active node, the resource's group is moved to an available node, where it can be started. In most cases, the failover process is either noticed by the clients as a short disruption of service or no disruption at all.
To avoid unwanted failover, power management should be disabled on each of the cluster nodes in the motherboard BIOS, on the network interface cards, and in the Power applet in the operating system's Control Panel. Power settings that allow a monitor to shut off are okay, but the administrator must make sure that the disks are configured to never go into standby mode.
Cluster nodes can monitor the status of resources running on their local system, and they can also keep track of other nodes in the cluster through private network communication messages called heartbeats. The heartbeats are used to determine the status of a node and send updates of cluster configuration changes to the cluster quorum resource.
The quorum resource contains the cluster configuration data necessary to restore a cluster to a working state. Each node in the cluster needs to have access to the quorum resource; otherwise, it will not be able to participate in the cluster. Windows Server 2003 provides three types of quorum resources, one for each cluster configuration model.
Using Network Load Balancing
The second clustering technology provided with the Windows Server 2003 Enterprise and Datacenter server platforms is network load balancing. NLB clusters provide high network performance and availability by balancing client requests across several servers. When client load increases, NLB clusters can easily be scaled out by adding more nodes to the cluster to maintain or provide better response time to client requests.
Two great features of network load balancing are that no proprietary hardware is needed, and an NLB cluster can be configured and up and running literally in minutes. NLB clusters can grow to 32 nodes, and if larger cluster farms are necessary, DNS round robin or a third-party solution should be investigated to meet this larger demand.
One important point to remember is that within NLB clusters, each server's configuration must be updated independently. The NLB administrator is responsible for making sure that application configuration and data are kept consistent across each node. Applications such as Microsoft's Application Center can be used to manage content and configuration data among those servers participating in the NLB cluster. To install network load balancing, proceed directly to the "Installing Network Load Balancing Clusters" section later in this chapter.
 This chapter is from Microsoft Windows Server 2003 Unleashed, by Rand Morimoto, et al. (Sams Publishing, 2004, ISBN: 0672326671). Check it out at your favorite bookstore today.
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