Planning High Availability Solutions

As briefly mentioned previously, Windows Server 2003 provides support for two different types of high availability, or clustering, technologies. Of course, the ability to implement high availability solutions does not come without a price. In the case of Windows Server 2003, you must be using either the Enterprise or Datacenter Server versions to have this capability available to you. In this section we first examine the types of clustering provided in Windows Server 2003. Next, we present key terms and operational modes that will be of importance to you as you plan and implement clustering solutions. Lastly, we implement and manage clusters and network load balancing using Windows Server 2003.

High Availability Solutions

Clustering is accomplished when you take a group of independent servers and assemble them together into one large collective entity that is accessed as if it were a single system. Incoming requests for service can be evenly distributed across multiple cluster members or may be handled by one specific cluster member.

The Microsoft Cluster Service ( MSCS ) in Windows Server 2003 provides high availability fault-tolerant systems through failover . When one of the cluster members (nodes) is unable to respond to client requests, the remaining cluster members respond by distributing the load among themselves , thus responding to all existing and new connections and requests for service. In this way, clients see little ”if any ”disruption in the service being provided by the cluster. Cluster nodes are kept aware of the status of other cluster nodes and their services through the use of heartbeats . A heartbeat is used to keep track of the status of each node and also to send updates in the configuration of the cluster. Clustering is most commonly used for database, messaging, and file/print servers. Windows Server 2003 supports up to eight nodes in a cluster.

Windows Server 2003 also provides network load balancing ( NLB ) in which all incoming connection requests are distributed using a mathematical algorithm to members of NLB cluster. NLB clustering is best used in situations in which clients can connect to any server in the cluster, such as Web sites, Terminal Services servers, and VPN servers. You can configure the way the client interacts with the NLB cluster as well, such as allowing the client to use multiple NLB cluster members during a single connection (acceptable for Web sites) or forcing the client to use the same cluster member for the entire connection period (a necessity for VPN and Terminal Services servers). Windows Server 2003 NLB clusters can contain as many as 32 nodes.

NOTE

Combining clustering and network load balancing Although you can use both clustering and NLB in your final design, such as in the case of an e-commerce site that uses NLB for front-end Web servers and clustering for back-end SQL servers, you cannot use both technologies on the same server.

Now that we've discussed clustering technologies provided in Windows Server 2003, let's look at some of the key clustering terminology and operational modes.

High Availability Terminology

A good understanding of the following MSCS and NLB clustering terminology is key to successfully implementing and managing any clustered solution. Although the following list of terms is not all-inclusive, it represents some of the more important ones you should understand before attempting to implement any high availability solution.

• Cluster ” A group of two or more independent servers that operate together and are viewed and accessed as a single resource.

• Cluster resource ” A network application, service, or hardware device (such as network adapters and storage systems) that is defined and managed by the cluster service.

• Cluster resource group ” The resources contained within a cluster. Cluster resource groups are used as failover units within a cluster; when a cluster resource group fails and cannot be automatically restarted by the cluster service, the entire cluster resource group is placed in an offline status and failed over to another node.

• Cluster virtual server ” A cluster resource group that has a network name and IP address assigned to it. Cluster virtual servers are thus accessible by their NetBIOS name, DNS name , and IP address.

• Convergence ” The process by which NLB clustering hosts determine a new, stable state among themselves and elect a new default host after the failure of one or more cluster nodes. During convergence, the total load on the NLB cluster is redistributed among all cluster nodes that share traffic handling on specific ports as determined by their port rules.

• Heartbeat ” A network communication sent among individual cluster nodes at intervals of no more than 500 milliseconds that is used to determine the status of all cluster nodes.

• Failback ” The process of a cluster group moving back to the preferred node after the preferred node has resumed cluster membership. For failback to occur, it must be configured for the cluster group, including the failback threshold and selection of the preferred node.

• Failover ” The process of a cluster group moving from the currently active node to another still functioning node in the cluster group. Failover typically occurs when the active node becomes unresponsive for any reason and cannot be recovered within the configured failure threshold.

• Node ” An individual server within a cluster.

• Quorum disk ” The disk drive that contains the definitive cluster configuration data. Clustering with MSCS requires the use of a quorum disk and requires continuous access to the data contained within the quorum disk. The quorum disk contains vital data about the nodes participating in the cluster, the applications and services defined within the cluster resource group, and the status of each node and cluster resource. The quorum disk is typically located on a shared storage device.

EXAM TIP

Clustering terminology Although you must understand the clustering terms presented here to be able to implement and support cluster solutions, you should not expect to be directly tested on them come exam day.

With the discussion of key clustering terminology now behind us, we can safely move forward and begin to plan for and implement high availability solutions. Because network load balancing clusters are the simpler of the two types to understand, deploy, and support, we start our discussion with them.