High-Level Overview of NLB and Failover Clustering in .NET

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failover cluster group can take over the service and ownership of the shared resources and provide high availability of the service to the client with no down time in the event of a server failure Because the shared resource or service instance can be serviced only by a single server in a cluster, there are active nodes that provide a service and passive nodes that are idling in a stand-by state, but ready to take over applications and services should the active node fail or need to be stopped for maintenance This is known as an active-passive cluster, and it is possible to have multiple active and passive nodes in a cluster Obviously, the passive nodes are effectively doing nothing most of the time, so many organizations will implement a four-node cluster with three active nodes and one passive node (where the passive node can be used to take over the roles of any of the three active nodes)
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FIGURE 18-1 The NLB process You can also have active-active clusters, where all nodes in a cluster are active Active-active clusters generally mean performance problems are an issue in the event of a node becoming unavailable the active node has to not only do the work it was already doing, but also that of the unavailable node It is important to size the nodes correctly if active-active is used to ensure all nodes have suf cient spare resources to handle the workload of other nodes in the cluster on top of their own Because a cluster offers a service that can be ful lled only by a single node, it is vital that all nodes in the cluster have a common membership
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view of the cluster and of which nodes should be active and offer what services To that end, a cluster has a quorum, which is responsible for maintaining a consistent view and agreed state of the cluster The quorum tells the nodes in the cluster which node(s) are active and, should nodes in the cluster fail to be able to contact each other, the quorum controls who is allowed to still run the application or service (Only one node can own the quorum) To facilitate the quorum, certain cluster resources can contribute to determining whether a con guration has a quorum by casting votes Votes can be assigned to nodes in the cluster, to a shared storage medium (such as a universally accessible SAN via ber, iSCSI, and so on), and to a le share In 2008, there is a single quorum model that has four modes (see the Failover Clustering section) The voting system governs the active status of a cluster and, with Windows Server 2008 (or 2003 SP1 with update 921181), node majority no longer has a three-node minimum You can now use a le share witness, which is a share on a server in the domain of the cluster that holds a vote for the cluster and helps maintain a quorum in the cluster As with NLB, the failover cluster has its own IP address and hostname After the cluster is con gured, cluster resources are added (such as le shares, print shares, disk resources, network names, and addresses) that form the service offered by the cluster They are the resources that failover when a passive node becomes active Other services, like Exchange and SQL, which are cluster-aware have speci c areas of functionality that are cluster-compatible and are added as cluster resources Figure 18-2 shows an example failover cluster for a le server with shared storage via a SAN, which could contain both the data being offered and the quorum Any client request to a share on the cluster is serviced by the active node, which owns the IP address, name of the cluster, and all the cluster resources for the cluster group it is active for (in this case, a le share) Only certain versions of Windows Server 2008 support failover cluster, and there are far less restrictions on the type of hardware supported in a cluster and the network and storage con gurations Make sure all components of a cluster are fully tested for cluster usage FIGURE 18-2 A basic two-node active-passive le cluster
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