barcode generator project source code in vb.net An active/passive fail-over clustering configuration in Software

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An active/passive fail-over clustering configuration
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An active/active clustering configuration
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Now that we have a good idea of how server nodes can participate in a cluster, let s look at how multiple machines can access the same storage devices.
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Sharing Storage Resources
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In the previous section, we considered how active and passive servers can be combined in a cluster. An important requirement in this type of clustering is that information stored on hard disks be available to the machines sharing that information. Generally, the disk array itself is protected using redundant hardware. But, if the entire server fails, another machine can access the same devices but take over the processing of other types of client interaction. For this to occur, there must be some way for a node in a cluster to assume control over disks formerly controlled by the failed server. Three main ways exist to configure disk sharing at the server level: w Shared-nothing configuration Each disk device is accessible to only one server at a time. If a failure of one node in the cluster occurs, the other one takes over these resources. Physically, the hard disks are usually attached via controllers that allow more than one server to access a disk array. Additionally, some disks might be owned by each node. Figure 8-26 illustrates a shared-nothing configuration. For example, consider a scenario that includes six physical disk devices. Server 1 might be responsible for owning disks 1, 2, and 4, while Server 2 owns disks 3, 5, and 6. (Recall that clients should see Server 1 and Server 2 as one logical machine.) When resources are requested from the cluster, the request is forwarded to the appropriate node. If Server 1 fails, Server 2 will assume control of all six disks and take on the extra workload necessary for ensuring that users maintain access to all needed resources.
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A shared-nothing disk configuration
Shared-everything configuration Each node in the cluster contains a copy of the same data. Physically, this might be implemented either as multiple servers that have access to the same disk devices using a shared controller, or through software methods that can keep data synchronized. We already looked at two ways to do this standby servers (using log-shipping) and replication. Mirrored disk configuration Each server controls its own disk devices, as shown in Figure 8-27. Whenever data is written to one disk, it is automatically written to the disks controlled by other nodes in the cluster. The main advantage is that special hardware that allows the same disk devices to be shared between physical servers is not required. However, a limitation of this method is that data will always be unsynchronized (although for a very brief period of time).
As we ll see later in this section, Enterprise Edition of SQL Server supports the use of shared-nothing disk configurations. The best method to use will be based on your business requirements.
Application Support
To take full advantage of clustering, applications should be cluster-aware. Although the basic clustering processes might be available at the OS level, many other issues must be considered. For example, active websites might store client information in session and application variables. This information must be kept synchronized between multiple servers if a transparent fail-over is expected to occur. If special provisions for this are not made, clients will temporarily lose their connection information. For SQL Server applica-
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Figure 8-27.
A mirrored disk configuration
tions, this means that all clients will need to reestablish their database connections before they can continue. A cluster-aware client application might automatically include error code that transparently reattempts to connect to a server before giving the end user an error message. As we ll see later in this section, features in SQL Server 7, Enterprise Edition have been designed to address such issues when running on Windows NT Server, Enterprise Edition.
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