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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 16: Network Load Balancing and Clustering
      9  Clustering

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Clustering
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Cluster Grouping and Dependency Configuration
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How Clustering Works

To understand cluster resources, it is first necessary to understand how the cluster actually works. Windows uses Active-Passive clustering, meaning that a cluster resource is “owned” by only one cluster node at any one time. The resource can be moved back and forth between cluster nodes as required, or automatically upon failure. As a simplified example, imagine server A and server B are clustered, using a quorum disk and a shared disk N. At any one time, either server A or server B can own and have access to the N drive. If server A owns the N drive, it is possible to browse that drive and to use it as a normal drive via Windows explorer on server A. At the same time, looking in “My Computer” on server B shows the N drive but attempting to access it produces an error message stating that the drive is inaccessible. If we then shut down server A, the N disk cluster resource is automatically failed over to server B and the above example is reversed.

OS Clustering and Application Clustering

Note: There is an important distinction between the Operating System clustering and application clustering. Whilst Windows only supports Active-Passive clustering, some applications support Active-Active clustering where an instance of the application runs on both cluster nodes simultaneously. Microsoft Exchange is one application that functions in this way. Active-Passive means only one node in the cluster is doing “work” at any one time, active-active means both nodes are doing “work”. If you’re designing a cluster, you need to understand the difference.


Cluster resources are objects that represent resources and services the cluster is providing. The cluster resources themselves are nothing more than representations of the actual resources and services the cluster is providing. For example, it is possible to add an IIS web site as a cluster resource. The web site itself is contained within IIS on the cluster nodes, and the cluster resource representing it is contained only within the cluster manager. It is possible to configure NetBIOS names, IP addresses, file shares and much more as cluster resources.


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Clustering
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Cluster Grouping and Dependency Configuration
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