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Creating Data Matrix ECC200 in Visual Basic .NET Lesson 3

Lesson 3
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Designing a Server Cluster
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In a multiple-instance application, more than one node in the cluster can be running the same application at the same time. When deploying multiple-instance applications, you either clone them or partition them. Cloned applications are rare on server clus ters. Most applications that require this type of deployment are stateless and are better suited to a Network Load Balancing cluster than to a server cluster. Partitioning an application means that you split the application s functionality into sep arate instances and deploy each one on a separate cluster node. For example, you can configure a database application on a four-node server cluster so that each node han dles requests for information from one fourth of the database, as shown in Figure 7-14. When an application provides a number of different services, you might be able to configure each cluster node to handle one particular service.
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Figure 7-14 A partitioned database application
Note With a partitioned application, some mechanism must distribute the requests to the appropriate nodes and assemble the replies from multiple nodes into a single response for the client. This mechanism, like the partitioning capability itself, is something that developers must build into the application; these functions are not provided by the clustering capability in Windows Server 2003 by itself.
Partitioning by itself can provide increased application efficiency, but it does not provide high availability. Failure of a node hosting one partition renders part of the database or certain services unavailable. In addition to partitioning the application, you must configure its failover capabilities. For example, in the four-node, partitioned database application mentioned earlier, you can configure each partition to fail over to one of the other nodes in the cluster. You can also add one or more passive nodes to func tion as standbys for the active nodes. Adding a single passive node to the four-node cluster would enable the application to continue running at full capacity in the event of a single node failure. It would be necessary for servers to run multiple partitions at once only if multiple server failures occurred.
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7
Clustering Servers
Planning Here again, you must decide what is the worst-case scenario for the cluster and
plan your server capacity accordingly. If you want the four-node cluster to be able to compen sate for the failure of three out of four nodes, you must be sure that each server is capable of running all four of the application s partitions at once.
If you plan to deploy more than one multiple-instance application on your cluster, the problem of configuring partitions, failover behavior, and server capacity becomes even more complex. You must plan for all possible failures and make sure that all the parti tions of each application have a place to run in the event of each type of failure.
Selecting a Quorum Model
Every node in a server cluster maintains a copy of the cluster database in its registry. The cluster database contains the properties of all the cluster s elements, including physical components such as servers, network adapters, and shared storage devices; and cluster objects such as applications and other logical resources. When a cluster node goes offline for any reason, its cluster database is no longer updated as the cluster s status changes. When the mode comes back online, it must have a current copy of the database to rejoin the cluster, and it obtains that copy from the cluster s quorum resource. A cluster s quorum contains all the configuration data needed for the recovery of the cluster, and the quorum resource is the drive where the quorum is stored. To create a cluster, the first node must be able to take control of the quorum resource, so that it can save the quorum data there. Only one system can have control of the quorum resource at any one time. Additional nodes must be able to access the quorum resource so that they can create the cluster database in their registries. Selecting the location for the quorum is a crucial part of creating a cluster. Server clusters running Windows Server 2003 support the following three types of quorum models:
Single-node cluster A cluster that consists of only one server. Because there is no need for a shared storage solution, the application data store and the quorum resource are located on the computer s local drive. The primary reason for creat ing single-node clusters is for testing and development. Single-quorum device cluster The cluster uses a single quorum resource, which is one of the shared storage devices accessible by all the nodes in the clus ter. This is the quorum model that most server cluster installations use. Majority node set cluster A separate copy of the quorum is stored in each clus ter node, with the quorum resource responsible for keeping all copies of the quorum consistent. Majority node set clusters are particularly well suited to geographically dispersed server clusters and clusters that do not have shared data storage devices.
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