Configuring and Planning a Windows Server Cluster - Page 2

 By Brien M. Posey
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In the past, I've worked for several organizations in which management deemed one or two applications to be mission critical. In these environments, management never wanted to see a network failure of any kind; but if the network did fail, they really didn't care what failed, as long as those essential applications were still running.

In such environments, load shedding is a great configuration. This configuration is especially effective because it not only guarantees that the application will be available under any circumstances, it also ensures that the application's performance won't suffer because of a bogged-down server.

In the load-shedding model, the clustered servers each run their own set of applications, just as you normally would on two separate servers (remember that the cluster is still seen as a single server by the rest of the network). The only difference is that the fail- over policy defines the critical applications. Now, suppose that one of the CPUs fails. During this failure, the second CPU would detect the failure and look at the fail-over policy. The fail-over policy would then tell the CPU to shut down all non-essential applications and to begin servicing any essential applications that were previously running on the failed CPU.

Cluster Planning

Once you have an idea of which cluster model is right for your environment, you have a lot more planning to do. The first part of this process is to create an exhaustive list of your applications. This list should include things like the current location of each application, any dependencies related to the application, and just how critical the application is. For example, if you have a critical customer management program, you might list the place that the program currently resides and indicate that the program is dependant on the sales database running in the background. Therefore, you'd also want to document the location of the sales database and flag both the program and the underlying database as critical applications. If you're questioning the critical status of the database, consider that the customer management program is critical and can't run without the database; therefore, the database is also critical.

While determining dependencies, you must also look for applications that have common dependencies. For example, suppose that you have two applications that both depend on the same underlying database. Because of the dependency structure, these applications and their dependencies must always be grouped together.

Finally, when designing your fail-over policy, you must consider the impact of that policy. For starters, if you make the second server take over running a critical application, will all the dependencies be in place for the application to run? You must also consider hardware-related issues, such as whether the CPUs have a fast enough processor and enough memory to handle the fail-over policy that you've designed without crashing or bogging down. As you can see, setting up a cluster can be a great way to protect your data or to increase the speed of a Web site. In this article, I've explained the type of clustering environment that's suitable for both situations. //

Brien M. Posey is an MCSE who works as a freelance writer. His past experience includes working as the director of information systems for a national chain of health care facilities and as a network engineer for the Department of Defense. Because of the extremely high volume of e-mail that Brien receives, it's impossible for him to respond to every message, although he does read them all.

This article was originally published on Oct 8, 2000
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