Choosing the Cluster Type that's Right For You

Network load-balancing cluster models or cluster servers with a shared hard disk array: how to determine which kind of Windows 2000 server cluster meets your technical and business needs.

 By Brien M. Posey
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In part 1 of this series, Is a Server Cluster Right for Your Organization?, I introduce you to the concept behind server clustering, and explain some of the basic hardware and software requirements for establishing a server-clustering environment. In this article, I'll continue with the server cluster planning and explain how different types of cluster server environments are suited to various business environments.

"Knowing what you're trying to accomplish before you start building a server cluster is a requirement from a technical standpoint, because of the various types of server clusters."

What Do You Want To Accomplish?

As I mentioned in the introduction, there are a couple of different types of cluster environments. Determining which one is right for you depends heavily on what you're trying to accomplish. For example, suppose your company has a Web server that has been getting an extremely high number of hits. It might make sense to cluster the Web server to reduce the workload placed on any single server. Normally, this involves using nothing more than a simple network load-balancing (NLB) cluster model, which means that each server is loaded with a copy of your Web site. A network load- balancing service directs incoming traffic to the server that's currently carrying the lightest load.

A load-balancing cluster model is the least complex and least expensive type of clustering, because each server maintains its own independent copy of the files that contain the Web page. With the possible exception of a database that resides on a separate server outside of the cluster, no files are shared. If a server drops offline, the other servers in the cluster pick up the slack. Because there are no shared files, there's no danger of a failed server making critical data unavailable. As the demand on your Web server array grows, you can easily add more servers to the cluster. Each new server will have its own copy of your Web site and be linked into the network load-balancing service.

Or your company might need a critical business application to be available 24 hours a day, 7 days a week. In a situation like this, you'd need much more complicated and expensive hardware than you would in the Web server situation I described earlier. You'd have to set up your cluster servers to share a common hard disk array. Because your servers share a disk array, this implementation can't be used for the technique in which each server has its own copy of the files. Instead, you'd have to set up the servers in such a way that the server any particular user utilizes for an application depends on which groups they belong to. If a user's assigned server were to fail, another server in the cluster could pick up the slack and respond to the user's request. Because all the servers in the cluster use the same hard disk array, there's no danger of a user losing access to a particular database record they're working on just because a server goes down.

Knowing what you're trying to accomplish before you start building a server cluster is a requirement from a technical standpoint, because of the various types of server clusters. Advanced planning is also a requirement from a business standpoint. In most of the companies that I've worked for, upper management tends to be a little afraid of computers. They often see new technology as expensive, unproven, and as a great opportunity to lose critical data and productivity during a crash. If you want to install a server cluster server in your organization, you'll usually end up having to convince the upper management that the server cluster isn't simply a expensive new toy for the IT department. To win your argument, you'll have to plan which type of server cluster is right for your environment so that you can explain to management how that type of clustering system can benefit your organization.

Now that you know what you're up against from a political and a technological standpoint, let's take a more technical look at the two major types of server clusters: the network load-balancing server and the full-blown server cluster. For the sake of clarity, I'll refer to these as NLB clusters and server clusters for the duration of the article.

This article was originally published on Oct 7, 2000
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