5.3 Cluster models

Many cluster system architectures have been used to design clustering products over the years. It became obvious to us late one night, as we were preparing to teach a seminar on clustering the next morning, that a cluster diagram is worth a thousand words when it comes to explaining the differences between the cluster designs in use today. A lot of thought was put into the drawings you will see on the next few pages to help us explain some of the configurations that are possible when designing clusters. As you study these drawings you might not see at first the differences between some of the configurations, because the differences between them are subtle. If you take the time to consider both the hardware and software elements in each configuration, these illustrations should be a valuable learning tool in helping you to understand the different solutions that have been implemented. The most important thing to remember as you look over these different architectures is that there is not necessarily only one correct answer. Once you understand these diagrams, the tradeoffs that are being made by the different vendors in their products will hopefully become clearer to you.

First, let's lay the groundwork by explaining the components that are typically used in clusters. In Figure 5.1 we show two servers referred to as Node A and Node B. Using our terminology, these two servers are referred to as cluster members. Like any normal enterprise server, they are connected...