Lesson 1

Understanding Clustering

7-9

For example, if you have two stateful applications that you want to deploy using server

clusters, the simplest method would be to create a single cluster and install both appli-

cations on every computer in the cluster, as shown in Figure 7-3. In this arrangement,

a single server failure affects both applications, and the remaining servers must be

capable of providing adequate performance for both applications by themselves.

App 1

App 1

App 1

App 1

App 2

App 2

App 2

App 2

Server

Server

Server

Server

F07pm03

Figure 7-3

A cluster with two applications running on each server

Another method is to create a separate cluster for each application, as shown in

Figure 7-4. In this model, each cluster operates independently, and a failure of one

server only affects one of the applications. In addition, the remaining servers in the

affected cluster only have to take on the burden of one application. Creating separate

clusters provides higher availability for the applications, but it can also be an expensive

solution, because it requires more servers than the first method.

App 2

App 2

App 1

App 1

Server

Server

Server

Server

F07pm04

Figure 7-4

Two separate clusters running two different applications

It is also possible to compromise between these two approaches by creating a single

cluster, installing each of the applications on a separate active node, and using one

passive node as the backup for both applications, as shown in Figure 7-5. In this

arrangement, a single server failure causes the passive node to take on the burden of

running only one of the applications. Only if both active nodes fail would the passive

node have to take on the full responsibility of running both applications. It is up to you

to evaluate the odds of such an occurrence and to decide whether your organization’s

availability requirements call for a passive node server with the capability of running

both applications at full performance levels, or whether a passive node scaled to run

only one of the applications is sufficient.