High Availability

Through its implementation of the shared nothing model, Application Center provides a high level of availability for all cluster members, including the controller. Remember that every member is, in essence, a mirror image of the cluster controller.

With integrated NLB installed, each member "hears" incoming client requests, so if one member fails, the other members can continue servicing these requests. Because a failed member is taken offline in the background and load balancing is automatically reconfigured to distribute client requests, Application Center provides an effective environment for providing computing availability. In addition, its transparent isolation mechanism shields users from the effects of server failure.

If a controller fails, cluster administrative activities such as synchronization cannot take place until a new controller is designated; however, load-balanced request handling continues to function.

When coupled with its automated e-mail notification and flexible administration features, the shared nothing implementation provided by Application Center ensures a highly available environment for clusters.

NOTE

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Although Application Center clusters provide high levels of server availability, fault tolerance, as it is defined, and the fault tolerant state are not supported. If a member crashes or is intentionally taken out of the cluster, all clients that rely on the state stored on that particular member will lose their state.

In the next chapter, the product's architecture and the underlying technologies that support Application Center are presented. You'll also get a behind-the-scenes look at each feature, explaining the sequence of events and processes that take place when it's used.