Data Replication in High Availability Clusters

Nelson Yount

The ultimate goal of high availability (HA) clustering is to improve the level of availability of applications and other services to their end-users. HA clusters generally consist of two or more computer systems in close physical proximity. The HA clustering software is designed to monitor the status of all of the systems in the cluster, and upon the failure of any of those systems, to restart on one or more of the remaining systems any applications that were running on the failed system. The clustering software will typically also move the network address associated with such an application to the backup system, allowing clients to easily continue to access the application.

Cluster Storage Models

For this application migration to work, it is necessary that the application data be accessible both to the computer system on which the application was running originally (the primary system) and to the system to which the application is moved (the backup system). This is usually accomplished with some form of shared storage, typically either an external storage array (to which both systems are connected by a common SCSI bus or a Fibre Channel network) or a network attached storage (NAS) device. By placing the application data on a shared external storage device, an application has access to its data regardless of whether it is running on the primary or backup system.

Cluster Configurations

The simplest HA cluster configuration consists of two systems, with one system actively running one or more applications, and the second system acting purely as a backup for the first. This configuration is known as a 2-node active/passive cluster. A more common and more useful configuration is the 2-node active/active cluster (see