High availability is a design for your deployment that operates with a small amount of planned and unplanned downtime. Typically, a highly available configuration is a cluster that is made up of two or more loosely coupled systems. Each system maintains its own processors, memory, and operating system. Storage is shared between the systems. Special software binds the systems together and allows them to provide fully automated recovery from a single point of failure. Messaging Server provides high-availability options that support both the Sun Cluster services and Veritas clustering solutions.
When you create your high availability plan, you need to weigh availability against cost. Generally, the more highly available your deployment is, the more its design and operation will cost.
High availability is an insurance against the loss of data access due to application services outages or downtime. If application services become unavailable, an organization might suffer from loss of income, customers, and other opportunities. The value of high availability to an organization is directly related to the costs of downtime. The higher the cost of downtime, the easier it is to justify the additional expense of having high availability. In addition, your organization might have service level agreements guaranteeing a certain level of availability. Not meeting availability goals can have a direct financial impact.
See Chapter 6, Designing for Service Availability for more information.