Does high availability mean that a fault-tolerant machine is needed?

Traditionally, fault tolerant machines were based around hardware redundancy, and able to survive hardware failures without any break in service.

High availability no longer requires such fault-tolerant machines. There are many solutions based on cluster architectures that are able to satisfy the majority of high availability needs.

It should be noted that the reliability and maintainability capabilities of the platforms used to construct a cluster system are of primary importance in obtaining a system able to meet extreme availability needs.

Fault-tolerant systems are used in situations when the time between a failure and complete resumption of services must be very short. Hardware reliability continues to improve (a trend that will continue for technology reasons), while software reliability marks time at best (granted, the quality of a given piece of software will improve over time�given appropriate management�but there always seems to be more software). Failures attributable to software substantially exceed failures due to hardware. Clusterbased solutions provide some protection against software failures, while purely hardware-based fault-tolerant machines can cope only with hardware failure.

Source of Information :  Elsevier Server Architectures 2005