| Almost every company needs to have high performance and high availability for their Web servers; however, there are many different aspects of high availability. At one time, the only avenue for achieving high availability in the UNIX realm was through fault tolerant technology. Fault tolerance relies on specialized hardware to detect a hardware fault and instantaneously switch to a redundant hardware component ”whether the failed component is a processor, memory board, power supply, I/O subsystem, or storage subsystem. Although this cutover is apparently seamless and offers non-stop service, a high premium is paid in both hardware cost and performance because the redundant components do no processing. More importantly, the fault tolerant model does not address software failures, by far the most common reason for down-time. High availability views availability not as a series of replicated physical components , but rather as a set of system-wide, shared resources that cooperate to guarantee essential services. High availability combines software with industry-standard hardware to minimize down-time by quickly restoring essential services when a system, component, or application fails. While not instantaneous, services are restored rapidly , often in less than a minute. The difference between fault tolerance and high availability, then, is this: A fault tolerant environment has no service interruption, while a highly available environment has a minimal service interruption. Many sites are willing to absorb a small amount of down-time with high availability rather than pay the much higher cost of providing fault tolerance. Additionally, in most highly available configurations, the backup processors are available for use during normal operation. Chapter 14 gives details on the different ways to achieve high availability for your Domino and WebSphere systems. |
