Types of high availability

Server clusters are designed to keep resources (such as applications, disks, and file shares) available. Availability is a measure of the ability of clients to connect with and use a resource. If a resource is not available, clients cannot use it.

It is possible to contrast high-availability with fault-tolerance, as different benchmarks for measuring availability:
Fault-tolerance is defined as 100% availability all of the time. Fault-tolerant systems are designed to guarantee resource availability.
A high-availability system maximizes resource availability. A highly available resource is available a high percentage of the time that might approach 100% availability, but a small percentage of down time is acceptable and expected.

In this way, high-availability can be defined as a highly available resource that is almost always operational and accessible to clients.

The section explains the following type of high availability: HACMP for AIX scenario - Backup domain manager