11.1. What is high availability?
Availability is an attribute of an entire system, and is always measured from the user's perspective. High availability is a complex term to define. Most people agree that high availability is an attribute of a system, whereby the system is available to its target users almost all the time. High availability starts with a minimum of 99.99% availability. That is, a particular server configuration is said to be “highly available” if the combination of both planned and unplanned outages in a year is less than one hour. All the pieces of a high-availability solution need to be simultaneously available, as shown in Figure 11-1.
Figure 11-1. A typical two-tier application solution
High availability does not imply that ...
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