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Chapter 1. Understanding High Availability > High Availability Explained

High Availability Explained

High availability refers to your Web application's capability to respond 99.99 percent of the time. You'll achieve this figure, which works out to about one hour of down time per year, by designing network architectures and Web applications that eliminate all single points of failure or that have a high degree of fault tolerance (redundancy at every level within the hosting provider, network, server, and Web-application architecture).

Here's an example: You have a basic Web site that contains a single Web server and a single database server. One day a power surge causes a power supply failure in the Web server, and the site goes down. If that server's running an e-commerce site, you might lose business irreparably. However, if you've built the site on a cluster of two or more Web servers, the end user can navigate the site normally and may never know that any component failure occurred. Ideally, all your servers would remain healthy all the time; however, that uptime percentage I mentioned earlier does not mean each server will maintain individual uptimes of 99.99 percent. Rather, this percentage refers to the Web application's total uptime as seen by the end user. See Table 1.1, which describes uptime percentage and downtime per year for an application running continuously 24 hours a day, 7 days a week, and 365 days a year.


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