User Guide

234 Chapter 11 Scalability and Availability Overview
What is Web Site Availability?
As you’ve already learned from the previous section, it’s critical to design, develop,
test, and deploy your Web applications so that they can scale well under heavy and
ever-increasing load. However, the reality is that in spite of the best-laid plans and
preparations, servers can fail for seemingly unknown reasons, causing your site to
become unavailable. If and when a server fails or becomes overloaded, regardless of
why it has, you want to ensure that it won’t adversely affect your business by
preventing your customers from accessing and using your Web application. If it does,
you risk jeopardizing your bottom line with lost sales and disgruntled customers who
will look to your competitors’ products for goods and services.
This section defines and describes Web site availability and failover. It contains the
following topics:
• “Availability and reliability” on page 234
• “Common failures” on page 235
• “A Web site availability scenario” on page 236
• “Failover considerations” on page 237
Availability and reliability
In the simplest of terms, availability and reliability means you can access your Web
site whenever you request it by entering the site’s URL in your browser and all of its
features work as intended. Thus, availability and reliability refers to the uptime of a
Web site, which is often directly related to the uptime of the Web server and other
dependent servers, such as a database server, an application server, or a file server.
All of the servers that provide your site’s functionality must work for a site to be
considered available.