Common Failures - MACROMEDIA COLDFUSION 5-ADVANCED ADMINISTRATION Manual

What is Web Site Availability?
For ColdFusion Web applications, it is particularly important that the ColdFusion
servers remain as highly available and responsive as the Web server and other
dependent servers. ColdFusion processes requests that are sent to it from the Web
server. Upon successfully processing the application logic, ColdFusion returns the
results back to the Web server, which in turn returns an HTML response back to the
browser.
Availability and reliability are concerned with keeping the relevant servers that
provide services to your Web application available at all times. However, if a server on
which your site depends becomes unavailable, it's critical that a sound redundancy
scheme makes certain that your site remains available. As your organization moves
into an e-business paradigm, you must plan, design, and implement load balancing
and failover strategies that guarantee that your servers will remain operational and
serving your customers.
If servers employ a good strategy for load balancing and failover, there's no reason
why they should not provide high availability and reliability to their users. In fact,
Internet Service Providers (ISPs) that host commercial Web sites and offer 24x7
technical support as a competitive service differentiator will typically specify in
written service-level agreements (SLA) a percentage of time that they guarantee a
Web site will be available. If the ISP has a sound scalability and failover strategy in
place, this figure is usually in the range of 99% or better.

Common failures

Following are typical types of failures that can negatively impact your Web
application's availability and reliability:
Hardware failures While less common than software failures, hardware failures
do occur and may include crashed hard drives, blown processors, and corrupted
network cards. Diagnosing and fixing these kinds of issues can be a lengthy
endeavor because of time spent procuring the parts and performing the labor. If
your Web application is mission-critical, you should ensure a sound hardware
redundancy strategy to avoid costly downtime. A sound strategy includes a
minimum of two Web servers but preferably three.
Software failures The types of software failures that will most likely affect a Web
application involve the Web server's operating system, the Web server software
itself, or the Web application software. If the operating system crashes or
becomes corrupt, the Web server cannot function properly (or perhaps at all),
causing your Web application's availability, reliability, and performance to be
compromised. Similarly, if the Web server software crashes or acts erratically, it
will likely cause the Web server to stop running when you didn't intend it to. It's
hard to prepare for software failures, but if you have mirrored secondary
hardware systems in place to account for failures, you'll minimize your Web
application's downtime.
Server failures In addition to the Web server, other servers on which your Web
application depends can also fail, causing either downtime or diminished
capabilities on your site. For example, for distributed applications, a proxy server
may go down, causing requests for your Web application's services to go
unanswered. Or, the database server can crash, making it impossible for users to
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