MACROMEDIA COLDFUSION 4.5-ADMINISTRING COLDFUSION SERVER Manual page 170

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Hardware planning
As illustrated in the availability example above, it's important to acquire all of the
necessary hardware and configure it before you deploy the application. All Web sites
have different requirements, feature sets, purposes, audiences, and budgets. It all
translates into determining appropriate needs. However, if your site is a business-
critical system that affects your company's bottom line, you must ensure an
appropriate redundancy strategy by having two or more redundant systems in place.
In fact, Allaire recommends that you use a minimum of three servers to support any
critical Web site so that you can take one server offline to perform update and
maintenance tasks while maintaining at least two servers in production at all times.
This scheme provides administrative flexibility while simultaneously protecting your
site from hardware or software failures.
Redundancy models
The two predominant redundancy models used today are:
Primary/Backup Servers
An example of this model would be an important Web application that receives
relatively little traffic. For instance, a corporate intranet. Typically, this
redundancy model uses an expensive, high-capacity server for the primary
server and uses an inexpensive, lower quality server for the backup server in
case the primary server fails.
Parallel Servers
This model is known as a classic load balancing/redundancy model and is used
most often for business-critical applications. Unlike the primary/secondary
scheme discussed above, the multiple servers used in a parallel scheme are
considered peers and are grouped together as a single entity to support one or
more applications.
You can use identical cloned hardware for creating your server clusters, or you
can mix hardware sizes and models. Cloned, higher capacity, higher-end
hardware may have greater up-front hardware costs but will help minimize
administration costs down the line. Conversely, mixing hardware models and
capacities may be less expensive up-front but can add administrative costs later
on.
If you plan to use a parallel model, Allaire recommends that you use many
middle range servers rather than fewer high-end ones or lots of inexpensive
ones. Servers that provide adequate capacity and are moderately priced can
generally accommodate all your needs just as well as expensive ones at a
fraction of the cost.
Systems monitoring
In addition to redundant hardware, you should ensure that your network and the
mission-critical sites that reside on its servers are supported by systems monitoring
software. This type of software actively and continuously monitors an application's
Administering ColdFusion Server