MACROMEDIA COLDFUSION MX-CLUSTERCATS Use Manual page 36

Hardware planning
As indicated in the availability example above, you must acquire all necessary hardware
and configure it before you deploy an application. All websites have different
requirements, feature sets, purposes, audiences, and budgets, and therefore different
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, Macromedia recommends that you use a minimum
of three servers to support a critical website, so 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 and protects your site from
hardware or software failures.
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, such as an intranet. Typically, this
redundancy model uses an expensive, high-capacity server for the primary server, and
an inexpensive, lower-quality server for the backup server in case the primary server
fails.
Parallel servers — this is a classic load-balancing/redundancy mode, and is used most
often for business-critical applications. Unlike the primary backup scheme, the
multiple servers in a parallel scheme are considered peers and are grouped as a single
entity to support one or more applications.
You can use identical cloned hardware in your server clusters, or you can mix
hardware sizes and models. Cloned, higher-capacity, higher-end hardware might have
greater up-front hardware costs, but help minimize long-term administration costs.
Conversely, mixing hardware models and capacities might be less expensive in the
short term, but could add administrative costs later on.
If you plan to use a parallel model, using many middle-range servers, rather than
fewer high-end ones, or many inexpensive ones, is recommended. Servers that
provide adequate capacity and are moderately priced can generally accommodate
your needs as well as expensive ones, but at a fraction of the cost.
Systems monitoring
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 availability and service levels. These monitoring
programs must be able to not only detect problems, but also route alerts to
administrators for immediate notification of problems.
Corrective actions
The third major failover consideration is the corrective actions that must occur if a failure
causes a server to become unavailable. Generally, if a server goes down and causes your
site to become unavailable, some level of human interaction is usually required to
effectively diagnose and correct the problem.
26 Chapter 2 Scalability and Availability Overview