User Guide

Chapter 6: Creating Scalable and Highly Available Web Sites 149
availability and its service levels. These monitoring programs must not only be able to
detect problems, but they must also be able to route alerts to the correct
administrators for immediate notification of problems.
Corrective actions
The third major failover consideration is the corrective actions that need to occur if a
failure causes a server to become unavailable. Generally speaking, 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.
However, before the analysis and repair can happen, the administrator needs to be
notified. Whatever failover system you put in place, it should include an automated
notification system that can route alerts via your telecommunications infrastructure
(e-mail, pagers, real time web-based alerts, etc.) to the appropriate administrator for
prompt attention.
Besides notifying the administrator that a problem has occurred, you also want your
failover solution to automatically redirect traffic intended for the unavailable server to
other available servers until the unavailable server is fixed. This crucial corrective
action is what keeps your Web site up and available to your users even if one of the
servers supporting it is experiencing problems.
Techniques for Creating Scalable & Highly Available Sites
Now that you have a fairly good understanding of scalability and availability, the next
step is to familiarize yourself with the techniques you can use to achieve scalable and
highly available Web sites.
This section describes the following topics:
• What is clustering?
• Hardware-based clustering solutions
• Software-based clustering solutions
• Combining hardware and software clustering solutions
What is clustering?
Clustering is a technique in which two or more Web servers supporting one or more
domains (www.yourcompany.com) are grouped together as a cluster of servers to
collectively accommodate increases in load and provide system redundancy.
The following figure shows an example of a server cluster for a sample Web site.