MACROMEDIA COLDFUSION 5-ADVANCED ADMINISTRATION, Manual

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240 Chapter 11 Scalability and Availability Overview
Clustering for failover relies on redundant servers to ensure that business-critical
applications remain available if one of the servers in a cluster fails. Intelligent
software-based failover solutions can detect when a server has failed and
automatically redirect new incoming HTTP requests to the cluster members that are
available. Some hardware-based failover devices that have less built-in intelligence
require an administrator’s intervention once the failure is detected.
Clustering can be accomplished using software-based solutions, such as
round-robin DNS by itself or together with a third-party package, a hardware-based
solution, such as a packet router, or a combination of the two.
Hardware-based clustering solutions
The most common and reliable hardware-based clustering solution is a device
known as a packet router. One of the most popular routers on the market is Cisco
System’s LocalDirector. A router sits in front of a cluster of Web servers and directs
incoming HTTP requests to available Web servers that form the cluster. A router
works by assessing the speed and volume of IP packet flow to and from the Web
servers and then selecting the best server to accommodate the traffic. This process is
fast and efficient. The router device in conjunction with the clustered Web servers
comprise what is known as a virtual server.
Routers are considered semi-intelligent devices because they can detect a server
failure and redirect requests to other servers. If a Web server fails or stops
responding, the router stops sending packets to the unresponsive server. Routers are
not considered fully intelligent because while they can redirect requests upon
discovering a failure, they do not allow you to configure redirection thresholds for
individual servers. They also do not provide for application-aware load balancing.