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Chapter 6: Creating Scalable and Highly Available Web Sites 139
server in a rote, sequential distribution manner. However, if a spike in user activity
occurs and causes servers to overload or fail, round-robin DNS will keep distributing
the requests among all of the servers, even if some of them are no longer operational.
In short, Internet DNS is limited in its capabilities, and its round-robin distribution
mechanism does not contain any intelligence that allows it to monitor, manage, and
react to overloaded or failed servers. Consequently, DNS by itself is not a sound load
balancing or failover solution for your business-critical sites. The load balancing and
failover technology that ColdFusion provides, ClusterCATS, compensates for DNS
limitations and allows you to create highly available, reliable, and scalable ColdFusion
Web applications.
DNS core elements
Following are core DNS elements that you must understand and be able to configure if
your ColdFusion applications are to work well with DNS:
•
Zones and domains
•
DNS record types, server aliases, and round-robin distribution
Zones and domains
A Domain Name System is composed of a distributed database of names. The names
in the DNS database establish a logical tree structure called the domain name space.
On the Internet, the root of the DNS database is managed by the Internet Network
Information Center (InterNIC). The top-level domains were originally assigned
organizationally and by country. Two-letter and three-letter abbreviations are used for
countries and various abbreviations are reserved for use by organizations.
A domain is a node on a network and all of the nodes below it (subdomains) that are
contained within the DNS database tree structure. Domains and subdomains can be
grouped into zones to allow distributed administration of the name space. More
specifically, a zone is some portion of the DNS name space whose database records
exist and are managed in a particular physical file. A single DNS server may be
configured to manage one or multiple zone files. Each zone is anchored at a specific
domain node. Zones are used for breaking up domains across multiple segments when
you need to distribute the management of the domain to multiple groups and for
replicating data more efficiently.