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18 Chapter 2 Scalability and Availability Overview
To understand why, let’s look at the e-commerce example. Even if you have planned
ahead and set up multiple servers to support this high-volume site, if you rely only on
DNS, it can only perform two tasks:
•
Translate natural language names to server IP address mappings so that users can find
the site
•
Distribute load among servers in a rote, sequential distribution manner, if you have
enabled round-robin distribution for multiserver load balancing
However, if a spike in user activity causes servers to overload or fail, round-robin DNS
keeps distributing requests among all servers, even if some are not operating.
In short, Internet DNS is limited in its capabilities, and its round-robin distribution
mechanism does not include intelligence for monitoring, managing, and reacting to
overloaded or failed servers. Consequently, DNS by itself is not a sound load-balancing or
failover solution for your business-critical sites. ClusterCATS compensates for DNS
limitations and lets you create highly available, reliable, scalable web applications.
DNS core elements
The following are core DNS elements that you must be able to configure if your web
applications are to work well with DNS:
•
“Zones and domains” on page 18
•
“DNS record types, server aliases, and round-robin distribution” on page 19
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. Some
abbreviations are reserved for use by organizations — for example, .com, .gov, and .edu
for business, government, and educational organizations, respectively.
A domain is a node on a network and all 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 a portion of the DNS name space whose database records exist and
are managed in one physical file. One DNS server may be configured to manage one or
multiple zone files. Each zone is anchored at a specific domain node. You use zones for
breaking up domains across multiple segments to distribute the management of the
domain to multiple groups, and to replicate data more efficiently.