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Chapter 6 The Cloud
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Networks

Content Delivery Networks

A second major use of the cloud is to deliver content from servers placed around the world.
A content delivery network (CDN) is a system of hardware and software that stores user data
in many different geographical locations and makes those data available on demand. A CDN pro-
vides a specialized type of PaaS but is usually considered in its own category, as it is here.
Consider CDN applications: A news organization could use a CDN to store copies of its news
articles. The CDN vendor replicates articles on servers, possibly worldwide, so as to speed response
time. When a news reader accesses an article, the request is transmitted to a routing server that
determines which CDN server is likely to deliver the article to the user the fastest. Because traffic
changes rapidly, especially for popular sites, such calculations are made in real time. A request for
content at one moment in time could be served by a computer in, say, San Diego, and a few moments
later, that same request from that same user might be served by a computer in Salt Lake City.
In addition to news articles, CDNs are often used to store and deliver content that seldom
changes. For example, the company banner on an organization’s Web page might be stored on
many CDN servers. Various pieces of the Web page could be obtained from different servers on the
CDN; all such decisions are made in real time to provide the fastest content delivery possible.
Figure 6-17 summarizes CDN benefits. The first two are self-explanatory. Reliability is increased
because data are stored on many servers. If one server fails, any of a potentially large number of
other servers can deliver the content. You will learn about denial-of-service (DOS) attacks in Chapter
10. For now, just understand that such security threats send so much data to a given server that the
server’s performance for legitimate traffic becomes unacceptable. By having multiple servers, CDNs
help to protect against such attacks.
In some cases, CDNs reduce access costs for mobile users (those who do have a limited data
account). By delivering the data faster, site connection charges can be reduced. Finally, many (but
not all) CDN services are offered on a flexible, pay-as-you-go basis. Customers need not contract
for fixed services and payments; they pay only for what they use, when they use it. Figure 6-18
shows an example of how CDN servers might be distributed. A number of vendors offer CDN.

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Barcelona, ES
0QTVJ #OGTKEC Galway, IE
Houston, TX London, GB
Indianapolis, IN Munich, DE #UKC
Hong Kong, HK
Kansas City, MO Paris, FR Kuala Lumpur, MY
Los Angeles, CA Rotterdam, NL Kyoto, JP
New York, NY Rome, IT Melbourne, AU
Roanoke, VA Uppsala, SE Sendai, JP
San Diego, CA
San Francisco, CA
Spokane, WA
Tampa, FL
Trenton, NJ
West Palm Beach, FL
Figure 6-18 5QWVJ #OGTKEC
Rio De Janiero, BR
Servers Used in a Typical CDN La Paz, BO
Bogota, CO
Service Buenos Aires, AR