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228 Chapter 6 The Cloud

to load whatever operating system they want, and Amazon.com licenses S3 (Simple Storage
Service), which provides unlimited, reliable data storage in the cloud.

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 provides 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. AllRoad would likely use a CDN to store Web site data during its
Academy Awards ad, as discussed in Q1.
In addition to news articles, CDNs are often used to store and deliver content that sel-
dom 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 differ-
ent servers on the CDN; all such decisions are made in real time to provide the fastest con-
tent delivery possible.
Figure 6-17 summarizes CDN benefits. The first two are self-explanatory. Reliability is in-
creased 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.

Using Web Services Internally
The third way that organizations can use cloud technology is to build internal information
systems using Web services. Strictly speaking, this is not using the cloud because it does not
provide elasticity nor the advantages of pooled resources. It does advantageously use cloud
standards, however, so we include it here.

Benefits of Content Delivery Networks
• Decreased, even guaranteed,load time
• Reduced load on origin server
• Increased reliability
• Protection form DOS attacks
Figure 6-17 • Reduced delivery costs for mobile users
Benefits of Content Delivery • Payasyougo
Networks