An Overview of Cloud Based Content Delivery Networks: Research Dimensions and State-of-the-Art - Transactions on Large-Scale-Data-and Knowledge-Centered Systems XX - page 137

grade of services delivered to the application consumer and administrator as speci ed

and agreed upon in the Service Level Agreement (SLA) document. The SLA guar-
antees scope and nature of an agreed QoS performance objective (also referred to as the
QoS targets) that the cloud application consumer and administrators can expect from
cloud service provider(s).
Though the notion of virtually unlimited resources is true in many aspects, there are
practical limitations to the realisation of this concept. For example, how to automati-
cally provision new resources as the demand for the service increases. Previous work
on resource provisioning in distributed computing environments [ 7 , 37 ] enables its
users to manually modify the hardware resources of their running job flows.

3 Cloud CDNs

CDNs have made a signi cant impact on how content is delivered via the Internet to

the end-users [ 21 ]. Traditionally content providers have relied on third-party CDNs to
deliver their content to end-users. With the ever changing landscape of content types
e.g. moving for standard de

nition, it is a
challenge for content providers who either supplement their existing delivery networks
with third-party providers or completely rely on them to understand and monitor the
performance of their service. Moreover, the performance of the CDN is impacted by
the geographical availability of the third-party infrastructure. A cloud CDN (CCDN)
provides a flexible solution allowing content providers to intelligently match and place
content on one or more cloud storage servers based on coverage, budget and QoS
preferences [ 23 ]. The key implication is economies of scale and the bene

nition video to high de nition to full high de

ts delivered
by the pay-as-you-go model. Using clouds the content providers have more agility in
managing situations such as flash crowds avoiding the need to invest in infrastructure
development.
As stated previously, clouds provide the end users with virtually in

nite pool of
compute and storage resources with no capital investment in terms of hardware and
software. Therefore, CCDN systems can be very valuable in data processing and
delivery of content over the Internet. The main advantage of such a system would be
that they provide a cheaper means of hosting and deploying multi-tiered applications
that can scale based on the usage demands. Further clouds offer not only cheaper
content storage and distribution functionality, but also compute functionality such that
application and data processing can also be performed on clouds. Lastly, cloud offers
pay-as-you-model whereby the end-users can start and terminate the cloud resources
based on the amount of money they are willing to spend hosting their services without
entering into a complex contract with the cloud provider. Migration from traditional
client/server based CDNs to cloud computing model is a major transformation that
introduces great opportunities and challenges. The major advantages and opportunities
introduced by CCDNs include: