Information Technology Reference
In-Depth Information
Also Reese (2009) notes a Cloud can be both software and infrastructure, and
stresses the way how Cloud services might be consumed:
“The [Cloud] service is accessible via a web browser (nonproprietary) or web services
API.; Zero capital expenditure is necessary to get started.; You pay only for what you use
as you use it.”
Foster et al. (2008) define Cloud Computing as
“[a] large-scale distributed computing paradigm that is driven by economies of scale, in
which a pool of abstracted, virtualized, dynamically-scalable, managed computing power,
storage, platforms, and services are delivered on demand to external customers over the
Internet.”
Two important aspects added by the definition of Foster et al. (2008) are virtu-
alization and scalability. Cloud Computing abstracts from the underlying hardware
and system software through virtualization. The virtualized resources are provided
through a defined abstracting interface (an Application Programming Interface
(API) or a service). Thus, at the raw hardware level, resources can be added or
withdrawn according to demand posted through the interface, while the interface to
the user is not changing. This architecture enables scalability and flexibility on the
physical layer of a Cloud without impact on the interface to the end user.
Finally, Vaquero et al. (2008) analysed no less than 22 definitions of Cloud
Computing, all proposed in 2008. Based on that analysis, Vaquero et al. (2008)
propose the following definition which aims to reflect how Cloud Computing is
currently conceived:
“Clouds are a large pool of easily usable and accessible virtualized resources (such as
hardware, development platforms and/or services). These resources can be dynamically
reconfigured to adjust to a variable load (scale), allowing also for an optimum resource
utilization. This pool of resources is typically exploited by a pay-per-use model in which
guarantees are offered by the Infrastructure Provider by means of customized SLAs.”
Further, Vaquero et al. (2008) summarized
scalability
,
pay-per-use utility model
and
virtualization
as the feature set that would most closely resemble a minimum defini-
tion of Clouds. However, while the definition of Vaquero et al. (2008) summarizes
other definitions with respect to the physical layer very well, it does not stress the
integration of hardware with Software-as-a-Service in sufficient manner.
All definitions illustrate that Cloud Computing is a phenomenon that comprises
a number of aspects and is related to a new paradigm of IT (hardware and applica-
tions) delivery and deployment. Generally, Cloud Computing concerns the delivery
of IT capabilities to external customers, or, from the perspective of a user, obtaining
IT capabilities from an external provider, as a service in a pay-per-use manner and
over the Internet. Further, scalability and virtualization are very often seen as key
characteristics of Cloud Computing (e.g. Foster et al. 2008, Sun 2009a, Vaquero
et al. 2009). Scalability refers to a dynamic adjustment of provisioned IT resources
to variable load, e.g. increasing or decreasing number of users, required storage
capacity or processing power. Virtualization, which is also regarded as the corner-
stone technology for all Cloud architectures (e.g. Sun 2009), is mainly used for
abstraction and encapsulation (Foster et al. 2008). Abstraction allows unifying
