Cloud computing is an emerging paradigm that aims at delivering hardware infrastructure and software applications as services, which users can consume on a pay per- use-basis. Cloud computing refers to the processing and storage of data through the Internet. Computing and storage become ‘services’ rather than physical resources. Files and other data can be stored in the cloud and be accessed from any Internet connection. It is a style of computing where IT-related capabilities are provided ‘as a service’, allowing users to access technology-enabled services from the Internet, or cloud, without knowledge of, expertise with, or control over the technology infrastructure that supports them. The cloud separate application and information resources from the infrastructure, and the mechanism used to deliver them. It’s a technology that uses the central remote severs and internet to main application and data, by allowing businesses and consumers to use applications without installation and also allows them to access their personal files at any computer with internet access. This paper present the framework for cloud computing.
Key Words: Software services, Platform service, Infrastructure service, Virtualization, cloud, SLA.
The interest towards Cloud computing solutions is rapid growing. As a result, they have already been adopted in different scenarios such as social networking, business applications, and content delivery networks. Cloud computing is the beginning of network based computing over the internet which is considered to be the element of two totally new computing models, the Client-Cloud computing and the Terminal-Cloud computing which would create whole generations of applications and business. It is also the beginning of a new Internet based service economy such as the Internet centric, Web based, on demand, Cloud applications and computing economy. A more structured definition is given by Buyya et al.  who define a Cloud as a “type of parallel and distributed system consisting of a collection of interconnected and virtualized computers that are dynamically provisioned and presented as one or more unified computing resources based on service-level agreement”. One of the key features characterizing Cloud computing is the ability of delivering both infrastructure and software as services.
Cloud computing is based on a very fundamental principal of `reusability of IT capabilities According to the IEEE Computer Society Cloud Computing is: “A paradigm in which information is permanently stored in servers on the Internet and cached temporarily on clients that include desktops, entertainment centres, table computers, notebooks, wall computers, handhelds, etc.
 The cloud computing draws on many existing technologies and architectures by integrating all these models, Centralizing computing power, utility computing, distributed computing and software as a service. This integration requires computing center of power to shift from processing unit to the network. Berkeley Report  released in Feb 2009 notes – “Cloud computing, the long-held dream of computing as a utility has the potential to transform a large part of the IT industry, making software even more attractive as a service”. Clouds aim to power the next generation data centers by architecting them as a network of virtual services (hardware, database, user-interface, application logic) so that users are able to access and deploy applications from anywhere in the world on demand at competitive costs depending on users Quality of Service (QoS) requirements .
Cloud Computing enhances collaboration, agility, scaling, and availability, and provides the potential for cost reduction through optimized and efficient computing.
More specifically, cloud describes the use of a collection of services, applications, information, and infrastructure comprised of pools of compute, network, information, and storage resources.
2. Cloud Computing Overview
Cloud computing is global and provides services to the mass, ranging from the end-users hosting their personal documents on the Internet to enterprises outsourcing their entire IT infrastructure to external data centers. Service Level Agreements (SLAs), which include QoS requirements, are set up between customers and Cloud providers. An SLA specifies the details of the service to be provided in terms of metrics agreed upon by all parties, and penalties for violating the expectations. SLAs act as a warranty for users, who can more comfortably move their business to the Cloud.The computing power in a Cloud computing environments is supplied by a collection of data centers, which are typically installed with hundreds to thousands of servers . Different solutions are available to move from the traditional science Grids and embrace the Cloud computing paradigm. Some vendors, such as Amazon Web Services and VMWare base their offering on hardware level virtualization and provide bare compute and storage resources on demand. Google AppEngine and Microsoft Azure are focused on application level virtualization. Other solutions provide end users with a platform for developing Cloud computing applications that can rely on, or compose, some of the existing solutions.
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Cloud computing focuses on delivery of reliable, secure, fault-tolerant, sustainable, and scalable infrastructures for hosting Internet-based application services, It can, to a certain extent, be regarded as the natural evolution of grid computing, considering that it was conceived to satisfy the new demands of users who, once accustomed to using the web 2.0 services, perceived the need to move much of their own data onto the web. Cloud computing customers do not generally own the physical infrastructure serving as host to the software platform in question. Instead, they avoid capital expenditure by renting usage from a third-party provider. They consume resources as a service and pay only for resources that they use. Many cloud-computing offerings employ the utility computing model, which is analogous to how traditional utility services (such as electricity) are consumed, while others bill on a subscription basis.
Cloud computing is the convergence of the major trends. -Virtualization – where applications are separated from infrastructure
-Utility computing where server capacity is
access across a grid as a vary price service.
-Software as a service – where applications are
available on demand basis.
Cloud computing can be the ability to rent a server or a thousand servers and run a geophysical modelling application on the most powerful systems available anywhere. It can be the ability to rent a virtual server, load software on it, turn it on and off at will, or clone it ten times to meet a sudden workload demand. It can be storing and securing immense amounts of data that is accessible only by authorized applications and users. It can be supported by a cloud provider that sets up a platform that includes the OS, Apache, a MySQLâ„¢ database, Perl, Python, and PHP with the ability to scale automatically in response to changing workloads. Cloud computing can be the ability to use applications on the Internet that store and protect data while providing a service – anything including email, sales force automation and tax preparation. It can be using a storage cloud to hold application, business, and personal data . Cloud computing users can avoid capital expenditure on hardware, software, and services when they pay a provider only for what they use. Consumption is usually billed on a utility (e.g. resources consumed, like electricity) or subscription (e.g. time based, like a newspaper) basis with little or no upfront cost. It also gives the power of flexibility and control to big business, delivering the heavy duty processing needed for a large network with many users and many different applications, while also giving the assurance of complete administrator control over security and access. Cloud computing can additionally further facilitate the ‘working from home’ revolution that we are already in the midst of. 
3. Cloud Computing service Models
Cloud service delivery is divided among three archetypal models and various derivative combinations. The three fundamental classifications are often referred to as the “SPI Model,” where ‘SPI’ refers to Software, Platform or Infrastructure (as a Service), respectively (Fig 1). Cloud computing delivers infrastructure, platform, and software (application) as services, which are made available as subscription-based services in a pay-as-yougo model to consumers. These services in industry are referred to as Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and Software as a Service (SaaS), respectively.
Figure 1. Cloud Computing service Models
Infrastructure as a Service (IaaS): Refer to the practice of delivering IT infrastructure based on virtual or physical resources as a commodity to customers. The capability provided to the consumer is to provision processing, storage, networks, and other fundamental computing resources where the consumer is able to deploy and run arbitrary software, which can include operating systems and applications. The consumer does not manage or control the underlying cloud infrastructure but has control over operating systems, storage, deployed applications, and possibly limited control of select networking components (e.g., host firewalls). Consumers are billed on a pay per use basis and have to set up their system on top of these resources that are hosted and managed in datacenters owned by the vendor. Amazon is one of the major players in providing IaaS solutions. Amazon Elastic Compute Cloud (EC2) provides a large computing infrastructure and a service based on hardware virtualization.
Platform as a Service (PaaS): The capability provided to the consumer is to deploy onto the cloud infrastructure consumer-created or acquired applications created using programming languages and tools supported by the provider. The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, or storage, but has control over the deployed applications and possibly application hosting environment configurations. It provides an application or development platform in which users can create their own application that will run on the Cloud.
PaaS implementations provide users with an application framework and a set of API that can be used by developers to program or compose applications for the Cloud. The two major players adopting this strategy are Google and Microsoft. It provides a set of APIs and an application model that allow developers to take advantage of additional services provided by Google such as Mail, Datastore, and others.
Software as a Service (SaaS): They provide end users with an integrated service comprising hardware, development platforms, and applications running on a cloud infrastructure. Users are not allowed to customize the service but get access to a specific application hosted in the Cloud. The applications are accessible from various client devices through a thin client interface such as a web browser (e.g., web-based email, services provided by Google for office automation, such as Google Document and Google Calendar, which are delivered for free to the Internet users and charged for professional quality services). The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, storage, or even individual application capabilities, with the possible exception of limited user specific application configuration settings
Cloud Deployment Models
There are four deployment models for cloud services, with derivative variations that address specific requirements:
Public Cloud: The cloud infrastructure is made available to the general public or a large industry group and is owned by an organization selling cloud services.
Private Cloud: The cloud infrastructure is operated solely for a single organization. It may be managed by the organization or a third party, and may exist on-premises or off premises.
Community Cloud: The cloud infrastructure is shared by several organizations and supports a specific community that has shared concerns (e.g., mission, security requirements, policy, or compliance considerations). It may be managed by the organizations or a third party and may exist on-premises or off-premises.
Hybrid Cloud: The cloud infrastructure is a composition of two or more clouds (private, community, or public) that remain unique entities but are bound together by standardized or proprietary technology that enables data and application portability (e.g., cloud bursting for load-balancing between clouds).
4. Advantages of Cloud Computing
It offers significant benefit to IT companies by freeing them from the low level tasks of setting up basic hardware (servers) and software infrastructures and thus enabling them to focus on innovation and creating business value for their services.
Cloud computing infrastructure allows enterprises to achieve more efficient use of their IT hardware and software investments: it increases profitability by improving resource utilization. Pooling resources into large clouds cuts costs and increases utilization by delivering resources only for as long as those resources are needed.
Cloud Computing is particularly beneficial for small and medium businesses, where effective and affordable IT tools are critical for helping them become more productive without spending a great deal of money on inhouse
Cloud computing can offer expediency, delivering instant IT network infrastructure to new users in any conceivable sector. Instead of using up considerable amounts of money and time establishing a brand new network, users can ‘plug-in’ to an existing cloud system and be up and running without delay. Infrastructure services address the problem of properly equipping data centers by assuring computing power when needed.
The time sharing style approach are low barriers to entry, shared infrastructure and costs, low management overhead, and immediate access to a broad range of applications. Users can generally terminate the contract at any time (thereby avoiding return on investment risk and uncertainty) and the services are often covered by service level agreements (SLAs) with financial penalties.
Agility improves with users able to rapidly and inexpensively re-provision technological infrastructure resources. The cost of overall computing is unchanged, however, and the providers will merely absorb up-front costs and spread costs over a longer period.
Reliability improves through the use of multiple redundant sites, which makes cloud computing suitable for business continuity and disaster recovery. Nonetheless, many major cloud computing services have suffered outages, and IT and business managers can at times do little when they are affected
Cloud computing offers extra level of security due to centralization of data, increased security-focused resources, etc., but concerns can persist about loss of control over certain sensitive data, and the lack of security for stored kernels. Security is often as good as or better than under traditional systems, in part because providers are able to devote resources to solving security issues that many customers cannot afford.
Cloud computing also offers additional security benefits. A single central mainframe only needs a centralised firewall and malware guard software application.
5. Cloud Computing Drawback
A side effect of this approach is that overall computer usage rises dramatically, as customers do not have to engineer for peak load limits. Additionally, “increased high-speed bandwidth” makes it possible to receive the same response times from centralized infrastructure at other sites.
Transparency; Entrusting mission critical applications and data to a third party means the customer has to know exactly how cloud providers handle key security and architectural issues. How transparent providers will be about those details remains an open question.
Problems currently impeding the growth of utility / cloud / grid computing include:
As an emerging technology, it takes time for service providers to get over the learning curve.
Service providers have been geared up for dedicated hosting for the last decade. Their infrastructure, sales channels, support systems, etc. are all built to focus on legacy systems. 
The overriding drawback was the lack of individuality for each user and the slower processing time created by having to house every application needed by the complete user group on one mainframe and then to transfer data back and forth.
Cloud Computing can be seen as a subset of grid computing as they share the same technologies and maintain the key concepts of the new distributed computing paradigm.
Cloud computing can offer expediency, delivering instant IT network infrastructure to new users in any conceivable sector. Instead of using up considerable amounts of money and time establishing a brand new network, users can ‘plug-in’ to an existing cloud system and be up and running without delay.
Cloud computing offers tremendous opportunities for SMBs and is likely to radically change the way they use the Internet in the coming years. By developing an overall adoption strategy, or simply just recognising the importance of the wider factors mentioned above, SMBs can reduce the potential risks and ensure they get the maximum possible benefit from their journey into the cloud.
Cloud computing will also have the potential to break geographical barriers to bring computing power to communities that previously did not have viable access. For example, through satellite broadband connections, remote third world locations can already gain access to first world mainframes. All they need is to be equipped with low cost basic laptop hardware.
In future more and more people will want to work from home in the coming years as commuting costs rise and companies will be happy to oblige to save on office rental. Cloud computing can solve the IT problems associated with home office setup, while also meeting the budget and security requirements of business.
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