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[[File:Cloud computing.svg|400px|thumb|right|Cloud computing metaphor: For a user, the network elements representing the provider-rendered services are invisible, as if obscured by a cloud.]]

'''Cloud computing''' is a recently evolved computing terminology or metaphor based on [[utility]] and consumption of [[Computing platform|computing resources]]. Cloud computing involves deploying groups of remote servers and software [[Computer network|networks]] that allow centralized data storage and online access to computer services or resources. Clouds can be classified as public, private or [[hybrid cloud|hybrid]].<ref>{{cite journal|last1=Hassan|first1=Qusay|title=Demystifying Cloud Computing|journal=The Journal of Defense Software Engineering|date=2011|volume=2011|issue=Jan/Feb|pages=16–21|url=http://www.crosstalkonline.org/storage/issue-archives/2011/201101/201101-Hassan.pdf|accessdate=11 December 2014|publisher=CrossTalk}}</ref><ref name=nist/>

==Overview==
Cloud computing<ref>{{cite web|title=Know Why Cloud Computing Technology is the New Revolution|url=http://www.fonebell.in/cloud-computing-technology-new-revolution/|publisher=By Fonebell|accessdate=8 January 2015}}</ref> relies on sharing of resources to achieve coherence and [[economies of scale]], similar to a utility (like the [[Electrical grid|electricity grid]]) over a network.<ref name="nist">{{cite web|title=The NIST Definition of Cloud Computing |url=http://csrc.nist.gov/publications/nistpubs/800-145/SP800-145.pdf|publisher=National Institute of Standards and Technology|accessdate=24 July 2011}}</ref>
At the foundation of cloud computing is the broader concept of [[converged infrastructure]] and [[shared services]].

Cloud computing, or in simpler shorthand just "the cloud", also focuses on maximizing the effectiveness of the shared resources. Cloud resources are usually not only shared by multiple users but are also dynamically reallocated per demand. This can work for allocating resources to users. For example, a cloud computer facility that serves European users during European business hours with a specific application (e.g., email) may reallocate the same resources to serve North American users during North America's business hours with a different application (e.g., a web server). This approach should maximize the use of computing power thus reducing environmental damage as well since less power, air conditioning, rack space, etc. are required for a variety of functions. With cloud computing, multiple users can access a single server to retrieve and update their data without purchasing licenses for different applications.

The term "moving to cloud" also refers to an organization moving away from a traditional [[Capital expenditure|CAPEX]] model (buy the dedicated hardware and depreciate it over a period of time) to the [[Operating expense|OPEX]] model (use a shared cloud infrastructure and pay as one uses it).

Proponents claim that cloud computing allows companies to avoid upfront infrastructure costs, and focus on projects that differentiate their businesses instead of on infrastructure.<ref name="aws.amazon">{{cite web|url=http://aws.amazon.com/what-is-cloud-computing/|title=What is Cloud Computing? |work=Amazon Web Services |date=2013-03-19 |accessdate=2013-03-20}}</ref> Proponents also claim that cloud computing allows enterprises to get their applications up and running faster, with improved manageability and less maintenance, and enables IT to more rapidly adjust resources to meet fluctuating and unpredictable business demand.<ref name="aws.amazon" /><ref>{{cite web|url=http://it.tmcnet.com/channels/cloud-storage/articles/211183-rising-cloud-storage-market-opportunity-strengthens-vendors.htm |title=Baburajan, Rajani, "The Rising Cloud Storage Market Opportunity Strengthens Vendors," infoTECH, August 24, 2011 |publisher=It.tmcnet.com |date=2011-08-24 |accessdate=2011-12-02}}</ref><ref>{{cite web|url=http://www.thectoforum.com/content/converged-infrastructure-0|author=Oestreich, Ken, |title=Converged Infrastructure |work=CTO Forum |publisher=Thectoforum.com |date=2010-11-15 |accessdate=2011-12-02}}</ref> Cloud providers typically use a "pay as you go" model. This can lead to unexpectedly high charges if administrators do not adapt to the cloud pricing model.<ref>{{cite web|url=http://www.forbes.com/sites/centurylink/2014/02/27/wheres-the-rub-cloud-computings-hidden-costs/ |title=Where's The Rub: Cloud Computing's Hidden Costs|date=2014-02-27 |accessdate=2014-07-14}}</ref>

The present availability of high-capacity networks, low-cost computers and storage devices as well as the widespread adoption of [[hardware virtualization]], [[service-oriented architecture]], and [[Autonomic Computing|autonomic]] and utility computing have led to a growth in cloud computing.<ref>{{Cite news|url= http://www.economist.com/displaystory.cfm?story_id=14637206|title= Cloud Computing: Clash of the clouds|date= 2009-10-15 | work = The Economist|accessdate=2009-11-03}}</ref><ref name="gartner">{{cite web|url=http://www.gartner.com/it/page.jsp?id=707508 |title= Gartner Says Cloud Computing Will Be As Influential As E-business |publisher=Gartner |accessdate=2010-08-22}}</ref><ref name="really">{{cite web|url =http://www.infoworld.com/d/cloud-computing/what-cloud-computing-really-means-031 |title=What cloud computing really means|last=Gruman|first= Galen|date = 2008-04-07 | work= [[InfoWorld]]| accessdate= 2009-06-02}}</ref>

Cloud vendors are experiencing growth rates of 50% per annum.<ref>{{cite web|title=The economy is flat so why are financials Cloud vendors growing at more than 90 percent per annum?|url=http://www.fsn.co.uk/channel_outsourcing/the_economy_is_flat_so_why_are_financials_cloud_vendors_growing_at_more_than_90_percent_per_annum#.UbmtsPlJPGA/ |publisher=FSN|date=March 5, 2013}}</ref>

==History of cloud computing==

===Origin of the term===
The origin of the term ''cloud computing'' is unclear. The expression ''cloud'' is commonly used in science to describe a large agglomeration of objects that visually appear from a distance as a cloud and describes any set of things whose details are not inspected further in a given context.<ref>{{cite book|last1=Liu|first1=[edited by] Hongji Yang, Xiaodong|authorlink1=Understanding Cloud Computing|title=Software reuse in the emerging cloud computing era|date=2012|publisher=Information Science Reference|location=Hershey, PA|isbn=9781466608979|pages=204–227|url=http://www.igi-global.com/chapter/software-reuse-emerging-cloud-computing/65173|accessdate=11 December 2014|chapter=9}}</ref> Another explanation is that the old programs to draw network schematics surrounded the icons for servers with a circle, and a cluster of servers in a network diagram had several overlapping circles, which resembled a cloud.<ref>{{cite book |last1=Schmidt |first1=Eric |last2=Rosenberg |first2=Jonathan |date=2014 |title=How Google Works |url=http://www.howgoogleworks.net/ |publisher=Grand Central Publishing |page=11 |isbn=978-1-4555-6059-2}}</ref>

In analogy to above usage the word ''cloud'' was used as a metaphor for the Internet and a standardized cloud-like shape was used to denote a network on telephony schematics and later to depict the Internet in [[computer network diagram]]s. With this simplification, the implication is that the specifics of how the end points of a network are connected are not relevant for the purposes of understanding the diagram. The cloud symbol was used to represent the Internet as early as 1994,<ref>Figure 8, "A network 70 is shown schematically as a cloud", US Patent 5,485,455, column 17, line 22, filed Jan 28, 1994</ref><ref>Figure 1, "the cloud indicated at 49 in Fig. 1.", US Patent 5,790,548, column 5 line 56–57, filed April 18, 1996</ref> in which servers were then shown connected to, but external to, the cloud.

References to cloud computing in its modern sense appeared early as 1996, with the earliest known mention in a [[Compaq]] internal document.<ref>
{{Cite news | title=Who Coined 'Cloud Computing'? | author=Antonio Regalado | date= 31 October 2011 | work= Technology Review | publisher= MIT | url=http://www.technologyreview.com/news/425970/who-coined-cloud-computing/ | accessdate=31 July 2013 }}
</ref>

The popularization of the term can be traced to 2006 when Amazon.com introduced the Elastic Compute Cloud.<ref>{{cite web|url=https://aws.amazon.com/about-aws/whats-new/2006/08/24/announcing-amazon-elastic-compute-cloud-amazon-ec2---beta/
|title=Announcing Amazon Elastic Compute Cloud (Amazon EC2) - beta |publisher=Amazon.com |date=2006-08-24 |accessdate=2014-05-31}}</ref>

===The 1950s===
The underlying concept of cloud computing dates to the 1950s, when large-scale [[mainframe computer]]s were seen as the future of computing, and became available in academia and corporations, accessible via thin clients/[[computer terminal|terminal]] computers, often referred to as "[[Computer terminal#Dumb terminals|dumb terminals]]", because they were used for communications but had no internal processing capacities. To make more efficient use of costly mainframes, a practice evolved that allowed multiple users to share both the physical access to the computer from multiple terminals as well as the [[CPU]] time. This eliminated periods of inactivity on the mainframe and allowed for a greater return on the investment. The practice of sharing CPU time on a mainframe became known in the industry as [[time-sharing]].<ref name="StracheyTSO">{{cite journal|last=Strachey|first=Christopher|title=Time Sharing in Large Fast Computers|journal=Proceedings of the International Conference on Information processing, UNESCO|date=June 1959|series=paper B.2.19|pages=336–341}}</ref> During the mid 70s, time-sharing was popularly known as RJE ([[Remote Job Entry]]); this nomenclature was mostly associated with large vendors such as [[IBM]] and [[Digital Equipment Corporation|DEC]]. IBM developed the [[VM (operating system)|VM Operating System]] to provide time-sharing services.

===The 1990s===
In the 1990s, telecommunications companies, who previously offered primarily dedicated point-to-point data circuits, began offering [[virtual private network]] (VPN) services with comparable quality of service, but at a lower cost. By switching traffic as they saw fit to balance server use, they could use overall network bandwidth more effectively. They began to use the cloud symbol to denote the demarcation point between what the provider was responsible for and what users were responsible for. Cloud computing extends this boundary to cover all servers as well as the network infrastructure.<ref>{{cite web|url=http://mirror.switch.ch/ftp/doc/ietf/ipatm/atm-minutes-93jul.txt | publisher = Switch | location = [[Switzerland|CH]] |title=July, 1993 meeting report from the IP over ATM working group of the IETF | accessdate=2010-08-22}}</ref>

As computers became more prevalent, scientists and technologists explored ways to make large-scale computing power available to more users through time-sharing. They experimented with algorithms to optimize the infrastructure, platform, and applications to prioritize CPUs and increase efficiency for end users.<ref name="MITCorbato">{{cite web|last=Corbató|first=Fernando J.|title=An Experimental Time-Sharing System|url=http://larch-www.lcs.mit.edu:8001/~corbato/sjcc62/|work=SJCC Proceedings|publisher=MIT|accessdate=3 July 2012}}</ref>

Since 2000 cloud computing has come into existence. In early 2008, [[OpenNebula]], enhanced in the RESERVOIR European Commission-funded project, became the first open-source software for deploying private and hybrid clouds, and for the federation of clouds.<ref>{{cite journal|last=Rochwerger|first=B.|coauthors=Breitgand, D.; Levy, E.; Galis, A.; Nagin, K.; Llorente, I. M.; Montero, R.; Wolfsthal, Y.; Elmroth, E.; Caceres, J.; Ben-Yehuda, M.; Emmerich, W.; Galan, F.|title=The Reservoir model and architecture for open federated cloud computing|journal=IBM Journal of Research and Development|volume=53|issue=4|pages=4:1–4:11|doi=10.1147/JRD.2009.5429058}}</ref> In the same year, efforts were focused on providing [[quality of service]] guarantees (as required by real-time interactive applications) to cloud-based infrastructures, in the framework of the IRMOS European Commission-funded project, resulting in a real-time cloud environment.<ref>{{cite journal|first=D|last=Kyriazis|author2=A Menychtas |author3=G Kousiouris |author4=K Oberle |author5=T Voith |author6=M Boniface |author7=E Oliveros |author8=T Cucinotta |author9=S Berger |title=A Real-time Service Oriented Infrastructure|journal=International Conference on Real-Time and Embedded Systems (RTES 2010)|location=Singapore|date=November 2010}}</ref> By mid-2008, Gartner saw an opportunity for cloud computing "to shape the relationship among consumers of IT services, those who use IT services and those who sell them"<ref>[http://www.networkworld.com/newsletters/itlead/2008/070708itlead1.html Keep an eye on cloud computing], Amy Schurr, Network World, 2008-07-08, citing the [[Gartner]] report, "Cloud Computing Confusion Leads to Opportunity". Retrieved 2009-09-11.</ref> and observed that "organizations are switching from company-owned hardware and software assets to per-use service-based models" so that the "projected shift to computing&nbsp;... will result in dramatic growth in IT products in some areas and significant reductions in other areas."<ref>{{cite web|url=http://www.gartner.com/it/page.jsp?id=742913|title=Gartner Says Worldwide IT Spending On Pace to Surpass Trillion in 2008|author=Gartner|date=2008-08-18}}</ref>

[[Microsoft Azure]] became available in late 2008.

In July 2010, [[Rackspace|Rackspace Hosting]] and [[NASA]] jointly launched an open-source cloud-software initiative known as [[OpenStack]]. The OpenStack project intended to help organizations offer cloud-computing services running on standard hardware. The early code came from NASA's [[Nebula (computing platform)|Nebula platform]] as well as from [[Rackspace Cloud#Cloud Files|Rackspace's Cloud Files]] platform.<ref>{{cite web|title=OpenStack History|url=http://wiki.riteme.site/wiki/OpenStack#History}}</ref>

On March 1, 2011, IBM announced the [[IBM cloud computing#IBM SmartCloud|IBM SmartCloud]] framework to support [[Smarter Planet]].<ref>{{cite web|title=Launch of IBM Smarter Computing|url=https://www-304.ibm.com/connections/blogs/IBMSmarterSystems/date/201102?lang=en_us|accessdate=1 March 2011}}</ref> Among the various components of the [[Smarter Computing]] foundation, cloud computing is a critical piece.

On June 7, 2012, Oracle announced the [[Oracle Corporation#Services|Oracle Cloud]].<ref>{{cite web|title=Launch of Oracle Cloud|url=http://www.theregister.co.uk/2012/06/07/oracle_cloud_rehash_platinum_services/|accessdate=28 February 2014}}</ref> While aspects of the Oracle Cloud are still in development, this cloud offering is posed to be the first to provide users with access to an integrated set of IT solutions, including the Applications ([[SaaS]]), Platform ([[PaaS]]), and Infrastructure ([[IaaS]]) layers.<ref>{{cite web|title=Oracle Cloud, Enterprise-Grade Cloud Solutions: SaaS, PaaS, and IaaS|url=https://cloud.oracle.com/home|accessdate=12 October 2014}}</ref><ref>{{cite web|title=Larry Ellison Doesn't Get the Cloud: The Dumbest Idea of 2013 | publisher = Forbes.com |url=http://www.forbes.com/sites/oracle/2012/10/09/larry-ellison-doesnt-get-the-cloud-the-dumbest-idea-of-2013/|accessdate=12 October 2014}}</ref><ref>{{cite web|title=Oracle Disrupts Cloud Industry with End-to-End Approach | publisher = Forbes.com |url=http://www.forbes.com/sites/oracle/2012/10/03/oracle-disrupts-cloud-industry-with-end-to-end-approach/|accessdate=12 October 2014}}</ref>

==Similar concepts==
<!-- this need not enumerate every different type of computing, just the top half-dozen that are provably linked to cloud computing -->

Cloud computing is the result of evolution and adoption of existing technologies and paradigms. The goal of cloud computing is to allow users to take benefit from all of these technologies, without the need for deep knowledge about or expertise with each one of them. The cloud aims to cut costs, and helps the users focus on their core business instead of being impeded by IT obstacles.<ref name=HAM2012>{{cite book|last=HAMDAQA|first=Mohammad|title=Cloud Computing Uncovered: A Research Landscape|year=2012|publisher=Elsevier Press|isbn=0-12-396535-7|pages=41–85|url=http://www.stargroup.uwaterloo.ca/~mhamdaqa/publications/Cloud_Computing_Uncovered.pdf}}</ref>

The main enabling technology for cloud computing is [[virtualization]]. Virtualization software separates a physical computing device into one or more "virtual" devices, each of which can be easily used and managed to perform computing tasks. With [[operating system–level virtualization]] essentially creating a scalable system of multiple independent computing devices, idle computing resources can be allocated and used more efficiently. Virtualization provides the agility required to speed up IT operations, and reduces cost by increasing infrastructure [[utilization]]. Autonomic computing automates the process through which the user can provision resources [[Code on demand|on-demand]]. By minimizing user involvement, automation speeds up the process, reduces labor costs and reduces the possibility of human errors.<ref name=HAM2012/>

Users routinely face difficult business problems. Cloud computing adopts concepts from [[Service-oriented Architecture]] (SOA) that can help the user break these problems into [[Service (systems architecture)|services]] that can be integrated to provide a solution. Cloud computing provides all of its resources as services, and makes use of the well-established standards and best practices gained in the domain of SOA to allow global and easy access to cloud services in a standardized way.

Cloud computing also leverages concepts from utility computing to provide [[Performance metric|metric]]s for the services used. Such metrics are at the core of the public cloud pay-per-use models. In addition, measured services are an essential part of the feedback loop in autonomic computing, allowing services to scale on-demand and to perform automatic failure recovery.

Cloud computing is a kind of [[grid computing]]; it has evolved by addressing the QoS (quality of service) and [[reliability (computer networking)|reliability]] problems. Cloud computing provides the tools and technologies to build data/compute intensive parallel applications with much more affordable prices compared to traditional [[parallel computing]] techniques.<ref name=HAM2012/

Cloud computing shares characteristics with:
* [[Client–server model]] — ''Client–server computing'' refers broadly to any [[distributed application]] that distinguishes between service providers (servers) and service requestors (clients).<ref>{{cite web|url=http://java.sun.com/developer/Books/jdbc/ch07.pdf|publisher=Sun Microsystem|title=Distributed Application Architecture|accessdate=2009-06-16}}</ref>
* [[Grid computing]] — "A form of distributed and parallel computing, whereby a 'super and virtual computer' is composed of a [[Cluster (computing)|cluster]] of networked, [[Loose coupling|loosely coupled]] computers acting in concert to perform very large tasks."
* [[Mainframe computer]] — Powerful computers used mainly by large organizations for critical applications, typically bulk data processing such as: [[census]]; industry and consumer statistics; police and secret intelligence services; [[enterprise resource planning]]; and financial [[transaction processing]].
* [[Utility computing]] — The "packaging of [[Computational resource|computing resources]], such as computation and storage, as a metered service similar to a traditional public utility, such as electricity."<ref name="It's you've">{{cite web|url=http://portal.acm.org/citation.cfm?id=1496091.1496100&coll=&dl=ACM&CFID=21518680&CFTOKEN=18800807|title=It's probable that you've misunderstood 'Cloud Computing' until now | publisher= TechPluto |accessdate=2010-09-14}}</ref><ref>{{cite web|last=Danielson |first=Krissi |url=http://www.ebizq.net/blogs/saasweek/2008/03/distinguishing_cloud_computing/ |title=Distinguishing Cloud Computing from Utility Computing |publisher=Ebizq.net |date=2008-03-26 |accessdate=2010-08-22}}</ref>
* [[Peer-to-peer]] — A distributed architecture without the need for central coordination. Participants are both suppliers and consumers of resources (in contrast to the traditional client–server model).

==Characteristics==
<!-- Pros and Cons are discussed as 'key features' as each typically has both -->
<!-- Separate pros and cons lists are very confusing, with the same topics, e.g., security, listed in both -->
Cloud computing exhibits the following key characteristics:
* '''Agility''' improves with users' ability to re-provision technological infrastructure resources.
* '''[[Application programming interface]]''' (API) accessibility to software that enables machines to interact with cloud software in the same way that a traditional user interface (e.g., a computer desktop) facilitates interaction between humans and computers. Cloud computing systems typically use Representational State Transfer ([[representational state transfer|REST]])-based APIs.
* '''Cost''' reductions claimed by cloud providers. A public-cloud delivery model converts capital expenditure to [[operational expenditure]].<ref>{{cite web|url=http://www.cloudave.com/link/recession-is-good-for-cloud-computing-microsoft-agrees |title=Recession Is Good For Cloud Computing – Microsoft Agrees |publisher=CloudAve |accessdate=2010-08-22}}</ref> This purportedly lowers [[barriers to entry]], as infrastructure is typically provided by a third party and does not need to be purchased for one-time or infrequent intensive computing tasks. Pricing on a utility computing basis is fine-grained, with usage-based options and fewer IT skills are required for implementation (in-house).<ref name="idc">{{cite web|url=http://blogs.idc.com/ie/?p=190 |title=Defining 'Cloud Services' and "Cloud Computing" |publisher=IDC |date=2008-09-23 |accessdate=2010-08-22}}</ref> The e-FISCAL project's state-of-the-art repository<ref>{{cite web|url=http://www.efiscal.eu/state-of-the-art |title=e-FISCAL project state of the art repository}}</ref> contains several articles looking into cost aspects in more detail, most of them concluding that costs savings depend on the type of activities supported and the type of infrastructure available in-house.
* '''[[Device independence|Device and location independence]]'''<ref name="yarmis">{{cite web|last=Farber |first=Dan |url=http://news.cnet.com/8301-13953_3-9977049-80.html |title=The new geek chic: Data centers |publisher=[[CNET News]] |date=2008-06-25 |accessdate=2010-08-22}}</ref> enable users to access systems using a web browser regardless of their location or what device they use (e.g., PC, mobile phone). As infrastructure is off-site (typically provided by a third-party) and accessed via the Internet, users can connect from anywhere.<ref name="idc" />
* '''[[Software maintenance|Maintenance]]''' of cloud computing applications is easier, because they do not need to be installed on each user's computer and can be accessed from different places.
* '''[[Multitenancy]]''' enables sharing of resources and costs across a large pool of users thus allowing for:
** '''centralization''' of infrastructure in locations with lower costs (such as real estate, electricity, etc.)
** '''peak-load capacity''' increases (users need not engineer for highest possible load-levels)
** '''utilisation and efficiency''' improvements for systems that are often only 10–20% utilised.<ref name="amazon">{{Cite journal | url = http://www.businessweek.com/magazine/content/06_46/b4009001.htm | title = Jeff Bezos' Risky Bet | journal = Business Week}}</ref><ref>{{cite journal|last=He|first=Sijin|coauthors=L. Guo, Y. Guo, M. Ghanem, |title=Improving Resource Utilisation in the Cloud Environment Using Multivariate Probabilistic Models|url=http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6253553 |publisher=2012 2012 IEEE 5th International Conference on Cloud Computing (CLOUD) |doi=10.1109/CLOUD.2012.66|isbn=978-1-4673-2892-0|pages=574–581}}</ref>
* '''[[Computer performance|Performance]]''' is monitored, and consistent and loosely coupled architectures are constructed using [[web services]] as the system interface.<ref name="idc" /><ref>He, Qiang, et al. "Formulating Cost-Effective Monitoring Strategies for Service-based Systems." (2013): 1-1.</ref><ref name="Elsevier.com">A Self-adaptive hierarchical monitoring mechanism for Clouds [http://dx.doi.org/10.1016/j.jss.2011.11.1043 Elsevier.com]</ref>
* '''[[Productivity]]''' may be increased when multiple users can work on the same data simultaneously, rather than waiting for it to be saved and emailed. Time may be saved as information does not need to be re-entered when fields are matched, nor do users need to install application software upgrades to their computer.<ref name="Smith2013">{{cite book|author=Heather Smith|authorlink=Heather_Smith_(author)|title=Xero For Dummies|url=http://books.google.com/books?id=drOF19aBKfgC&pg=PT37|date=23 May 2013|publisher=John Wiley & Sons|isbn=978-1-118-57252-8|pages=37–}}</ref>
* '''Reliability''' improves with the use of multiple redundant sites, which makes well-designed cloud computing suitable for [[business continuity]] and [[disaster recovery]].<ref>{{cite web|last=King |first=Rachael |url=http://www.businessweek.com/technology/content/aug2008/tc2008083_619516.htm |title=Cloud Computing: Small Companies Take Flight |work=Bloomberg BusinessWeek |date=2008-08-04 |accessdate=2010-08-22}}</ref>
* '''Scalability and [[Elasticity (cloud computing)|elasticity]]''' via dynamic ("on-demand") [[provisioning]] of resources on a fine-grained, self-service basis in near real-time<ref name="vmstartuptime2012">{{cite journal|last=Mao|first=Ming|author2=M. Humphrey|title=A Performance Study on the VM Startup Time in the Cloud|journal=Proceedings of 2012 IEEE 5th International Conference on Cloud Computing (Cloud2012)|year=2012|url=http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6253534&isnumber=6253471|doi=10.1109/CLOUD.2012.103|isbn=978-1-4673-2892-0|page=423}}</ref><ref>Dario Bruneo, Salvatore Distefano, Francesco Longo, Antonio Puliafito, Marco Scarpa: Workload-Based Software Rejuvenation in Cloud Systems. IEEE Trans. Computers 62(6): 1072-1085 (2013)[http://doi.ieeecomputersociety.org/10.1109/TC.2013.30]</ref> (Note, the VM startup time varies by VM type, location, OS and cloud providers<ref name="vmstartuptime2012"/>), without users having to engineer for peak loads.<ref>{{cite web|title=Defining and Measuring Cloud Elasticity|url=http://digbib.ubka.uni-karlsruhe.de/volltexte/1000023476|publisher=KIT Software Quality Departement|accessdate=13 August 2011}}</ref><ref>{{cite web|title=Economies of Cloud Scale Infrastructure|url=https://www.youtube.com/watch?v=nfDsY3f4nVI|publisher=Cloud Slam 2011|accessdate=13 May 2011}}</ref><ref name="He 15–22">{{cite journal|last=He|first=Sijin|author2=L. Guo |author3=Y. Guo |author4=C. Wu |author5=M. Ghanem |author6=R. Han |title=Elastic Application Container: A Lightweight Approach for Cloud Resource Provisioning|url=http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6184989 |publisher=2012 IEEE 26th International Conference on Advanced Information Networking and Applications (AINA) |doi=10.1109/AINA.2012.74|isbn=978-1-4673-0714-7|pages=15–22}}</ref>
* '''[[Computer security|Security]]''' can improve 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 other traditional systems, in part because providers are able to devote resources to solving security issues that many customers cannot afford to tackle.<ref>{{cite web|last=Mills |first=Elinor |url=http://news.cnet.com/8301-1009_3-10150569-83.html |title=Cloud computing security forecast: Clear skies |publisher=CNET News |date=2009-01-27 |accessdate=2010-08-22}}</ref> However, the complexity of security is greatly increased when data is distributed over a wider area or over a greater number of devices, as well as in multi-tenant systems shared by unrelated users. In addition, user access to security [[audit log]]s may be difficult or impossible. Private cloud installations are in part motivated by users' desire to retain control over the infrastructure and avoid losing control of information security.

The [[National Institute of Standards and Technology]]'s definition of cloud computing identifies "five essential characteristics":{{quote|text=''On-demand self-service.'' A consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with each service provider.

''Broad network access.'' Capabilities are available over the network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g., mobile phones, tablets, laptops, and workstations).

''Resource pooling.'' The provider's computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to consumer demand.&nbsp;

''Rapid elasticity.'' Capabilities can be elastically provisioned and released, in some cases automatically, to scale rapidly outward and inward commensurate with demand. To the consumer, the capabilities available for provisioning often appear unlimited and can be appropriated in any quantity at any time.

''Measured service.'' Cloud systems automatically control and optimize resource use by leveraging a metering capability at some level of abstraction appropriate to the type of service (e.g., storage, processing, bandwidth, and active user accounts). Resource usage can be monitored, controlled, and reported, providing transparency for both the provider and consumer of the utilized service.|sign=National Institute of Standards and Technology<ref name="nist"/>}}

==Service models==
Cloud computing providers offer their services according to several fundamental models:<ref name="nist" /><ref name="Buyya">
{{cite book
|title=Cloud Computing: Principles and Paradigms
|editor=R. Buyya, J. Broberg, A.Goscinski
|date=February 2011
|publisher=Wiley Press
|location=New York, USA
|isbn=978-0-470-88799-8
|url=http://media.johnwiley.com.au/product_data/excerpt/90/04708879/0470887990-180.pdf
|chapter = Introduction to Cloud Computing
|first1 = William
|last1 = Voorsluys
|first2 = James
|last2 = Broberg
|first3 = Rajkumar
|last3 = Buyya
|pages=1–44
}}
</ref>
[[File:Cloud computing layers.png|right]]

===Infrastructure as a service (IaaS)===
{{See also|Category:Cloud infrastructure}}

In the most basic cloud-service model & according to the IETF (Internet Engineering Task Force), providers of IaaS offer computers – physical or (more often) virtual machines – and other resources. (A [[hypervisor]], such as [[Xen]], [[VirtualBox|Oracle VirtualBox]], [[Kernel-based Virtual Machine|KVM]], [[VMware ESX|VMware ESX/ESXi]], or [[Hyper-V]] runs the virtual machines as guests. Pools of hypervisors within the cloud operational support-system can support large numbers of virtual machines and the ability to scale services up and down according to customers' varying requirements.) IaaS clouds often offer additional resources such as a virtual-machine [[disk image]] library, raw [[block storage]], and file or [[object storage]], firewalls, load balancers, IP addresses, [[VLAN|virtual local area networks]] (VLANs), and software bundles.<ref name="DHAC">
{{cite book
|title='''Developing and Hosting Applications on the Cloud'''
|date=July 2012
|publisher=IBM Press
|isbn=978-0-13-306684-5
|url=http://www.ibmpressbooks.com/bookstore/product.asp?isbn=9780133066845
|chapter = ''Infrastructure as a Service Cloud Concepts''
|first1 = Alex
|last1 = Amies
|first2 = Harm
|last2 = Sluiman
|first3 = Qiang Guo
|last3 = Tong
|first4 = Guo Ning
|last4 = Liu
}}
</ref>
IaaS-cloud providers supply these resources on-demand from their large pools installed in [[data centers]]. For [[Wide area network|wide-area]] connectivity, customers can use either the Internet or [[carrier cloud]]s (dedicated virtual private networks).

To deploy their applications, cloud users install operating-system images and their application software on the cloud infrastructure. In this model, the cloud user patches and maintains the operating systems and the application software. Cloud providers typically bill IaaS services on a utility computing basis: cost reflects the amount of resources allocated and consumed.<ref>{{cite web|title=Amazon EC2 Pricing|url=http://aws.amazon.com/ec2/pricing/|accessdate=7 July 2014}}</ref><ref>{{cite web|title=Compute Engine Pricing|url=https://cloud.google.com/products/compute-engine/#pricing|accessdate=7 July 2014}}</ref><ref>{{cite web|title=Microsoft Azure Virtual Machines Pricing Details|url=http://azure.microsoft.com/en-us/pricing/details/virtual-machines/|accessdate=7 July 2014}}</ref>

===Platform as a service (PaaS)===
{{Main|Platform as a service}}
{{See also|Category:Cloud platforms}}

In the PaaS models, cloud providers deliver a [[computing platform]], typically including operating system, programming language execution environment, database, and web server. Application developers can develop and run their software solutions on a cloud platform without the cost and complexity of buying and managing the underlying hardware and software layers. With some PaaS offers like [[Microsoft Azure]] and [[Google App Engine]], the underlying computer and storage resources scale automatically to match application demand so that the cloud user does not have to allocate resources manually. The latter has also been proposed by an architecture aiming to facilitate real-time in cloud environments.<ref>{{Citation | author = Boniface, M. et al. | title = Platform-as-a-Service Architecture for Real-Time Quality of Service Management in Clouds | series = 5th International Conference on Internet and Web Applications and Services (ICIW) | year = 2010 | pages = 155–160 | place = Barcelona, Spain | publisher = IEEE | url = http://dx.doi.org/10.1109/ICIW.2010.91 | doi = 10.1109/ICIW.2010.91}}</ref> Even more specific application types can be provided via PaaS, e.g., such as media encoding as provided by services as bitcodin transcoding cloud<ref>[http://www.bitcodin.com bitcodin cloud transcoding platform]</ref> or media.io.<ref>[http://www.media.io/ media.io]</ref>

===Software as a service (SaaS)===
{{Main|Software as a service}}

In the [[business model]] using software as a service (SaaS), users are provided access to application software and databases. Cloud providers manage the infrastructure and platforms that run the applications. SaaS is sometimes referred to as "on-demand software" and is usually priced on a pay-per-use basis or using a subscription fee.

In the SaaS model, cloud providers install and operate application software in the cloud and cloud users access the software from cloud clients. Cloud users do not manage the cloud infrastructure and platform where the application runs. This eliminates the need to install and run the application on the cloud user's own computers, which simplifies maintenance and support. Cloud applications are different from other applications in their scalability—which can be achieved by cloning tasks onto multiple [[virtual machines]] at run-time to meet changing work demand.<ref name="hamdaqa">{{cite book
|title = A Reference Model for Developing Cloud Applications
|first = Mohammad
|last = Hamdaqa
|url = http://www.stargroup.uwaterloo.ca/~mhamdaqa/publications/A%20REFERENCEMODELFORDEVELOPINGCLOUD%20APPLICATIONS.pdf
}}</ref> [[Load balancer]]s distribute the work over the set of virtual machines. This process is transparent to the cloud user, who sees only a single access point. To accommodate a large number of cloud users, cloud applications can be ''[[multitenant]]'', that is, any machine serves more than one cloud user organization.

The pricing model for SaaS applications is typically a monthly or yearly flat fee per user,<ref name="Chou">
{{cite book
|title = Introduction to Cloud Computing: Business & Technology
|first = Timothy
|last = Chou
|url = http://www.scribd.com/doc/64699897/Introduction-to-Cloud-Computing-Business-and-Technology
}}
</ref> so price is scalable and adjustable if users are added or removed at any point.<ref>{{cite web|title=HVD: the cloud's silver lining|url=http://www.intrinsictechnology.co.uk/FileUploads/HVD_Whitepaper.pdf|publisher=Intrinsic Technology|accessdate=30 August 2012}}</ref>

Proponents claim SaaS allows a business the potential to reduce IT operational costs by outsourcing hardware and software maintenance and support to the cloud provider. This enables the business to reallocate IT operations costs away from hardware/software spending and personnel expenses, towards meeting other goals. In addition, with applications hosted centrally, updates can be released without the need for users to install new software. One drawback of SaaS is that the users' data are stored on the cloud provider's server. As a result, there could be unauthorized access to the data. For this reason, users are increasingly adopting intelligent third-party key management systems to help secure their data.

==Cloud clients==
{{See also|Category:Cloud clients}}

Users access cloud computing using networked client devices, such as [[desktop computers]], [[laptop]]s, [[tablet computer|tablet]]s and [[smartphones]]. Some of these devices – ''cloud clients'' – rely on cloud computing for all or a majority of their applications so as to be essentially useless without it. Examples are [[thin clients]] and the browser-based [[Chromebook]]. Many cloud applications do not require specific software on the client and instead use a web browser to interact with the cloud application. With [[Ajax (programming)|Ajax]] and [[HTML5]] these [[Web user interface]]s can achieve a similar, or even better, [[look and feel]] to native applications. Some cloud applications, however, support specific client software dedicated to these applications (e.g., [[desktop virtualization|virtual desktop]] clients and most email clients). Some legacy applications (line of business applications that until now have been prevalent in thin client computing) are delivered via a screen-sharing technology.

==Deployment models==
<!-- TODO: find source & add community cloud -->
[[File:Cloud computing types.svg|thumb|395px|right|Cloud computing types]]

===Private cloud===
Private cloud is cloud infrastructure operated solely for a single organization, whether managed internally or by a third-party, and hosted either internally or externally.<ref name="nist" /> Undertaking a private cloud project requires a significant level and degree of engagement to virtualize the business environment, and requires the organization to reevaluate decisions about existing resources. When done right, it can improve business, but every step in the project raises security issues that must be addressed to prevent serious vulnerabilities.<ref>{{cite web|url=http://cloudandcompute.com/private-cloud/private-cloud-more-secure/|title=Is The Private Cloud Really More Secure?|publisher=CloudAndCompute.com | accessdate=12 October 2014}}</ref> Self-run data centers<ref>{{cite web |url= http://www.govconnection.com/IPA/PM/Info/Cloud-Computing/Self-Run-Private-Cloud.htm |title=Self-Run Private Cloud Computing Solution - GovConnection |first= |last= |work=govconnection.com |year=2014 |accessdate=April 15, 2014}}</ref> are generally capital intensive. They have a significant physical footprint, requiring allocations of space, hardware, and environmental controls. These assets have to be refreshed periodically, resulting in additional capital expenditures. They have attracted criticism because users "still have to buy, build, and manage them" and thus do not benefit from less hands-on management,<ref name="iwpc">{{cite web|last=Foley |first=John |url=http://www.informationweek.com/news/services/business/showArticle.jhtml?articleID=209904474 |title=Private Clouds Take Shape |work=InformationWeek |accessdate=2010-08-22}}
</ref>
essentially "[lacking] the economic model that makes cloud computing such an intriguing concept".<ref>
{{cite web|last=Haff |first=Gordon |url=http://news.cnet.com/8301-13556_3-10150841-61.html |title=Just don't call them private clouds |publisher=CNET News |date=2009-01-27 |accessdate=2010-08-22}}</ref><ref>{{cite web|url=http://www.informationweek.com/cloud-computing/theres-no-such-thing-as-a-private-cloud/229207922 |title=There's No Such Thing As A Private Cloud |work=InformationWeek |date=2010-06-30 |accessdate=2010-08-22}}</ref>

===Public cloud===
A cloud is called a "public cloud" when the services are rendered over a network that is open for public use. Public cloud services may be free.<ref>{{cite web|first=Margaret|last=Rouse|title=What is public cloud?|accessdate=12 October 2014|url=http://searchcloudcomputing.techtarget.com/definition/public-cloud|publisher=Definition from Whatis.com}}</ref> Technically there may be little or no difference between public and private cloud architecture, however, security consideration may be substantially different for services (applications, storage, and other resources) that are made available by a service provider for a public audience and when communication is effected over a non-trusted network. Saasu is a large public cloud. Generally, public cloud service providers like Amazon AWS, Microsoft and Google own and operate the infrastructure at their [[data center]] and access is generally via the Internet. AWS and Microsoft also offer direct connect services called "AWS Direct Connect" and "Azure ExpressRoute" respectively, such connections require customers to purchase or lease a private connection to a peering point offered by the cloud provider.<ref name="idc"/>

===Hybrid cloud===
Hybrid cloud is a composition of two or more clouds (private, community or public) that remain distinct entities but are bound together, offering the benefits of multiple deployment models. Hybrid cloud can also mean the ability to connect collocation, managed and/or dedicated services with cloud resources.<ref name="nist" />

[[Gartner|Gartner, Inc.]] defines a hybrid cloud service as a cloud computing service that is composed of some combination of private, public and community cloud services, from different service providers.<ref>http://blogs.gartner.com/thomas_bittman/2012/09/24/mind-the-gap-here-comes-hybrid-cloud/</ref> A hybrid cloud service crosses isolation and provider boundaries so that it can’t be simply put in one category of private, public, or community cloud service. It allows one to extend either the capacity or the capability of a cloud service, by aggregation, integration or customization with another cloud service.

Varied use cases for hybrid cloud composition exist. For example, an organization may store sensitive client data in house on a private cloud application, but interconnect that application to a business intelligence application provided on a public cloud as a software service.<ref>{{cite web|url=http://www.cio.com/article/753708/Business_Intelligence_Takes_to_Cloud_for_Small_Businesses |title=Business Intelligence Takes to Cloud for Small Businesses |publisher=CIO.com |date=2014-06-04 |accessdate=2014-06-04}}</ref> This example of hybrid cloud extends the capabilities of the enterprise to deliver a specific business service through the addition of externally available public cloud services. Hybrid cloud adoption depends on a number of factors such as data security and compliance requirements, level of control needed over data, and the applications an organization uses.<ref>http://www.techradar.com/news/internet/cloud-services/hybrid-cloud-is-it-right-for-your-business--1261343</ref>

Another example of hybrid cloud is one where IT organizations use public cloud computing resources to meet temporary capacity needs that can not be met by the private cloud.<ref>Metzler, Jim; Taylor, Steve. (2010-08-23) "Cloud computing: Reality vs. fiction," Network World. [http://www.networkworld.com/newsletters/frame/2010/082310wan1.html]</ref> This capability enables hybrid clouds to employ cloud bursting for scaling across clouds.<ref name="nist" /> Cloud bursting is an application deployment model in which an application runs in a private cloud or data center and "bursts" to a public cloud when the demand for computing capacity increases. A primary advantage of cloud bursting and a hybrid cloud model is that an organization only pays for extra compute resources when they are needed.<ref>Rouse, Margaret. "Definition: Cloudbursting," May 2011. SearchCloudComputing.com. [http://searchcloudcomputing.techtarget.com/definition/cloud-bursting]</ref> Cloud bursting enables data centers to create an in-house IT infrastructure that supports average workloads, and use cloud resources from public or private clouds, during spikes in processing demands.<ref>Vizard, Michael. "How Cloudbursting 'Rightsizes' the Data Center", (2012-06-21). Slashdot. [http://slashdot.org/topic/datacenter/how-cloudbursting-rightsizes-the-data-center/]</ref>

=== Others===

====Community cloud====
[[Community cloud]] shares infrastructure between several organizations from a specific community with common concerns (security, compliance, jurisdiction, etc.), whether managed internally or by a third-party, and either hosted internally or externally. The costs are spread over fewer users than a public cloud (but more than a private cloud), so only some of the cost savings potential of cloud computing are realized.<ref name="nist" />

====Distributed cloud====
Cloud computing can also be provided by a distributed set of machines that are running at different locations, while still connected to a single network or hub service. Examples of this include distributed computing platforms such as [[BOINC]] and [[Folding@Home]].
An interesting attempt in such direction is Cloud@Home, aiming at implementing cloud computing provisioning model on top of voluntarily shared resources <ref>Vincenzo D. Cunsolo, Salvatore Distefano, Antonio Puliafito, Marco Scarpa: Volunteer Computing and Desktop Cloud: The Cloud@Home Paradigm. IEEE International Symposium on Network Computing and Applications, NCA 2009, pp 134-139 [http://doi.ieeecomputersociety.org/10.1109/NCA.2009.41]</ref>

====Intercloud====
{{Main|Intercloud}}
The [[Intercloud]]<ref>{{Cite journal|date=2009-05-24|title= Blueprint for the Intercloud – Protocols and Formats for Cloud Computing Interoperability|publisher=IEEE Computer Society|pages=328–336|url=http://www2.computer.org/portal/web/csdl/doi/10.1109/ICIW.2009.55|doi=10.1109/ICIW.2009.55|last1=Bernstein|first1=David|last2=Ludvigson|first2=Erik|last3=Sankar|first3=Krishna|last4=Diamond|first4=Steve|last5=Morrow|first5=Monique|isbn=978-1-4244-3851-8}}</ref> is an interconnected global "cloud of clouds"<ref name="kk">{{cite web|url=http://www.kk.org/thetechnium/archives/2007/11/a_cloudbook_for.php |title=Kevin Kelly: A Cloudbook for the Cloud |publisher=Kk.org |accessdate=2010-08-22}}</ref><ref>{{cite web|url=http://samj.net/2009/06/intercloud-is-global-cloud-of-clouds.html |title=Intercloud is a global cloud of clouds |publisher=Samj.net |date=2009-06-22 |accessdate=2010-08-22}}</ref> and an extension of the Internet "network of networks" on which it is based. The focus is on direct interoperability between public cloud service providers, more so than between providers and consumers (as is the case for hybrid- and multi-cloud).<ref>{{cite web|url=http://www.readwriteweb.com/archives/vint_cerf_despite_its_age_the.php?mtcCampaign=2765 |title=Vint Cerf: Despite Its Age, The Internet is Still Filled with Problems |publisher=Readwriteweb.com |accessdate=2010-08-22}}</ref><ref>{{cite web|url=http://blogs.cisco.com/sp/comments/from_india_to_intercloud/ |title=SP360: Service Provider: From India to Intercloud |publisher=Blogs.cisco.com |accessdate=2010-08-22}}</ref><ref>{{Cite news|author=Canada |url=http://www.theglobeandmail.com/servlet/story/LAC.20071129.TWLINKS29/TPStory/Business |title=Head iaaan the clouds? Welcome to the future |work=The Globe and Mail |date= 2007-11-29|accessdate=2010-08-22 |location=Toronto}}</ref>

====Multicloud====
{{Main|Multicloud}}
Multicloud is the use of multiple cloud computing services in a single heterogeneous architecture to reduce reliance on single vendors, increase flexibility through choice, mitigate against disasters, etc. It differs from hybrid cloud in that it refers to multiple cloud services, rather than multiple deployment modes (public, private, legacy).<ref name=rouse>{{cite web|last1=Rouse|first1=Margaret|title=What is a multi-cloud strategy|url=http://searchcloudapplications.techtarget.com/definition/multi-cloud-strategy|publisher=SearchCloudApplications|accessdate=3 July 2014}}</ref><ref name=king>{{cite web|last1=King|first1=Rachel|title=Pivotal's head of products: We're moving to a multi-cloud world|url=http://www.zdnet.com/pivotals-head-of-products-were-moving-to-a-multi-cloud-world-7000030737/|publisher=ZDnet|accessdate=3 July 2014}}</ref>

==Architecture<!--'Cloud architecture' redirects here-->==
[[File:CloudComputingSampleArchitecture.svg|thumb|325px|right|Cloud computing sample architecture]]

'''Cloud architecture''',<ref>{{cite web|url=http://developer.amazonwebservices.com/connect/entry.jspa?externalID=1632&categoryID=100 |title=Building GrepTheWeb in the Cloud, Part 1: Cloud Architectures |publisher=Developer.amazonwebservices.com |accessdate=2010-08-22}}</ref> the [[systems architecture]] of the [[software systems]] involved in the delivery of cloud computing, typically involves multiple ''cloud components'' communicating with each other over a loose coupling mechanism such as a messaging queue. Elastic provision implies intelligence in the use of tight or loose coupling as applied to mechanisms such as these and others.

===Cloud engineering===
'''[[Cloud engineering]]''' is the application of [[engineering]] disciplines to cloud computing. It brings a systematic approach to the high-level concerns of commercialization, standardization, and governance in conceiving, developing, operating and maintaining cloud computing systems. It is a multidisciplinary method encompassing contributions from diverse areas such as [[systems engineering|systems]], [[software engineering|software]], [[web engineering|web]], [[performance engineering|performance]], [[information engineering|information]], [[security engineering|security]], [[platform engineering|platform]], [[Risk analysis (engineering)|risk]], and [[Quality control|quality]] engineering.

==Security and privacy==
{{Main|Cloud computing issues}}

Cloud computing poses privacy concerns because the service provider can access the data that is on the cloud at any time. It could accidentally or deliberately alter or even delete information.<ref name="ryan">{{cite web|url= http://cacm.acm.org/magazines/2011/1/103200-cloud-computing-privacy-concerns-on-our-doorstep/fulltext |title= Cloud Computing Privacy Concerns on Our Doorstep }}</ref> Many cloud providers can share information with third parties if necessary for purposes of law and order even without a warrant. That is permitted in their privacy policies which users have to agree to before they start using cloud services.<ref>{{cite web|url=https://wiki.riteme.site/wiki/Cloud_computing_issues#Sharing_information_without_a_warrant |title=Sharing information without a warrant|accessdate=2014-12-05}}</ref> Solutions to privacy include policy and legislation as well as end users' choices for how data is stored.<ref name="ryan"/> Users can encrypt data that is processed or stored within the cloud to prevent unauthorized access.<ref name="ryan"/>

According to the [[Cloud Security Alliance]], the top three threats in the cloud are “Insecure Interfaces and API’s”, Data Loss & Leakage”, and “Hardware Failure” which accounted for 29%, 25% and 10% of all cloud security outages respectively - together these form shared technology vulnerabilities. In a cloud provider platform being shared by different users there may be a possibility that information belonging to different customers resides on same data server. Therefore Information leakage may arise by mistake when information for one customer is given to other.<ref>{{cite journal|last1=Chhibber|first1=A|title=SECURITY ANALYSIS OF CLOUD COMPUTING|journal=International Journal of Advanced Research in Engineering and Applied Sciences|date=2013|volume=2|issue=3|page=2278-6252|url=http://garph.co.uk/IJAREAS/Mar2013/6.pdf|accessdate=27 February 2015}}</ref> Additionally, [[Eugene Schultz]], chief technology officer at Emagined Security, said that hackers are spending substantial time and effort looking for ways to penetrate the cloud. "There are some real Achilles' heels in the cloud infrastructure that are making big holes for the bad guys to get into”. Because data from hundreds or thousands of companies can be stored on large cloud servers, hackers can theoretically gain control of huge stores of information through a single attack — a process he called "hyperjacking".

There is the problem of legal ownership of the data (If a user stores some data in the cloud, can the cloud provider profit from it?). Many Terms of Service agreements are silent on the question of ownership.<ref>{{cite news |url=http://articles.latimes.com/2012/apr/26/business/la-fi-tech-savvy-cloud-services-20120426 |title=Who owns your stuff in the cloud? |date=26 April 2012 |first=Michelle |last=Maltais |newspaper=Los Angeles Times |deadurl=no |accessdate=2012-12-14}}</ref>

Physical control of the computer equipment (private cloud) is more secure than having the equipment off site and under someone else's control (public cloud). This delivers great incentive to public cloud computing service providers to prioritize building and maintaining strong management of secure services.<ref>{{cite web|url=http://www.networkworld.com/news/2010/022210-virtualization-cloud-security-debate.html |title=Security of virtualization, cloud computing divides IT and security pros |publisher=Network World |date=2010-02-22 |accessdate=2010-08-22}}</ref> Some small businesses that don't have expertise in IT security could find that it's more secure for them to use a public cloud.

There is the risk that end users don't understand the issues involved when signing on to a cloud service (persons sometimes don't read the many pages of the terms of service agreement, and just click "Accept" without reading). This is important now that cloud computing is becoming popular and required for some services to work, for example for an [[intelligent personal assistant]] (Apple's [[Siri]] or [[Google Now]]).

Fundamentally private cloud is seen as more secure with higher levels of control for the owner, however public cloud is seen to be more flexible and requires less time and money investment from the user.<ref>{{cite web|title= The Bumpy Road to Private Clouds| url=http://www.computerworld.com/article/2549867/data-center/the-bumpy-road-to-private-clouds.html |accessdate=2014-10-08}}</ref>

==The future==
According to Gartner's [[Hype cycle]], cloud computing has reached a maturity that leads it into a productive phase. This means that most of the main issues with cloud computing have been addressed to a degree that clouds have become interesting for full commercial exploitation. This however does not mean that all the problems listed above have actually been solved, only that the according risks can be tolerated to a certain degree.<ref>http://blog.kaseya.com/blog/2014/10/06/realistic-look-cloud-computing/</ref> Cloud computing is therefore still as much a research topic, as it is a market offering.<ref name=ghc>{{cite web|last1=Smith|first1=David Mitchell|title=Hype Cycle for Cloud Computing, 2013|url=https://www.gartner.com/doc/2573318/hype-cycle-cloud-computing-|publisher=Gartner|accessdate=3 July 2014}}</ref> What is clear through the evolution of Cloud Computing services is that the CTO is a major driving force behind Cloud adoption.<ref>http://www.hello-cirro.co.uk/evolution-of-cloud-computing/</ref> The major Cloud technology developers continue to invest billions a year in Cloud R&D, in 2011 Microsoft for example committed 90% of its $9.6bn R&D budget to Cloud<ref>http://cloudtimes.org/2011/04/12/microsoft-says-to-spend-90-of-rd-on-cloud-strategy/</ref>

==See also==
{{Portal|Computer networking}}

{{Div col||25em}}
* [[:Category:Cloud computing providers]]
* [[:Category:Cloud platforms]]
* [[Cloud computing comparison]]
* [[Cloud management]]
* [[Cloud research]]
* [[Cloud storage]]
* [[Edge computing]]
* [[Fog computing]]
* [[Grid computing]]
* [[eScience]]
* [[iCloud]]
* [[Mobile cloud computing]]
* [[Personal cloud]]
* [[Robot as a Service]]
* [[Service-Oriented Architecture]]
* [[Synaptop]]
* [[Ubiquitous computing]]
* [[Web computing]]
{{Div col end}}

==References==
{{Reflist|30em}}

==External links==

{{Commons category|Cloud computing}}

{{Cloud computing}}

{{DEFAULTSORT:Cloud Computing}}
[[Category:Cloud computing| ]]
[[Category:Cloud infrastructure]]

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