Gartner defines Cloud computing as a style of computing in which scalable and elastic IT-enabled capabilities are delivered as a service to external customers using Internet technologies. This is a slight revision of Gartner's original definition published in 2008. Gartner has removed "massively scalable" and replaced it with "scalable and elastic" as an indicator that the important characteristic of scale is the ability to scale up and down, not just to massive size.
"We recognize that services may adhere to some attributes more effectively than others," said David Mitchell Smith, vice president and Gartner Fellow, in a statement. "The degree to which the service exhibits all these characteristics indicates how much it adheres to the Cloud computing model. One must examine a combination of these attributes to evaluate Cloud services. Focusing on one attribute in isolation is not recommended."
Service Based: Consumer concerns are abstracted from provider concerns through service interfaces that are well-defined. The interfaces hide the implementation details and enable a completely automated response by the provider of the service to the consumer of the service.
The service could be considered "ready to use" or "off the shelf" because the service is designed to serve the specific needs of a set of consumers, and the technologies are tailored to that need rather than the service being tailored to how the technology works. The articulation of the service feature is based on service levels and IT outcomes―availability, response time, performance vs. price, and clear and predefined operational processes―rather than technology and its capabilities.
In other words, what the service needs to do is more important than how the technologies are used to implement the solution.
Scalable and Elastic: The service can scale capacity up or down as the consumer demands at the speed of full automation (which may be seconds for some services and hours for others). Elasticity is a trait of shared pools of resources. Scalability is a feature of the underlying infrastructure and software platforms. Elasticity is associated with not only scale but also an economic model that enables scaling in both directions in an automated fashion. This means that services scale on demand to add or remove resources as needed.
Shared: Services share a pool of resources to build economies of scale. IT resources are used with maximum efficiency. The underlying infrastructure, software or platforms are shared among the consumers of the service (usually unknown to the consumers). This enables unused resources to serve multiple needs for multiple consumers, all working at the same time.
Metered by Use: Services are tracked with usage metrics to enable multiple payment models. The service provider has a usage accounting model for measuring the use of the services, which could then be used to create different pricing plans and models. These may include pay-as-you-go plans, subscriptions, fixed plans and even free plans. The implied payment plans will be based on usage, not on the cost of the equipment. These plans are based on the amount of the service used by the consumers, which may be in terms of hours, data transfers or other use-based attributes delivered.
Uses Internet Technologies: The service is delivered using Internet identifiers, formats and protocols, such as URLs, HTTP, IP and representational state transfer Web-oriented architecture. Many examples of Web technology exist as the foundation for Internet based services. Google's Gmail, Amazon.com's book buying, eBay's auctions and Lolcats' picture sharing all exhibit the use of Internet and Web technologies and protocols.
This document is included in the Gartner Special Report The What, Why, and When of Cloud Computing.