Private cloud. The cloud infrastructure is operated solely for an organization. It may be managed by the organization or a third party and may exist on premise or off premise.
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, and compliance considerations). It may be managed by the organizations or a third party and may exist on premise or off premise.
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.
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).
Public Clouds: In its simplest definition, a public cloud exists externally to its end user and is generally available with little restriction as to who may pay to use it. As a result, the most common forms of public clouds are ones that are accessed via the Internet. There has been tremendous development in the public cloud space, resulting in very sophisticated Infrastructure-as-a-Service offerings from companies like Amazon, with their Elastic Compute Cloud (EC2), Rackspace’s Cloud Offerings, and IBM’s BlueCloud. Other forms of public cloud offerings can take the form at more of the application layer, or Platform-as-a-Service, like Google’s AppEngine and Windows’ Azure Services platform, as well as Amazon’s service-specific cloud hosting SimpleDB, Cloud Front, and S3 Simple Storage.
Private Clouds: In contrast to a public cloud, a private cloud is internally hosted. The hallmark of a private cloud is that it is usually dedicated to an organization. Although there is no comingling of data or sharing of resources with external entities, different departments within the organization may have strong requirements to maintain data isolation within their shared private cloud. Organizations deploying private clouds often do so utilizing virtualization technology within their own data centers. A word of caution here: “Describing private cloud as releasing you from the constraints of public cloud only does damage to the cloud model. It’s the discipline in cloud implementations that makes them more interesting (and less costly) than conventional IT. Private clouds could very well be more constrained than their public counterparts and probably will be to meet those needs that public clouds cannot address.”
Community Clouds: The promise of community clouds is that they allow multiple independent entities to gain the cost benefits of a shared nonpublic cloud while avoiding security and regulatory concerns that might be associated with using a generic public cloud that did not address such concerns in its SLA. This model has tremendous potential for entities or companies that are subject to identical regulatory, compliance, or legal restrictions. Different kinds of community clouds are being considered in the United States and the European Union by governments at the national and local levels. This makes great sense since there are multiple benefits to both the individual entities as well as collectively. For instance, when multiple government agencies that transact business with each other have their processing colocated in a single facility, they can achieve both savings and increased security in terms of reducing the amount of traffic that would otherwise need to traverse the Internet. Continuity of operations can also be enhanced at a lower overall cost to all parties when multiple data centers are used to implement such a community cloud.
Hybrid Clouds: Hybrid clouds are just as the name implies. They are formed when an organization builds out a private cloud and wishes to leverage public or community clouds in conjunction with its private cloud for a particular purpose; the linking of the two clouds is what would be called a hybrid cloud. (Actually, a hybrid cloud could be formed by any combination of the three cloud types: public, private, and community.) Many organizations deploy an internal private cloud for their critical infrastructure but find certain needs that just aren’t economical to build out internally. A common example would be for testing or quality assurance purposes. For instance, an internal cloud might be used to run the infrastructure of a business, but the business may need to test an upgrade or roll out of a new system. It might be advantageous to pay for capacity of a public cloud for a few months to complete the testing, and when their own private cloud is upgraded, discontinue the public cloud usage.