A cloud deployment model defines where the physical servers are deployed and who manages them.
Public clouds, like Amazon Web Services, provide access to compute and storage resources for the general public over the Internet but the resources themselves are owned by the organization selling the cloud services. Cloud deployment in this case means customers self-provision resources, typically via a web service interface. Customers rent access to resources as needed on a pay-as-you-go basis, usually by the hour. Public clouds offer access to large pools of highly scalable resources on a temporary basis without the need for capital investment in data center infrastructure.
With public cloud deployment, infrastructure costs are shared across customers, which result in economies of scale. Data control might be an issue depending on a number of factors, including the type and sensitivity of the data as well as the industry and local laws concerning the data.
Private cloud deployment, like Eucalyptus, give users immediate access to computing resources hosted within an organization's infrastructure behind the firewall and the resources are dedicated solely for that organization's use. Users self-provision and scale collections of resources drawn from the private cloud, typically via web service interface, just as with a public cloud. However, because it is deployed within the organization's existing data center—and behind the organization's firewall—a private cloud is subject to the organization's physical, electronic, and procedural security measures and thus offers a higher degree of security, governance, and control over sensitive code and data.
With a private cloud deployment, performance of physical hardware can be controlled and tuned by the organization, and can thus markedly improve data center efficiency while reducing operational expense.
Hybrid cloud deployments combine one or more public clouds and one or more private clouds by technology that enables data and application migration between them. Hybrid clouds typically use a shared API to enable hybrid operation. Eucalyptus and AWS provide a ready-made hybrid cloud environment since Eucalyptus maintains high fidelity with AWS APIs, including EC2, S3, EBS, IAM, Auto Scaling, Elastic Load Balancing, and CloudWatch.
With a hybrid cloud deployment, companies can utilize the cost benefits of a public cloud and, when needed, protect confidential data in a private cloud.