We've settled in on a few deployment models for cloud computing: private, public, community, and hybrid. Defined by the National Institute for Standards and Technology, these models have been adopted by many organizations in very different and more complex directions. Lately, the deployment action has been around the concept of hybrid clouds, or the mixing and matching of various cloud computing patterns in support of the architecture.
Hybrid clouds, as the name suggests, are a mixture of public, private, and/or community clouds for the target architecture. Most organizations adopting cloud computing will use a hybrid cloud model, considering that not all assets can be placed in public clouds (for security and control reasons), and many will opt for the value of both private and public together, sometimes adding community clouds to the mix as well.
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What's interesting here is not the concept of a having a private and a public cloud, but of a private and a public cloud working together. Here's a classic example: leveraging storage, database, and processing services within a private cloud, and from time to time leveraging a public cloud to handle spikes in processing requirements without having to purchase additional hardware for stand-by capacity. Many organizations are moving toward this type of "cloud-bursting" architecture for the value it brings to the bottom line.
Considering that public clouds charge you only for the capacity you use, cloud-bursting becomes a very cheap way to handle spikes in demand. Example application domains include retail, whose processing requirements go way up around the holidays -- or any business with a seasonal uptick.
However, hybrid clouds also provide a good foundation for other purposes beyond on-demand scalability, such as disaster recovery. This means your private cloud is using a public cloud as an on-demand failover platform, to be turned on when needed. That's a very cost-effective idea. Another good idea: When using public clouds as your platform of choice, opt for other public clouds as failover platforms.