Rackspace was one of the first players in the cloud arena. The company recognized early that enterprises wanted faster, simpler ways to spin up and spin down servers. If the bosses are going to be fickle and impulsive, there will always be a market for companies that make it easy for the people curating the data to pivot. If the corporate vision is going to morph, the IT shops will want a way to morph with it.
At Rackspace, the meaning of "cloud" has always been a bit simpler and more straightforward, and the philosophy a bit more open and pragmatic, than at other cloud providers. While some of the others spun elaborate metaphors, abstracted away the old files, and portrayed the opaqueness of their mechanism as a feature, Rackspace sold real instances that felt more like real computers. From the beginning, Rackspace's cloud was just a fast way to buy extra machines for an hour, then turn them off.
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Lately the company has been adding new products and features to create what it calls the Next Gen cloud. You can still access the First Gen cloud and use the original cloud software, but it won't offer all of the new features such as better data storage, public IPv6 support, and the ability to change a server's metadata.