For some time now, the "cloud-o-sphere" has generally agreed on a basic classification scheme for cloud computing, typically called the SPI model. SPI stands for "SaaS, PaaS and IaaS", or Software-as-a-Service, Platform-as-a-Service and Infrastructure-as-a-Service, respectively.
A few weeks ago, Randy Bias (the CTO of cloud vendor GoGrid) wrote a great post differentiating two classes of Infrastructure-as-a-Service offerings: infrastructure Web services versus cloud centers. I but I've since come across another cloud model that leads me to believe there is a third category to consider.
I spent some time with Scott Roza, CEO of Skytap, and his product development manager. Skytap is a Seattle-based start-up targeting a "lab in the cloud" concept (my term, not theirs) in which enterprises can acquire resources for on-demand development, test, training, proof-of-concept and demonstration workloads. What differentiates their offering is a sophisticated front end, targeted at what lab managers need to control resource availability and costs, in front of what would normally be thought of as an Infrastructure-as-a-Service offering. If that sounds good to you, check out their Web site for more information.
Roza pointed out one interesting facet of his business: from a pricing perspective, they operate a lot like an SaaS company, with a monthly pricing scheme that covers a specific level of usage, but an "IaaS"-like pricing for usage above the base levels. At first I didn't understand why they would do this, but he pointed out that they provide an application--in this case, a lab management tool--with sophisticated features and specialized support. From the customer perspective, the service really is very SaaS-like.
However, it's also IaaS, as what is being sold behind that interface is servers and storage. The target market they are pursuing has just led them to begin to specialize the tools used to acquire that infrastructure.
So my theory: perhaps there are three categories of Infrastucture-as-a-Service:
Cloudcenters, or "pure" Infrastructure-as-a-Service: Offerings focused on raw resources for system administrator types.
Infrastructure Web Services, or "PaaS fronts IaaS": API-heavy infrastucture clouds targeted at those with development skills
Infrastructure Application Services, or "SaaS fronts IaaS": Application-fronted infrastructure clouds targeted at specific markets requiring specialized management and interface tools
Specialization in IaaS is blurring the lines within the SPI model. In general, the model remains highly useful, but its getting harder to firmly put vendors in one box or another. I've thought of Amazon as a PaaS/IaaS hybrid for some time now; Skytap is the first SaaS/IaaS hybrid I've heard of, but I am sure there are others in development or even on the market.
No doubt all this somewhat complicates taxonomy, so please, don't take the bullets above as a call for the industry to adopt those terms. However, there is a distinction that I find useful. Some of you may find it useful as well.