I recently sat in on a cloud-101 type presentation by Dan Koffler and, in his presentation, he discussed the interrelationship between IaaS, PaaS, and SaaS. To sum it up, IaaS supports both PaaS and SaaS implementations, and PaaS supports SaaS implementations. This brings up an interesting point: does SaaS conform to the NIST standard definition of cloud computing? Here's an excerpt of the NIST definition:
The footnote at the end of that sentence reads:Software as a Service (SaaS). The capability provided to the consumer is to use the provider’s applications running on a cloud infrastructure 2.
And, as we all know, one of the essential characteristics is "Rapid elasticity". The fact that the upper service model(s) are served by the lower one(s) indicates that any true SaaS implementation would necessarily be elastic. This is the true test of whether a product is SaaS or an ASP (application service provider) implementation where the software is simply hosted on a server.2 A cloud infrastructure is the collection of hardware and software that enables the five essential characteristics of cloud computing.
So, the next time a vendor pitches you on a SaaS product, ask them this question: "Does the product's resources (i.e., compute power, RAM, storage) scale automatically as my organization reaches predetermined usage thresholds (e.g., % utilization of resources or number of users)? Or do I have to call you to increase these resources?" If the answer is "Yes," and "No," in that order, it's a true SaaS implementation. If the answer is "No," and "Yes," it's ASP and you should definitely ask what the SLA is on the vendor completing the request.
Whatever the answer, I assure you that the answer to this question will be telling. Whether the sales rep knows the answer or not will interesting in and of itself.