I have already opined in the introduction that cloud computing has led to a ‘marketing fog’, and this is no better illustrated than with the multitude of definitions for the term. I considered a number of well-known definitions of cloud computing, but I have chosen the NIST (National Institute of Standards and Technology) draft definition as my preferred definition because it is publicly available, it reflects the IT market and it is relatively simple, in my opinion. Quoting from draft number 15 (Mell and Grance, 2009):
Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (eg networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction. This cloud model promotes availability and is composed of five essential characteristics, three service models, and four deployment models.
The NIST draft definition goes on to describe these five essential characteristics (on-demand self-service, broad network access, resource pooling, rapid elasticity and measured service), three service models (Software as a Service – SaaS, Platform as a Service – PaaS and Infrastructure as a Service – IaaS), and four deployment models (Private, Community, Public and Hybrid Cloud); but this chapter provides alternative descriptions. See Figure 1.2 for a visual representation of the NIST definition and visit http://csrc.nist.gov/groups/SNS/cloud-computing for the latest draft.
Now, some industry experts do not like the use of the SaaS, PaaS and IaaS acronyms, but they are so firmly embedded in the cloud computing literature that they cannot be ignored so I will continue to refer to them in this book. For an alternative viewpoint I refer you to ‘Above the Clouds’, the 2009 technical report from the University of Berkeley (Armbrust et al, 2009).