Essential Characteristics of the Cloud

I need a total of 8 used and useful references.  Understand Quality references.

On-demand, broad network access, resource pooling, rapid elasticity, measure service, charge by usage (“pay as you go”) are the essential characteristics of the Cloud.  Do some research on the topic and provide four (4) substantive reference that addresses the subject. 

Describe essential characteristics to the cloud.  Do some research on the topic and provide four (4) substantive reference that supports this type of functionality.