21. What is CloudStack? Free (as in speech, and beer) Infrastructure as a Service implementation that supports multiple hypervisors, complex network and firewall configuration, high-availability, and complex multi-tenant offerings.
49. Robust API CloudStack is replete with a robust RESTful API (output in XML or JSON) Allows external applications such as monitoring to perform actions such as spinning up new instances in response to load. Allows you to loosely couple external applications such as billing and PaaS to CloudStack.