OpenStack is one of the widely used cloud computing platform. Here is guide written by Jeff Cogswell, published in linux.com that provides a good introduction to OpenStack API.
OpenStack is an open-source, standard cloud computing platform. Included with OpenStack is an API that you can use for writing software that manages a cloud. You can write scripts that run as jobs to automatically allocate servers, for example, or write applications that allow a user to manage their own cloud servers. The possibilities are endless. But in order to accomplish such tasks, your software needs to communicate with the OpenStack installation using the API. In this tutorial I get you up and running using the OpenStack API.There are two ways you can use OpenStack--API and SDK--and I've found that some people get a bit confused about this, so I'll try to clarify it. At the basic level, OpenStack services provide an API through which your software can communicate with the service.