The API Server provides an interface to the outside world to interact with the cloud infrastructure. API server is the only component that the outside world uses to manage the infrastructure. The management is done through web services calls using EC2 API. The API Server then, in turn, communicates with the relevant components of the cloud infrastructure through the Message Queue. Also there is an OpenStack API as well now, as an alternative to the EC2 API.
OpenStack ManualsOpenStack Compute Starter Guide - diablo
