OpenStack Compute consists of several main components. A "cloud controller" contains many of these components, and it represents the global state and interacts with all other components. An API Server acts as the web services front end for the cloud controller. The compute controller provides compute server resources and typically contains the compute service, The Object Store component optionally provides storage services. An auth manager provides authentication and authorization services when used with the Compute system, or you can use the Identity Service (keystone) as a separate authentication service. A volume controller provides fast and permanent block-level storage for the compute servers. A network controller provides virtual networks to enable compute servers to interact with each other and with the public network. A scheduler selects the most suitable compute controller to host an instance.
OpenStack Compute is built on a shared-nothing, messaging-based architecture. You can run all of the major components on multiple servers including a compute controller, volume controller, network controller, and object store (or image service). A cloud controller communicates with the internal object store via HTTP (Hyper Text Transfer Protocol), but it communicates with a scheduler, network controller, and volume controller via AMQP (Advanced Message Queue Protocol). To avoid blocking each component while waiting for a response, OpenStack Compute uses asynchronous calls, with a call-back that gets triggered when a response is received.
To achieve the shared-nothing property with multiple copies of the same component, OpenStack Compute keeps all the cloud system state in a database.
