OpenStack Cloud Controller communicates with other nova components such as Scheduler, Network Controller, and Volume Controller via AMQP(Advanced Message Queue Protocol). Nova uses asynchronous calls for request response, with a call-back that gets triggered once a response is received. Since asynchronous communication is used, none of the end points get stuck for long in a waiting state. This is especially true since many actions expected by the API calls such as launching an instance or uploading an image are time consuming.
OpenStack ManualsOpenStack Compute Starter Guide - cactus
