Nova is built on a shared-nothing, messaging-based architecture. All of the major nova components can be run on multiple servers. This means that most component to component communication must go via message queue. In order to avoid blocking each component while waiting for a response, we use deferred objects, with a callback that gets triggered when a response is received.
Nova recently moved to using a sql-based central database that is shared by all components in the system. The amount and depth of the data fits into a sql database quite well. For small deployments this seems like an optimal solution. For larger deployments, and especially if security is a concern, nova will be moving towards multiple data stores with some kind of aggregation system.
Below you will find a helpful explanation of the different components.
/- ( LDAP )
[ Auth Manager ] ---
| \- ( DB )
|
|
|
[ Web Dashboard ] -> [ api ] -- < AMQP > ------ [ network ] - ( Flat/Vlan )
| \
< HTTP > [ scheduler ] - [ compute ] - ( libvirt/xen )
| |
[ objectstore ] < - retrieves images