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Nova System Architecture
========================

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.

Components
----------

Below you will find a helpful explanation of the different components.

.. image:: ./images/architecture.svg
   :width: 100%

* DB: sql database for data storage.
* API: component that receives HTTP requests, converts commands and communicates with other components via the **oslo.messaging** queue or HTTP
* Scheduler: decides which host gets each instance
* Network: manages ip forwarding, bridges, and vlans
* Compute: manages communication with hypervisor and virtual machines.
* Conductor: handles requests that need coordination(build/resize), acts as a
  database proxy, or handles object conversions.

While all services are designed to be horizontally scalable, you should have significantly more computes then anything else.
