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      Administrator of the National Aeronautics and Space Administration.
      All Rights Reserved.

      Licensed under the Apache License, Version 2.0 (the "License"); you may
      not use this file except in compliance with the License. You may obtain
      a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

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.. _service_manager_driver:

Services, Managers and Drivers
==============================

The responsibilities of Services, Managers, and Drivers, can be a bit confusing to people that are new to nova.  This document attempts to outline the division of responsibilities to make understanding the system a little bit easier.

Currently, Managers and Drivers are specified by flags and loaded using utils.load_object().  This method allows for them to be implemented as singletons, classes, modules or objects.  As long as the path specified by the flag leads to an object (or a callable that returns an object) that responds to getattr, it should work as a manager or driver.


The :mod:`nova.service` Module
------------------------------

.. automodule:: nova.service
    :noindex:
    :members:
    :undoc-members:
    :show-inheritance:


The :mod:`nova.manager` Module
------------------------------

.. automodule:: nova.manager
    :noindex:
    :members:
    :undoc-members:
    :show-inheritance:


Implementation-Specific Drivers
-------------------------------

A manager will generally load a driver for some of its tasks. The driver is responsible for specific implementation details.  Anything running shell commands on a host, or dealing with other non-python code should probably be happening in a driver.

Drivers should not touch the database as the database management is done inside ``nova-conductor``.

It usually makes sense to define an Abstract Base Class for the specific driver (i.e. VolumeDriver), to define the methods that a different driver would need to implement.
