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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 nova.service Module
nova.service
The nova.manager Module
nova.manager
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.