Hooks

Hooks provide a mechanism to extend Nova with custom code through a plugin mechanism.

Named hooks are added to nova code via a decorator that will lazily load plugin code matching the name. The loading works via setuptools entry points.

What are hooks good for?

Hooks are good for anchoring your custom code to Nova internal APIs.

What are hooks NOT good for?

Hooks should not be used when API stability is a key factor. Internal APIs may change. Consider using a notification driver if this is important to you.

Declaring hooks in the Nova codebase

The following example declares a resize_hook around the resize_instance method:

from nova import hooks

@hooks.add_hook("resize_hook")
def resize_instance(self, context, instance, a=1, b=2):
    ...

Hook objects can now be attached via entry points to the resize_hook.

Adding hook object code

1. Setup a Python package with a setup.py file.

2. Add the following to the setup.py setup call:

   entry_points = {
       'nova.hooks': [
           'resize_hook=your_package.hooks:YourHookClass',
       ]
   },
   

3. YourHookClass should be an object with pre and/or post methods:

   class YourHookClass(object):
   
       def pre(self, *args, **kwargs):
           ....
   
       def post(self, rv, *args, **kwargs):
           ....