Joe Gordon a1e8fc6dd9 Update docs layout

* Explain these docs are for trunk (copied from ironic)
* All the docs in this repo are meant to be developer docs, so having a
  devref inside of the docs is redundant and just makes the docs more
  complicated to navigate. Move everything out of the devref folder and
  link to everything from main index.
* Move man pages into separate section. The man pages are pretty sparse
* right now, we should either make them useful or just delete them
* Remove dead docs from unused_docs list in doc/source/conf.py
* Shuffle docs landing page, move common referees to the top (API,
  hypervisor support matrix), Add a introduction section and more. The
  hope is the updated layout makes this document easier to navigate.
* Use maxdepth of 1
* Rename a few sections with what are hopefully better names

The next step is to prune out outdated documents and further cleanup
this page.

Change-Id: Iff453e47ccc902a0e72b1a5f6ce1ee939ff3a1a0

2015-05-15 10:47:18 -07:00

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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

Setup a Python package with a setup.py file.

Add the following to the setup.py setup call:

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

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):
        ....

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