e99653b60c
Hooks add the ability to insert custom code around operations that
declare a named hook:
e.g.
@hooks.add_hooks('create_instance')
def create_instance(....):
....
The above hook allows Hook objects to be run 'pre' and 'post' the
execution of create_instance()
Hook objects are discovered via the setuptools entry point group
'nova.hooks'.
Change-Id: I3961df12ef415085de7459438967edacc34500c2
58 lines
1.5 KiB
ReStructuredText
58 lines
1.5 KiB
ReStructuredText
Hooks
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=====
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Hooks provide a mechanism to extend Nova with custom code through a plugin
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mechanism.
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Named hooks are added to nova code via a decorator that will lazily load
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plugin code matching the name. The loading works via setuptools
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`entry points`_.
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.. _`entry points`: http://packages.python.org/distribute/pkg_resources.html#entry-points
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What are hooks good for?
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------------------------
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Hooks are good for anchoring your custom code to Nova internal APIs.
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What are hooks NOT good for?
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----------------------------
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Hooks should not be used when API stability is a key factor. Internal APIs may
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change. Consider using a notification driver if this is important to you.
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Declaring hooks in the Nova codebase
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------------------------------------
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The following example declares a *resize_hook* around the *resize_instance* method::
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from nova import hooks
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@hooks.add_hook("resize_hook")
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def resize_instance(self, context, instance, a=1, b=2):
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...
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Hook objects can now be attached via entry points to the *resize_hook*.
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Adding hook object code
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-----------------------
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1. Setup a Python package with a setup.py file.
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2. Add the following to the setup.py setup call::
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entry_points = [
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'nova.hooks': [
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'resize_hook': your_package.hooks.YourHookClass,
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]
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]
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3. *YourHookClass* should be an object with *pre* and/or *post* methods::
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class YourHookClass(object):
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def pre(self, *args, **kwargs):
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....
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def post(self, rv, *args, **kwargs):
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....
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