how to read openstack code: service plugin

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　　We have learned core plugin, service plugin and extension in last post. Now let`s review:
　　Core Plugin
　　Core plugin manage core resources which are network, subnet, port and subnetpool.
　　Service Plugin
　　Service plugin manage higher services.
　　extension
　　Extensions are called API Extensions. There are three types of extension

• resource extension which define new resources
  
• action extension which define actions for resource
  
• request extension which can add more parameter to request
  

　　Normally a new feature will be implemented by extension first. When the feature is stable, the community will move it to official api and may implement it in plugin.
　　We write our core plugin in the previous post now we are going to write our service plugin.

What is the difference between core plugin and service plugin
　　Core plugin manage core resource in neutron. The code structure is different from service plugin. But the community are considering transfer core plugin into one kind of service plugin. You will see the trend in code

Design our service plugin
　　Our service plugin is called "ZOO". This plugin will manage some resource like "tiger". We are going to do API call like CREATE/UPDATE/DELETE/GET tiger with this service plugin.

Write our service plugin

　　Service plugin must be inherited from the>　　Below are my service plugin
　　

from neutron.services import service_base　　

　　

　　
class ZooPlugin(service_base.ServicePluginBase):
　　supported_extension_aliases = ["zoo"]
　　

　　def __init__(self):
　　super(ZooPlugin, self).__init__()
　　

　　def get_plugin_name(self):
　　return "ZOO"
　　

　　def get_plugin_type(self):
　　# should be under neutron/plugins/common/constants.py
　　return "ZOO"
　　

　　def get_plugin_description(self):
　　return ("ZOO")
　　

　　def create_tiger(self, context, tiger):
　　return "tiger created"
　　

　　def get_tigers(self, context, filters, fields):
　　return {}
　　

　　supported_extension_aliases is necessary since we need an extension to generate the resource.
　　We only support get_tigers and create_tiger here for simplicity purpose.
　　Because the plugin is third-party code, so we have to register it under certain entry point so neutron can load it. So our code structure will be like :
　　

[iyunv@liberty-controller01 tmp]# tree zooServicePlugin　　
zooServicePlugin/
　　
├── setup.py
　　
└── zoo
　　├── __init__.py
　　├── zoo_plugin.py
　　

　　The content of setup.py is like
　　

from setuptools import setup, find_packages　　

　　
setup(
　　name='zoo',
　　version='1.0',
　　

　　packages=find_packages(),
　　

　　entry_points={
　　'neutron.service_plugins': [
　　'zoo = zoo.zoo_plugin:ZooPlugin',
　　],
　　},
　　
)
　　

　　The key point here is to register the plugin under neutron.service_plugins namespace.

Write the extension
　　We have service plugin ready to manage the tiger resource. But we do not have the tiger resource yet. One option is to modify the neutron/api/v2/attribute.py which is not suggested. The recommended way is to generate the resource by extension like below
　　

from neutron.api import extensions　　
from neutron.api.v2 import base
　　
from neutron import manager
　　

　　

　　
EXT_PREFIX = '/zoo'
　　
RESOURCE_NAME = 'tiger'
　　
COLLECTION_NAME = '%ss' % RESOURCE_NAME
　　
RESOURCE_ATTRIBUTE_MAP = {
　　'tiger': {
　　'id': {'allow_post': False, 'allow_put': False,
　　'validate': {'type:uuid': None},
　　'is_visible': True,
　　'primary_key': True},
　　'name': {'allow_post': True,
　　'allow_put': False,
　　'is_visible': True,
　　'default': ''},
　　'tenant_id': {'allow_post': True, 'allow_put': False,
　　'required_by_policy': True,
　　'validate': {'type:string': None},
　　'is_visible': True}
　　}
　　
}
　　

　　

　　
class Zoo(extensions.ExtensionDescriptor):
　　@classmethod
　　def get_name(cls):
　　return "zoo"
　　

　　@classmethod
　　def get_alias(cls):
　　return 'zoo'
　　

　　@classmethod
　　def get_description(cls):
　　return "zoo"
　　

　　@classmethod
　　def get_updated(cls):
　　return "2017-02-08T10:00:00-00:00"
　　

　　@classmethod
　　def get_resources(cls):
　　# This method registers the URL and the dictionary  of
　　# attributes on the neutron-server.
　　exts = list()
　　plugin = manager.NeutronManager.get_service_plugins()['ZOO']
　　resource_name = RESOURCE_NAME
　　collection_name = COLLECTION_NAME
　　params = RESOURCE_ATTRIBUTE_MAP.get(resource_name)
　　controller = base.create_resource(collection_name, resource_name,
　　plugin, params, allow_bulk=False)
　　ex = extensions.ResourceExtension(collection_name, controller, path_prefix=EXT_PREFIX)
　　exts.append(ex)
　　return exts
　　

　　The RESOURCE_ATTRIBUTE_MAP is used for define resource tiger. The tenant_id attribute is necessary for auth.
　　An extension must inherited from extensions.ExtensionDescriptor
　　get_alias method is really important because plugin will use this value to find the extension. This value must be in the supported_extension_alias of plugin
　　The get_resources method is necessary for an extension who define new resource. We will see the detail in later post.

Config
　　Now we have our service plugin and extension we need to install our service plugin by python setup.py install and put the extension under neutron/extensions
　　Also config the /etc/neutron/neutron.conf
　　

service_plugins = ...,zoo　　

　　Restart your neutron server and run below API
　　

curl -g -i  "http://liberty-controller01:9696/v2.0/zoo/tigers" -H "Content-Type: application/json" -H "Accept: application/json" -H "X-Auth-Token:$token"　　
HTTP/1.1 200 OK
　　
Content-Type: application/json;
　　
Content-Length: 14
　　
X-Openstack-Request-Id: req-82ed8ccc-da9d-46d9-8fd9-beb01a24385b
　　
Date: Wed, 08 Feb 2017 11:09:13 GMT
　　

　　
{"tigers": []}
　　

　　It work