** Also affects: neutron
   Importance: Undecided
       Status: New

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  [RFE] Plugin support for API resource attribute extensions

Status in neutron:
Status in python-openstackclient:
Status in OpenStack SDK:

Bug description:
  The purpose of this bug is to facilitate a discussion. While I'm willing to 
do all the work to implement the feature request herein, I'd first like some 
agreement that:
  - We want to solve this problem.
  - Core(s) on the respective projects (python-openstackclient and 
python-openstacksdk) are willing to provide some guidance on the best 
high-level approach.

  Some OpenStack REST APIs such as neutron and nova support API extensions.
  While the implementation details for API extensions may differ from project 
to project, the basic extension support across all projects includes the 
ability for pluggable extensions to augment the REST API in the following ways:
  (a) Introducing new RESTful resources (APIs). These may be new top-level 
resources (e.g. /v1/theapi/{extension_resource}) or sub-resources (e.g. 
  (b) Adding new attributes (fields) to existing RESTful resources. For example 
neutron's net-mtu extension adds a 'mtu' attribute to networks [1] and nova's 
extended status adds attributes to servers [2].

  While some API extensions may exist in-tree (e.g. right in neutron or
  nova), they can also live in out-of-tree projects that implement

  Hopefully we can all agree that a CLI should encompass the means to
  support both #a and #b above in some form or another in order to
  obtain the pluggability our consumers expect with OpenStack.

  For sake of argument lets consider #a and #b as use cases from an OpenStack 
CLI (python-openstackclient and python-openstacksdk) perspective. Albeit we are 
talking from a contributor perspective here; but the ability to contribute 
drive features users can consume.

  (a) This case should already be supported via the existing OSC plugin
  model. For example [3].

  (b) This case is covered for stadium projects; they can add logic
  right into python-openstackclient[sdk] as needed. However for non-
  stadium projects, this case is not covered; today there's no easy way
  to extend existing OSC resources/actions/options/etc. in a reusable
  manner. For example, suppose a non-stadium neutron (plugin) project
  implements the network API, extends the network resource with
  additional attribute(s) and wants to provide CLI support for these
  attributes. Ideally they should be able to reuse the existing neutron
  network OSC logic in a pluggable manner such that they need not
  reimplement the OSC "actions" and associated logic.

  From a technical perspective, case #b roughly requires the ability for 
non-stadium projects (with such attribute extensions) to:
  (1) Add attributes to python-openstacksdk resource(s). This allows the 
extension attributes to be "collected" from an API response body.
  (2) Extend existing python-openstackclient commands by;
   a) Adding parser options if needed to support the attribute extension(s).
   b) Adding custom "take action" logic enabling the extension to process 
attribute extension options if needed.
   c) Displaying the attribute extension(s).

  In neutron alone, the following out-of-tree projects implement extensions 
falling under case #b:
  - vmware-nsx
  - gbpservice
  - quark
  - networking-cisco
  - networking-bigswitch
  - networking-h3c

  This support was provided in an ad-hoc fashion with the classic python
  clients (e.g. python-neutronclient) via it's ability to
  pass/handle/display arbitrary key/values. For example an API extension
  that adds a 'meta' boolean attribute to networks can be handled with
  the neutron client without any changes. The sample output snippets
  below show GET and POST (notice the 'meta' attribute that's added by
  the sample extension).

  stack@bvm2:~/devstack$ neutron --debug net-show my-meta-net
  | Field                     | Value                                |
  | admin_state_up            | True                                 |
  | availability_zone_hints   |                                      |
  | availability_zones        | nova                                 |
  | created_at                | 2017-07-21T17:31:23Z                 |
  | description               |                                      |
  | id                        | e824c797-1707-458c-9b71-8ee52f2c46c7 |
  | ipv4_address_scope        |                                      |
  | ipv6_address_scope        |                                      |
  | meta                      | True                                 |

  stack@bvm2:~/devstack$ neutron --debug net-create meta-net-2 --meta=false
  DEBUG: keystoneauth.session REQ: curl -g -i -X POST -H "User-Agent: python-neutronclient" 
-H "Content-Type: application/json" -H "Accept: application/json" -H 
"X-Auth-Token: {SHA1}4f63823e9b5041c7c2ac209a8068b2637ec717cf" -d '{"network": 
{"meta": "false", "name": "meta-net-2", "admin_state_up": true}}'

  Using the OSC, there's no way to achieve what was shown with the
  neutron client in the snippets above without updating python-
  openstacksdk's Network resource to add the attribute and then adding
  logic into neutron's network logic in the python-openstackclient.


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