Quota based on placement resource class

This is a spec about quota count based on placement resource
class.

specs/quota-based-on-placement-resource-class.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=============================================
+Quota Check Based on Placement Resource Class
+=============================================
+
+Include the URL of your launchpad blueprint:
+
+https://blueprints.launchpad.net/nova/+spec/example
+
+This spec proposes a way to count the quota based on the placement resource
+class.
+
+Problem description
+===================
+
+There are two problems need to be resolved for the current quota system in
+Nova:
+
+* Nova only supports very few built-in resources quota checks. But actually,
+  Nova isn't limited to manage just those few build-in resources. Currently,
+  Nova also supports GPU, PCI/SRIOV. With placement service, the user also can
+  define a custom resource, but also expected to be count by the quota system.
+  Just like the Baremetal users, the operator wants to limit the usages for a
+  specific kind of Baremetal node.
+
+Use Cases
+---------
+
+* The bare-metal operator wants to count the quota of specific kind of BM node.
+* The operator wants to count the quota for a custom resources which is
+  traced by the placement.
+
+Proposed change
+===============
+
+The proposal is going to:
+* Keep the existed built-in quota works as before.
+* Re-use the existed Quota REST API and DB table to store the quota limit.
+* Count the usages by the Placement API.
+* When booting the instance, the resource classes in the flavor extra spec
+  will be count by the quota system.
+
+In this proposal, there is nothing change for the existed build-in quotas, it
+will works as before and basically same code path.
+
+Reuse the existed `/os-quota-sets` API to query and update the quota limit,
+the API schema for `PUT /os-quota-sets/{project_id}?user_id={user_id}` will be
+allowed to specific a quota limit for a specific placement resource class.
+
+Using the Placement `GET /usages` API to implement the quota
+usage count.
+
+For a specific user and project's resource usage, which is traced by the
+placement, can be queried by::
+
+  GET /usages?project_id=<project_id>&user_id=<user_id>
+
+  Responses:
+
+  {
+    "usages": {
+        "CUSTOM_RC_BAREMETAL_GOLD": 2,
+        "DISK_GB": 2,
+        "MEMORY_MB": 256,
+        "VCPU": 2
+    }
+  }
+
+This API is already supported in Pike release with placement microversion 1.10.
+
+When booting an instance, if there is any resource class specified by the
+flavor extra spec, and there is quota limit for that resource class, then the
+quota limit will be checked for this resource class.
+
+Alternatives
+------------
+
+N/A
+
+Data model impact
+-----------------
+
+N/A
+
+REST API impact
+---------------
+
+This spec proposes to change the API schema of
+`PUT /os-quota-sets/{project_id}` to accept extra properties than the built-in
+quotas.
+
+To create/update a quota limit for a resource class, the request is as below::
+
+  PUT /os-quota-sets/{project_id}
+  
+  {
+     'CUSTOM_RC_BAREMETAL_GOLD': 10
+  }
+
+Then you can query the quota and the usages by the API `GET /quotas` and
+`GET /quotas/detail``.
+
+Security impact
+---------------
+
+N/A
+
+Notifications impact
+--------------------
+
+N/A
+
+Other end user impact
+---------------------
+
+N/A
+
+Performance Impact
+------------------
+
+When checking the quota for a resource class, there is REST API call to the
+placement API to get the usages. All the resource classes` usage can be
+done that single API call. The performance should be ok.
+
+Other deployer impact
+---------------------
+
+* The existed quotas still work as before, so there is nothing change.
+* To check quota for a resource class, the deployer needs to ensure the
+  flavor consume for that resource class, and there is quota limit in the
+  system for that resource class.
+
+Developer impact
+----------------
+
+N/A
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Who is leading the writing of the code? Or is this a blueprint where you're
+throwing it out there to see who picks it up?
+
+If more than one person is working on the implementation, please designate the
+primary author and contact.
+
+Primary assignee:
+  <launchpad-id or None>
+
+Other contributors:
+  <launchpad-id or None>
+
+Work Items
+----------
+
+* Add PlacementCountableResource to the quota system
+* Enable os-quota-sets API to accept resource class
+* Count the quota for the resource class in the flavor
+* Enable python-novacilent work with API change.
+
+Dependencies
+============
+
+N/A
+
+Testing
+=======
+
+Add related unit tests and functional tests.
+
+Documentation Impact
+====================
+
+N/A
+
+References
+==========
+
+N/A
+
+History
+=======
+
+Optional section intended to be used each time the spec is updated to describe
+new design, API or any database schema updated. Useful to let reader understand
+what's happened along the time.
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Pike
+     - Introduced