api-guide: add a doc on users

To help add color to the API doc, we use a common set of persona when
talking about the use cases behind each API.

This is really just a quick sketch that adds TODOs for others to follow
up on, in order to complete the guide.

blueprint complete-todo-in-api-concept-doc

Change-Id: I71a07caa27f1b17d3256a5439e867c40fc9d0abd

api-guide/source/index.rst

@@ -64,6 +64,7 @@ Contents
 .. toctree::
     :maxdepth: 2
 
+    users
     versions
     extensions
     microversions

api-guide/source/users.rst

@@ -0,0 +1,62 @@
+..
+      Copyright 2015 OpenStack Foundation
+
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+=====
+Users
+=====
+
+The Compute API includes all end user and administrator API calls.
+
+Role based access control
+=========================
+
+Keystone middleware is used to authenticate users and identify their roles.
+
+The Compute API uses these roles, along with oslo.policy, to decide
+what the user is authorized to do.
+
+TODO - link to compute admin guide for details.
+
+Personas used in this guide
+===========================
+
+While the policy can be configured in many ways, to make it easy to understand
+the most common use cases the API have been designed for, we should
+standardize on the following types of user:
+
+* application deployer: creates/deletes servers, directly or indirectly via API
+* application developer: creates images and applications that run on the cloud
+* cloud administrator: deploys, operates and maintains the cloud
+
+Now in reality the picture is much more complex. Specifically, there are
+likely to be different roles for observer, creator and administrator roles for
+the application developer. Similarly, there are likely to be various levels of
+cloud administrator permissions, such as a read-only role that is able to view
+a lists of servers for a specific tenant but is not able to perform any
+actions on any of them.
+
+Note: this is not attempting to be an exhaustive set of personas that consider
+various facets of the different users, but instead aims to be a minimal set of
+users, such that we use a consistent terminology throughout this document.
+
+TODO - could assign names to these users, or similar, to make it more "real".
+
+Discovering Policy
+==================
+
+An API to discover what actions you are authorized to perform is still a work
+in progress. Currently this reported by a HTTP 403 error.
+
+TODO - link to the doc on errors.