add concept doc for rbac (#571)

* add concept doc for rbac

* fix comments

docs/concepts/registry-access-control.md

@@ -0,0 +1,82 @@
+---
+layout: default
+title: Registry Access Control
+parent: Feathr Concepts
+---
+
+# Feathr Registry Access Control
+
+A project level role based access control (RBAC) plugin is available to help you manage who have access to **Feathr Registry**.
+
+It provides a simple authorization system built on OAuth tokens along with a SQL database as backend storage for `userrole` records.
+
+
+## How Registry Access Control works?
+
+With Registry Access Control, you can
+- Share your project metadata(read-only) by assigning `consumer` role.
+- Invite contributors to update features in this project by assigning `producer` role.
+- Transfer project admin by assigning `admin` role.
+
+For `admin`, `producer`, `consumer` explanation, please refer to [Role](#role) section. 
+
+### Scope
+
+_Scope_ is the set of resources that the permission applies to. In Feathr, you can specify a scope at 2 levels: **global**, **project**. Lower levels inherit role permissions from higher levels.
+
+```mermaid
+flowchart TD
+    A[Global] --> B[Project A];
+    A[Global] --> C[Project B];
+    A[Global] --> D[...Project N];
+```
+
+When you specify the scope in role assignment, please use `global` for global-level scope and `{project name}` for project-level scope.
+
+Feature level access control is **NOT** supported yet. Users are encouraged to group features with similar access control rules in one project.
+
+### Role
+
+A _role_ is a collection of permissions. We have 3 built-in roles with different permissions:
+| Role     | Description                | Permissions         |
+| -------- | -------------------------- | ------------------- |
+| Admin    | The owner of project       | Read, Write, Manage |
+| Producer | The contributor of project | Read, Write         |
+| Consumer | The reader of project      | Read                |
+
+### Permission
+_permission_ refers to the a certain kind of access to registry metadata or role assignment records.
+| Permission | Description                                               |
+| ---------- | --------------------------------------------------------- |
+| Read       | Read registry meta data; `GET` Registry APIs              |
+| Write      | Write registry meta data; `POST` Registry APIs            |
+| Manage     | Create and manage role assignment records with management APIs |
+
+### User
+A _user_ can be an email account or an [Azure AppId](https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app). 
+
+All Registry API requests requires `token` in header to identify the requestor and validate the permission.
+- Feathr UI uses the id token of login account. User credentials will be auto generated with [@azure/msal-browser](https://www.npmjs.com/package/@azure/msal-browser)
+- Feathr Client let users to pass their own credentials. 
+- In particular, Feathr samples get token with [DefaultAzureCredential()](https://docs.microsoft.com/en-us/python/api/azure-identity/azure.identity.defaultazurecredential?view=azure-python).
+
+Please make sure your token is valid when getting `500` or `401` Exceptions.
+
+_AAD Group_ is **NOT** supported yet. 
+
+### Role Assignment
+
+A _Role Assignment_ is the process of add a `user-role` mapping record into backend storage table.
+
+[Feature Registry](https://linkedin.github.io/feathr/concepts/feature-registry.html#access-control-management-page) section briefly introduced the access control management page, where project admins can manage role assignments.
+Management APIs are not exposed in Feathr Client by design. As we don't want to put control plane together with data plane. 
+
+
+## How to enable Registry Access Control?
+[Azure Resource Provisioning](https://linkedin.github.io/feathr/how-to-guides/azure-deployment-arm.html) section has detailed instructions on resource provisioning. For RBAC specific, you will need to manually:
+1. Choose `Yes` for `Enable RBAC` in ARM Template, and provision the resources.
+2. Create a `userrole` table in provisioned SQL database with [RBAC Schema](../../registry/access_control/scripts/schema.sql).
+3. Initialize the `userrole` table refer to commands in [test data](../../registry/access_control/scripts/test_data.sql).
+4. Login to the Web UI and navigate to the management page, and the roles you initialized in #3 should be in table. 
+
+For more details, please refer to the [Feathr Registry Access Control Gateway Specifications](../../../feathr/registry/access_control/README.md).