Add Cascading Settings developer documentation

doc/development/README.md

@@ -207,6 +207,7 @@ In these cases, use the following workflow:
 - [Newlines style guide](newlines_styleguide.md)
 - [Image scaling guide](image_scaling.md)
 - [Export to CSV](export_csv.md)
+- [Cascading Settings](cascading_settings.md)
 
 ## Performance guides
 

doc/development/cascading_settings.md

+---
+stage: Manage
+group: Access
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Cascading Settings
+
+> Introduced in [GitLab 13.11](https://gitlab.com/gitlab-org/gitlab/-/issues/321724).
+
+The cascading settings framework allows groups to essentially inherit settings 
+values from ancestors (parent group on up the group hierarchy) and from 
+instance-level application settings. The framework also allows settings values
+to be enforced on groups lower in the hierarchy. 
+
+Cascading settings can currently only be defined within `NamespaceSetting`, though
+the framework may be extended to other objects in the future.
+
+## Add a new cascading setting
+
+Settings are not cascading by default. To define a cascading setting, take the following steps:
+
+1. In the `NamespaceSetting` model, define the new attribute using the `cascading_attr`
+   helper method. You can use an array to define multiple attributes on a single line.
+   
+    ```ruby
+    class NamespaceSetting
+      include CascadingNamespaceSettingAttribute
+   
+      cascading_attr :delayed_project_removal
+    end
+    ```
+
+1. Create the database columns.
+   
+   You can use the following database migration helper for a completely new setting. 
+   The helper creates four columns, two each in `namespace_settings` and 
+   `application_settings`.
+   
+    ```ruby
+    class AddDelayedProjectRemovalCascadingSetting < ActiveRecord::Migration[6.0]
+      include Gitlab::Database::MigrationHelpers::CascadingNamespaceSettings
+    
+      def up
+        add_cascading_namespace_setting :delayed_project_removal, :boolean, default: false, null: false
+      end
+   
+      def down
+       remove_cascading_namespace_setting :delayed_project_removal
+      end
+    end
+    ```
+   
+   Existing settings being converted to a cascading setting will require individual
+   migrations to add columns and change existing columns. Use the specifications 
+   below to create migrations as required: 
+   
+    1. Columns in `namespace_settings` table:
+        - `delayed_project_removal`: No default value. Null values allowed. Use any column type.
+        - `lock_delayed_project_removal`: Boolean column. Default value is false. Null values not allowed.
+    1. Columns in `application_settings` table:
+        - `delayed_project_removal`: Type matching for the column created in `namespace_settings`. 
+          Set default value as desired. Null values not allowed. 
+        - `lock_delayed_project_removal`: Boolean column. Default value is false. Null values not allowed. 
+        
+## Convenience methods
+
+By defining an attribute using the `cascading_attr` method, a number of convenience
+methods are automatically defined. 
+
+**Definition:**
+
+```ruby
+cascading_attr :delayed_project_removal
+```
+
+**Convenience Methods Available:**
+
+- `delayed_project_removal`
+- `delayed_project_removal=`
+- `delayed_project_removal_locked?`
+- `delayed_project_removal_locked_by_ancestor?`
+- `delayed_project_removal_locked_by_application_setting?`
+- `delayed_project_removal?` (Boolean attributes only)
+- `delayed_project_removal_locked_ancestor` - (Returns locked namespace settings object [namespace_id])
+
+The attribute reader method (`delayed_project_removal`) returns the correct
+cascaded value using the following criteria:
+
+1. Returns the dirty value, if the attribute has changed. This allows standard
+   Rails validators to be used on the attribute, though `nil` values *must* be allowed. 
+1. Return locked ancestor value.
+1. Return locked instance-level application settings value.
+1. Return this namespace's attribute, if not nil.
+1. Return value from nearest ancestor where value is not nil.
+1. Return instance-level application setting.