Document code design of approval rules

In this development document, major parts of the application being
used to implement the approval rules feature is explained in a
high level manner.

The goal is to help contributors understand the code design easier
and to also help those familiar with the codebase to see if there
are parts to improve.

This doesn't document the entire feature yet as it's missing other
parts specified in the TODO section of the document.

doc/development/README.md

@@ -100,6 +100,7 @@ Complementary reads:
 - [Renaming features](renaming_features.md)
 - [Windows Development on GCP](windows.md)
 - [Code Intelligence](code_intelligence/index.md)
+- [Approval Rules](approval_rules.md)
 
 ## Performance guides
 

doc/development/approval_rules.md

+# Approval Rules **(STARTER)**
+
+This document explains the backend design and flow of all related functionality
+about [merge request approval rules](../user/project/merge_requests/merge_request_approvals.md).
+
+This should help contributors to understand the code design easier and to also
+help see if there are parts to improve as the feature and its implementation
+evolves.
+
+It's intentional that it doesn't contain too much implementation detail as they
+can change often. The code should explain those things better. The components
+mentioned here are the major parts of the application for the approval rules
+feature to work.
+
+NOTE: **Note:**
+This is a living document and should be updated accordingly when parts
+of the codebase touched in this document changed/removed or when new components
+are added.
+
+## Data Model
+
+```mermaid
+erDiagram
+  Project ||--o{ MergeRequest: " "
+  Project ||--o{ ApprovalProjectRule: " "
+  ApprovalProjectRule }o--o{ User: " "
+  ApprovalProjectRule }o--o{ Group: " "
+  ApprovalProjectRule }o--o{ ProtectedBranch: " "
+  MergeRequest ||--|| ApprovalState: " "
+  ApprovalState ||--o{ ApprovalWrappedRule: " "
+  MergeRequest ||--o{ Approval: " "
+  MergeRequest ||--o{ ApprovalMergeRequestRule: " "
+  ApprovalMergeRequestRule }o--o{ User: " "
+  ApprovalMergeRequestRule }o--o{ Group: " "
+  ApprovalMergeRequestRule ||--o| ApprovalProjectRule: " "
+```
+
+### `Project` and `MergeRequest`
+
+`Project` and `MergeRequest` models are defined in `ee/app/models/ee/project.rb`
+and `ee/app/models/ee/merge_request.rb`. They extend the non-EE versions since
+approval rules is an EE only feature. Associations and other related stuff to
+merge request approvals are defined here.
+
+### `ApprovalState`
+
+```mermaid
+erDiagram
+  MergeRequest ||--|| ApprovalState: " "
+```
+
+`ApprovalState` class is defined in `ee/app/models/approval_state.rb`. It's not
+an actual `ActiveRecord` model. This class encapsulates all logic related to the
+state of the approvals for a certain merge request like:
+
+- Knowing the approval rules that are applicable to the merge request based on
+  its target branch.
+- Knowing the approval rules that are applicable to a certain target branch.
+- Checking if all rules were approved.
+- Checking if approval is required.
+- Knowing how many approvals were given or still required.
+
+It gets the approval rules data from the project (`ApprovalProjectRule`) or the
+merge request (`ApprovalMergeRequestRule`) and wrap it as `ApprovalWrappedRule`.
+
+### `ApprovalProjectRule`
+
+```mermaid
+erDiagram
+  Project ||--o{ ApprovalProjectRule: " "
+  ApprovalProjectRule }o--o{ User: " "
+  ApprovalProjectRule }o--o{ Group: " "
+  ApprovalProjectRule }o--o{ ProtectedBranch: " "
+```
+
+`ApprovalProjectRule` model is defined in `ee/app/models/approval_project_rule.rb`.
+
+A record is created/updated/deleted when an approval rule is added/edited/removed
+via project settings or the [project level approvals API](../api/merge_request_approvals.md#project-level-mr-approvals).
+The `ApprovalState` model get these records when approval rules are not
+overwritten.
+
+The `protected_branches` attribute is set and used when a rule is scoped to
+protected branches. See [Scoped to Protected Branch doc](../user/project/merge_requests/merge_request_approvals.md#scoped-to-protected-branch-premium)
+for more information about the feature.
+
+### `ApprovalMergeRequestRule`
+
+```mermaid
+erDiagram
+  MergeRequest ||--o{ ApprovalMergeRequestRule: " "
+  ApprovalMergeRequestRule }o--o{ User: " "
+  ApprovalMergeRequestRule }o--o{ Group: " "
+  ApprovalMergeRequestRule ||--o| ApprovalProjectRule: " "
+```
+
+`ApprovalMergeRequestRule` model is defined in `ee/app/models/approval_merge_request_rule.rb`.
+
+A record is created/updated/deleted when a rule is added/edited/removed via merge
+request create/edit form or the [merge request level approvals API](../api/merge_request_approvals.md#merge-request-level-mr-approvals).
+
+The `approval_project_rule` is set when it is based from an existing `ApprovalProjectRule`.
+
+An `ApprovalMergeRequestRule` doesn't have `protected_branches` as it inherits
+them from the `approval_project_rule` if not overridden.
+
+### `ApprovalWrappedRule`
+
+```mermaid
+erDiagram
+  ApprovalState ||--o{ ApprovalWrappedRule: " "
+```
+
+`ApprovalWrappedRule` is defined in `ee/app/modes/approval_wrapped_rule.rb` and
+is not an `ActiveRecord` model. It's used to wrap an `ApprovalProjectRule` or
+`ApprovalMergeRequestRule` for common interface. It also has the following sub
+types:
+
+- `ApprovalWrappedAnyApprovalRule` - for wrapping an `any_approver` rule.
+- `ApprovalWrappedCodeOwnerRule` - for wrapping a `code_owner` rule.
+
+This class delegates most of the responsibilities to the approval rule it wraps
+but it's also responsible for:
+
+- Checking if the approval rule is approved.
+- Knowing how many approvals were given or still required for the approval rule.
+
+It gets this information from the approval rule and the `Approval` records from
+the merge request.
+
+### `Approval`
+
+```mermaid
+erDiagram
+  MergeRequest ||--o{ Approval: " "
+```
+
+`Approval` model is defined in `ee/app/models/approval.rb`. This model is
+responsible for storing information about an approval made on a merge request.
+Whenever an approval is given/revoked, a record is created/deleted.
+
+## Controllers and Services
+
+The following controllers and services below are being utilized for the approval
+rules feature to work.
+
+### `API::ProjectApprovalSettings`
+
+This private API is defined in `ee/lib/api/project_approval_settings.rb`.
+
+This is used for the following:
+
+- Listing the approval rules in project settings.
+- Creating/updating/deleting rules in project settings.
+- Listing the approval rules on create merge request form.
+
+### `Projects::MergeRequests::CreationsController`
+
+This controller is defined in `app/controllers/projects/merge_requests/creations_controller.rb`.
+
+The `create` action of this controller is used when create merge request form is
+submitted. It accepts the `approval_rules_attributes` parameter for creating/updating/deleting
+`ApprovalMergeRequestRule` records. It passes the parameter along when it executes
+`MergeRequests::CreateService`.
+
+### `Projects::MergeRequestsController`
+
+This controller is defined in `app/controllers/projects/merge_requests_controller.rb`.
+
+The `update` action of this controller is used when edit merge request form is
+submitted. It's like `Projects::MergeRequests::CreationsController` but it executes
+`MergeRequests::UpdateService` instead.
+
+### `API::MergeRequestApprovals`
+
+This API is defined in `ee/lib/api/merge_request_approvals.rb`.
+
+The [Approvals API endpoint](../api/merge_request_approvals.md#get-configuration-1)
+is requested when merge request page loads.
+
+The `/projects/:id/merge_requests/:merge_request_iid/approval_settings` is a
+private API endpoint used for the following:
+
+- Listing the approval rules on edit merge request form.
+- Listing the approval rules on the merge request page.
+
+When approving/unapproving MR via UI and API, the [Approve Merge Request](../api/merge_request_approvals.md#approve-merge-request)
+API endpoint and the [Unapprove Merge Request](../api/merge_request_approvals.md#unapprove-merge-request)
+API endpoint are requested. They execute `MergeRequests::ApprovalService` and
+`MergeRequests::RemoveApprovalService` accordingly.
+
+### `API::ProjectApprovalRules` and `API::MergeRequestApprovalRules`
+
+These APIs are defined in `ee/lib/api/project_approval_rules.rb` and
+`ee/lib/api/merge_request_approval_rules.rb`.
+
+Used to list/create/update/delete project and merge request level rules via
+[Merge request approvals API](../api/merge_request_approvals.md).
+
+Executes `ApprovalRules::CreateService`, `ApprovalRules::UpdateService`,
+`ApprovalRules::ProjectRuleDestroyService`, and `ApprovalRules::MergeRequestRuleDestroyService`
+accordingly.
+
+### `ApprovalRules::ParamsFilteringService`
+
+This service is defined in `ee/app/services/approval_rules/params_filtering_service.rb`.
+
+It is called only when `MergeRequests::CreateService` and
+`MergeRequests::UpdateService` are executed.
+
+It is responsible for parsing `approval_rules_attributes` parameter to:
+
+- Remove it when user can't update approval rules.
+- Filter the user IDs whether they are members of the project or not.
+- Filter the group IDs whether they are visible to user.
+- Identify the `any_approver` rule.
+- Append hidden groups to it when specified.
+- Append user defined inapplicable (rules that does not apply to MR's target
+  branch) approval rules.
+
+## Flow
+
+These flowcharts should help explain the flow from the controllers down to the
+models for different functionalities.
+
+Some CRUD API endpoints are intentionally skipped because they are pretty
+straightforward.
+
+### Creating a merge request with approval rules via web UI
+
+```mermaid
+graph LR
+  Projects::MergeRequests::CreationsController --> MergeRequests::CreateService
+  MergeRequests::CreateService --> ApprovalRules::ParamsFilteringService
+  ApprovalRules::ParamsFilteringService --> MergeRequests::CreateService
+  MergeRequests::CreateService --> MergeRequest
+  MergeRequest --> db[(Database)]
+  MergeRequest --> User
+  MergeRequest --> Group
+  MergeRequest --> ApprovalProjectRule
+  User --> db[(Database)]
+  Group --> db[(Database)]
+  ApprovalProjectRule --> db[(Database)]
+```
+
+When updating, same flow is followed but it starts at `Projects::MergeRequestsController`
+and executes `MergeRequests::UpdateService` instead.
+
+### Viewing the merge request approval rules on an MR page
+
+```mermaid
+graph LR
+  API::MergeRequestApprovals --> MergeRequest
+  MergeRequest --> ApprovalState
+  ApprovalState --> id1{approval rules are overridden}
+  id1{approval rules are overridden} --> |No| ApprovalProjectRule & ApprovalMergeRequestRule
+  id1{approval rules are overridden} --> |Yes| ApprovalMergeRequestRule
+  ApprovalState --> ApprovalWrappedRule
+  ApprovalWrappedRule --> Approval
+```
+
+This flow gets initiated by the frontend component. The data returned will
+then be used to display information on the MR widget.
+
+### Approving a merge request
+
+```mermaid
+graph LR
+  API::MergeRequestApprovals --> MergeRequests::ApprovalService
+  MergeRequests::ApprovalService --> Approval
+  Approval --> db[(Database)]
+```
+
+When unapproving, same flow is followed but the `MergeRequests::RemoveApprovalService`
+is executed instead.
+
+## TODO
+
+1. Add information related to other rule types (e.g. `code_owner` and `report_approver`).
+1. Add information about side effects of approving/unapproving merge request.