Approval rules development guidelines

This document explains the backend design and flow of all related functionality about merge request approval rules.

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: This is a living document and should be updated accordingly when parts of the codebase touched in this document are changed or removed, or when new components are added.

Data Model

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, because approval rules are an EE-only feature. Associations and other related stuff to merge request approvals are defined here.

`ApprovalState`

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 wraps it as ApprovalWrappedRule.

`ApprovalProjectRule`

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. The ApprovalState model gets 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 Approvals for protected branches for more information about the feature.

`ApprovalMergeRequestRule`

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.

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`

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`

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 used 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 the 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 the 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 is requested when a 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 an MR via UI and API, the Approve Merge Request API endpoint or the 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.

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 the approval_rules_attributes parameter to:

Remove it when a 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 do not apply to the merge request'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

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, the 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

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 is used to display information on the MR widget.

Approving a merge request

graph LR
  API::MergeRequestApprovals --> MergeRequests::ApprovalService
  MergeRequests::ApprovalService --> Approval
  Approval --> db[(Database)]

When unapproving, the same flow is followed but the MergeRequests::RemoveApprovalService is executed instead.

TODO

Add information related to other rule types, such as code_owner and report_approver.
Add information about side effects of approving/unapproving a merge request.

Approval rules development guidelines

Data Model

Project and MergeRequest

ApprovalState

ApprovalProjectRule

ApprovalMergeRequestRule

ApprovalWrappedRule

Approval