Feature-Sliced Design (FSD)

The architectural pattern enforced by Apex AI Brain.

What is FSD?

Feature-Sliced Design is an architectural pattern that organizes code by features, not by technical layers.

Structure

force-app/main/default/classes/
├── leads/
│   ├── domain/        ← Domain model
│   ├── service/       ← Business logic
│   ├── selector/      ← Query layer
│   ├── handler/       ← Event handlers
│   └── test/          ← Tests
├── accounts/
│   ├── domain/
│   ├── service/
│   └── ...
└── shared/
    └── utilities/

Benefits

Clear Organization - Easy to find code Scalability - Add features independently
Testability - Isolated test coverage Collaboration - Teams work independently

Layer Responsibilities

Layer	Purpose	Example
Selector	Read data	LeadSelector.getById()
Domain	Business logic	LeadDomain.score()
Service	Public API	LeadService.scoreLeads()
Handler	Event response	LeadTriggerHandler
Test	Validation	LeadServiceTest

Dependencies

Valid dependency directions:

Handler → Service 
Service → Domain 
Service → Selector 
Domain → Selector 
Selector → nothing 

Handler → Selector ❌ (skip layer)
Domain → Service ❌ (backwards)

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See Also:

• Guardrails Guide
• Architecture Overview