Architecture (Feature-Sliced Design)

Priority: P2 (MEDIUM)

Adopt Feature-Sliced Design (FSD) for scalable applications. Warning: FSD introduces boilerplate. Use it only if the project is expected to grow significantly (e.g., 20+ features). For smaller projects, a simple module-based structure is preferred.

Workflow: Create a New Feature Slice

1. Create feature folder — src/features/auth/login/ with ui/, model/, api/ segments.
2. Add public API — Export via src/features/auth/login/index.ts.
3. Wire into page — Import the feature widget in app/login/page.tsx (thin page).
4. Verify imports — Ensure no upward or cross-slice imports violate the layer hierarchy.

Layer Hierarchy

App (app/) -> Widgets -> Features -> Entities -> Shared

See implementation examples for a thin page example.

Strategy

1. RSC Boundaries: Enforce strict serialization rules for props passed from Server to Client. See RSC Boundaries & Serialization.
2. App Layer is Thin: The app/ directory (App Router) is only for Routing.
   • Rule: page.tsx should only import Widgets/Features. No business logic (useEffect, fetch) directly in pages.
3. Slices over Types: Group code by Business Domain (User, Product, Cart), not by File Type (Components, Hooks, Utils).
   • Bad: src/components/LoginForm.tsx, src/hooks/useLogin.ts
   • Good: src/features/auth/login/ containing both.
4. Layer Hierarchy: Code can only import from layers below it.
   • App -> Widgets -> Features -> Entities -> Shared.
5. Avoid Excessive Entities: Do not preemptively create Entities.
   • Rule: Start logic in Features or Pages. Move to Entities only when data/logic is strictly reused across multiple differing features.
   • Rule: Simple CRUD belongs in shared/api, not entities.
6. Standard Segments: Use standard segment names within slices.
   • ui (Components), model (State/actions), api (Data fetching), lib (Helpers), config (Constants).
   • Avoid: components, hooks, services as segment names.

Structure Reference

For the specific directory layout and layer definitions, see the reference documentation.

• FSD Folder Structure
• Bundling & Compatibility
• Runtime Selection (Edge/Node)
• Debug Tricks & MCP

Architecture Checklist (Mandatory)

• Layer Imports: Does any layer import from a layer ABOVE it? (App > Widgets > Features > Entities > Shared)

• Page Logic: Is page.tsx thin, containing only Widgets/Features and zero useEffect/fetch?

• RSC Boundaries: Are Server Components isolated from Client Components with proper 'use client' boundaries?

• Public API: Is all access to a slice performed via the top-level index.ts (public API)?

• Cross-Slice: Do slices within the same layer (e.g., two features) import from each other directly? (Prohibited)

• Server Actions: Place them in the model/ folder of a Feature (e.g., features/auth/model/actions.ts).

• Data Access (DAL): Place logic in the model/ folder of an Entity (e.g., entities/user/model/dal.ts).

• UI Components: Base UI (shadcn) belongs in shared/ui. Feature-specific UI belongs in features/*/ui.

Anti-Patterns

• No cross-slice imports: Slices in the same layer must not import from each other directly.
• No business logic in page.tsx: Pages import Widgets/Features only; zero useEffect/fetch.
• No file-type folders: Group by domain (features/auth/), not type (components/, hooks/).
• No premature Entity creation: Start in Features; move to Entities only on strict reuse.