Architecture (Feature-Sliced Design)

Priority: P2 (MEDIUM)

Warning: FSD introduces boilerplate. Use it only if project expected to grow significantly (e.g., 20+ features). For smaller projects, simple module-based structure preferred.

Workflow: Create 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 feature widget in app/login/page.tsx (thin page).
4. Verify imports — Ensure no upward or cross-slice imports violate layer hierarchy.

Layer Hierarchy

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

See implementation examples for 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 Thin: app/ directory (App Router) 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: not preemptively create Entities.

• Rule: Start logic in Features or Pages. Move to Entities only when data/logic 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 specific directory layout and layer definitions, see reference documentation.

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

Architecture Checklist (Mandatory)

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

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

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

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

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

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

• Data Access (DAL): Place logic in model/ folder of 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 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.