Feature-Sliced Design (FSD) Architecture

The frontend uses Feature-Sliced Design. This skill documentation helps you understand the architecture and correctly create new features.

Layer Hierarchy (STRICT)

app/        → widgets, features, entities, shared
widgets/    → features, entities, shared
features/   → entities, shared
entities/   → shared
shared/     → (only external libs)

Import direction: ONLY downward!

Project Structure

frontend/src/
├── app/                        # Next.js App Router (outside FSD)
│
├── widgets/                    # Composite UI Blocks
│   └── header/
│       ├── ui/
│       │   ├── header.tsx
│       │   └── mode-toggle.tsx
│       └── index.ts            # Public API
│
├── features/                   # User Interactions / Use Cases
│   ├── auth/
│   │   ├── ui/
│   │   │   ├── login-form.tsx
│   │   │   └── register-form.tsx
│   │   ├── model/
│   │   │   └── use-auth-sync.ts
│   │   └── index.ts
│   └── stats/
│       ├── ui/
│       │   └── stats-grid.tsx
│       ├── model/
│       │   └── use-sse.ts
│       └── index.ts
│
├── entities/                   # Business Objects
│   └── user/
│       ├── ui/
│       │   └── user-info.tsx
│       ├── model/
│       │   └── types.ts
│       └── index.ts
│
└── shared/                     # Reusable Code
    ├── ui/                     # shadcn/ui components
    ├── api/                    # Orval-generated
    │   ├── endpoints/
    │   ├── models/
    │   └── custom-fetch.ts
    ├── lib/
    │   ├── auth-client/        # Client-safe Auth
    │   ├── auth-server/        # Server-only Auth
    │   ├── query-client.ts
    │   └── utils.ts
    └── config/
        ├── providers.tsx
        └── theme-provider.tsx

TypeScript Path Aliases

// Always use layer aliases:
import { Button } from "@shared/ui/button"
import { useAuthSync } from "@features/auth"
import { SessionUser } from "@entities/user"
import { Header } from "@widgets/header"

Slice Segments

Each slice can have these segments:

Segment	Purpose	Example
ui/	React Components	login-form.tsx
model/	Hooks, State, Types	use-auth-sync.ts
api/	API Calls (rare, mostly in shared)	user-api.ts
lib/	Utilities for the slice	validation.ts

Public API Pattern (IMPORTANT!)

Every slice MUST have an index.ts:

// features/auth/index.ts
export { LoginForm } from "./ui/login-form"
export { RegisterForm } from "./ui/register-form"
export { useAuthSync, broadcastSignOut } from "./model/use-auth-sync"

Always import via index.ts:

// ✅ Correct
import { LoginForm } from "@features/auth"

// ❌ Wrong (Public API Sidestep)
import { LoginForm } from "@features/auth/ui/login-form"

Creating a New Feature

1. Create folder structure

mkdir -p src/features/<name>/ui
mkdir -p src/features/<name>/model  # if hooks/state needed

2. Create UI Component

// src/features/<name>/ui/<name>-form.tsx
"use client"

import { Button } from "@shared/ui/button"
import { Card } from "@shared/ui/card"

export function NameForm() {
  return (
    <Card>
      <Button>Action</Button>
    </Card>
  )
}

3. Create Model/Hook (optional)

// src/features/<name>/model/use-<name>.ts
"use client"

import { useQueryClient } from "@tanstack/react-query"
import { useCallback } from "react"

export function useName() {
  const queryClient = useQueryClient()
  // ...
  return { /* ... */ }
}

4. Create Public API

// src/features/<name>/index.ts
export { NameForm } from "./ui/name-form"
export { useName } from "./model/use-name"

5. Use in App

// app/(protected)/page.tsx
import { NameForm } from "@features/<name>"

export default function Page() {
  return <NameForm />
}

Creating a New Entity

// src/entities/<name>/model/types.ts
export interface Product {
  id: string
  name: string
  price: number
}

// src/entities/<name>/ui/product-card.tsx
"use client"

import type { Product } from "../model/types"
import { Card } from "@shared/ui/card"

export function ProductCard({ product }: { product: Product }) {
  return <Card>{product.name}</Card>
}

// src/entities/<name>/index.ts
export type { Product } from "./model/types"
export { ProductCard } from "./ui/product-card"

Import Rules

// ✅ ALLOWED
// In app/:
import { Header } from "@widgets/header"
import { LoginForm } from "@features/auth"
import { SessionUser } from "@entities/user"
import { Button } from "@shared/ui/button"

// In widgets/:
import { useAuthSync } from "@features/auth"
import { SessionUser } from "@entities/user"
import { Button } from "@shared/ui/button"

// In features/:
import { SessionUser } from "@entities/user"
import { Button } from "@shared/ui/button"

// In entities/:
import { Button } from "@shared/ui/button"

// ❌ FORBIDDEN
// In shared/ NEVER import features/!
// In entities/ NEVER import features/!
// In features/ NEVER import other features/!

Steiger Linting

FSD rules are automatically checked:

# Integrated in lint
bun run lint

# Only Steiger
bunx steiger src

Common Steiger Errors

Error	Cause	Solution
no-public-api-sidestep	Direct import instead of via index.ts	Import via index.ts
insignificant-slice	Slice has only 1 reference	Use more or move to widget/higher layer
forbidden-imports	Import from higher layer	Fix import direction

Server vs Client Components

// Server Component (no "use client")
// → Can only import shared/, no hooks

// Client Component
"use client"
// → Can use hooks, but NEVER server-only code

Auth Separation

// In Client Components:
import { signIn, signOut } from "@shared/lib/auth-client"

// In Server Components:
import { getSession } from "@shared/lib/auth-server"

Summary

1. Respect layers: Only import downward
2. Public API: Always export/import via index.ts
3. Segments: ui/, model/, api/, lib/ as needed
4. Aliases: @shared/, @entities/, @features/, @widgets/
5. Linting: bun run lint checks FSD rules