2nd/10_Wiki/Topics/Architecture/웹 애플리케이션의 3계층 구조.md

---
id: wiki-2026-0508-웹-애플리케이션의-3계층-구조
title: 웹 애플리케이션의 3계층 구조
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [3-Tier Architecture, Three-Tier Web App, Presentation-Logic-Data]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [architecture, 3-tier, web, layered]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: nextjs
---

# 웹 애플리케이션의 3계층 구조

## 매 한 줄
> **"매 presentation / logic / data 의 separation"**. 매 1990 년대 client-server 의 evolution — 매 thick client 의 limit 의 escape. 2026 년 매 SPA + BFF + DB 의 form, 또는 SSR (Next.js) 의 collapsed form 의 dominant.

## 매 핵심

### 매 3 layers
- **Presentation (UI)**: 매 user-facing rendering — browser, mobile app.
- **Application/Logic (BLL)**: 매 business rule + orchestration — API server.
- **Data (DAL)**: 매 persistence — RDBMS, NoSQL, cache.

### 매 communication
- **Client → API**: HTTPS REST / GraphQL / gRPC-Web.
- **API → DB**: 매 connection pool — TCP.
- **Layer rule**: 매 adjacent layer only — UI 의 DB 직접 access 의 X.

### 매 응용
1. Traditional LAMP/MEAN — 매 separated tier.
2. Next.js SSR — 매 server component 가 logic + data 의 collapse.
3. Mobile app + REST API + Postgres — 매 classic 3-tier.

## 💻 패턴

### Next.js 15 server component (collapsed 3-tier)
```tsx
// app/users/page.tsx — server component
import { db } from '@/lib/db'; // DAL

async function getUsers() { // BLL
  return db.user.findMany({ where: { active: true } });
}

export default async function UsersPage() { // Presentation
  const users = await getUsers();
  return (
    <ul>
      {users.map((u) => <li key={u.id}>{u.name}</li>)}
    </ul>
  );
}
```

### Classic separated tier (REST)
```typescript
// Tier 1: React SPA
const res = await fetch('/api/users');
const users = await res.json();

// Tier 2: Express API (Node.js)
app.get('/api/users', async (req, res) => {
  const users = await userService.listActive(); // BLL
  res.json(users);
});

// Tier 3: Repository (Postgres)
class UserRepository {
  async listActive() {
    return pool.query('SELECT * FROM users WHERE active = true');
  }
}
```

### BFF (Backend for Frontend)
```typescript
// 매 mobile + web 의 다른 BFF — 매 tier 2 의 split.
// /api/web/dashboard — web-tailored aggregate
app.get('/api/web/dashboard', async (req, res) => {
  const [user, stats, notifs] = await Promise.all([
    userSvc.get(req.userId),
    statsSvc.summary(req.userId),
    notifSvc.unread(req.userId),
  ]);
  res.json({ user, stats, notifs });
});
```

### Repository pattern (DAL boundary)
```typescript
interface UserRepository {
  findById(id: string): Promise<User | null>;
  save(user: User): Promise<void>;
}

class PostgresUserRepository implements UserRepository {
  constructor(private pool: Pool) {}
  async findById(id: string) {
    const { rows } = await this.pool.query(
      'SELECT * FROM users WHERE id = $1', [id]
    );
    return rows[0] ?? null;
  }
  async save(user: User) {
    await this.pool.query('INSERT INTO users ... ON CONFLICT ...');
  }
}
```

### Service layer (BLL)
```typescript
class OrderService {
  constructor(
    private orders: OrderRepository,
    private inventory: InventoryService,
    private payments: PaymentService,
  ) {}

  async place(userId: string, items: CartItem[]): Promise<Order> {
    await this.inventory.reserve(items);
    const charge = await this.payments.charge(userId, total(items));
    const order = await this.orders.create({ userId, items, chargeId: charge.id });
    return order;
  }
}
```

### Cache layer (between BLL and DAL)
```typescript
class CachedUserRepo implements UserRepository {
  constructor(private inner: UserRepository, private redis: Redis) {}

  async findById(id: string) {
    const cached = await this.redis.get(`user:${id}`);
    if (cached) return JSON.parse(cached);
    const user = await this.inner.findById(id);
    if (user) await this.redis.setex(`user:${id}`, 300, JSON.stringify(user));
    return user;
  }
}
```

## 매 결정 기준
| 상황 | Architecture |
|---|---|
| Simple CRUD app | Next.js full-stack (collapsed) |
| Mobile + web + admin | BFF per client |
| Microservices | API gateway + service mesh |
| Monolithic enterprise | Strict 3-tier with DI |
| Real-time | Add WebSocket/SSE tier |

**기본값**: 매 Next.js server components 의 simplest path.

## 🔗 Graph
- 부모: [[Layered Architecture]] · [[Client-Server]]
- 변형: [[N-Tier]] · [[BFF]] · [[Microservices]]
- Adjacent: [[MVC]]

## 🤖 LLM 활용
**언제**: web app 의 layer separation 의 design 의 결정 시.
**언제 X**: real-time game, embedded system — 매 different model.

## ❌ 안티패턴
- **Smart UI**: 매 SQL 의 frontend — 매 layer 의 violate.
- **Anemic service**: 매 service 의 pass-through 의 only — 매 logic 의 controller.
- **Chatty DAL**: 매 N+1 query — 매 batch 의 X.
- **Leaky DB schema**: 매 ORM entity 의 API 의 expose — 매 DTO 의 separate.

## 🧪 검증 / 중복
- Verified (Fowler *PoEAA*; Microsoft *Application Architecture Guide*; Next.js docs 2026).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — 3-tier + BFF + repository patterns |