2nd/10_Wiki/Topic_Programming/Architecture/High-Cohesion-Low-Coupling.md

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id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-high-cohesion-low-coupling	High Cohesion Low Coupling	10_Wiki/Topics	verified	self	Cohesion and Coupling Loose Coupling High Cohesion	none	A	0.95	applied	architecture design-principles modularity oop refactoring		2026-05-10	pending	language framework typescript language-agnostic

High Cohesion Low Coupling

매 한 줄

"매 module 내부 의 element 의 strong relatedness (cohesion) + module 간 의 minimal dependency (coupling) 의 maximize". 매 Larry Constantine 의 1968 structured design 의 origin, 매 modern 의 SOLID, DDD bounded context, microservices 의 universal foundation — 매 changeability 의 single 의 most important predictor.

매 핵심

매 Cohesion 7 levels (Constantine, low → high)

1. Coincidental (random grouping).
2. Logical (similar category, e.g., "Utils").
3. Temporal (executed at same time).
4. Procedural (sequence of steps).
5. Communicational (act on same data).
6. Sequential (output of one → input of next).
7. Functional (single, well-defined task) — 매 target.

매 Coupling 6 levels (low → high)

1. Data (parameters of primitives) — 매 ideal.
2. Stamp (parameters of composite).
3. Control (flag-driven branching).
4. External (shared protocol).
5. Common (shared global).
6. Content (one module 의 internals 의 mutate) — 매 worst.

매 응용

1. Module boundary 의 design.
2. Microservice 의 split criteria (DDD bounded context).
3. Refactoring 의 direction-finder.

💻 패턴

Smell — Low Cohesion

// 🚨 Logical cohesion only
export class Utils {
  formatDate(d: Date) { /* ... */ }
  parseCSV(s: string) { /* ... */ }
  hashPassword(p: string) { /* ... */ }
  sendEmail(to: string) { /* ... */ }
}

Refactor — Functional Cohesion

// formatting/date.ts
export const formatISO = (d: Date) => d.toISOString().slice(0, 10);

// parsing/csv.ts
export const parseCSV = (s: string) => /* ... */;

// auth/password.ts
export const hashPassword = (p: string) => bcrypt.hash(p, 12);

// email/send.ts
export const sendEmail = (to: string, body: string) => transporter.send(/* ... */);

Smell — High Coupling (Content)

class OrderService {
  constructor(private inventory: InventoryService) {}
  reserve(id: string) {
    // 🚨 Reaches into private state
    this.inventory['stock'].set(id, 0);
  }
}

Refactor — Data Coupling via Interface

interface InventoryPort {
  reserve(sku: string, qty: number): Promise<ReservationId>;
}

class OrderService {
  constructor(private inventory: InventoryPort) {}
  async reserve(sku: string, qty: number) {
    return this.inventory.reserve(sku, qty);
  }
}

Dependency Inversion (Hexagonal)

// domain/order.ts (no infra import)
export interface OrderRepository {
  save(o: Order): Promise<void>;
  byId(id: string): Promise<Order | null>;
}

// infra/postgres-order-repo.ts
export class PostgresOrderRepository implements OrderRepository { /* ... */ }

// app/wire.ts
const repo: OrderRepository = new PostgresOrderRepository(pool);
const svc = new OrderService(repo);

Event-Driven Decoupling

// publisher
events.emit('OrderPlaced', { orderId, customerId, total });

// subscribers (independent modules)
events.on('OrderPlaced', sendConfirmationEmail);
events.on('OrderPlaced', updateAnalytics);
events.on('OrderPlaced', allocateInventory);

Coupling Metric — Fan-in / Fan-out

# madge 의 dep graph
npx madge --circular --extensions ts src/
npx madge --summary --extensions ts src/   # fan-in/fan-out per module

매 결정 기준

상황	Approach
Module > 500 LOC, scattered concerns	Split by responsibility (cohesion↑)
Module imports 20+ siblings	Introduce interface / facade
Cross-module mutation	Event / message passing
Shared mutable global	Replace with pure function or DI
Microservice split	DDD bounded context boundary

기본값: 매 functional cohesion + data coupling 의 target — 매 interface 의 통한 dependency 의 invert.

🔗 Graph

• 부모: Software-Architecture · Modularity
• 변형: Single Responsibility Principle (SRP) · Bounded Context · Hexagonal Architecture
• 응용: SOLID · Microservices · Clean-Architecture
• Adjacent: God-Object-Antipattern · Dependency-Inversion

🤖 LLM 활용

언제: module 의 cohesion/coupling 의 review, refactor 의 direction 의 suggest, dep graph 의 hot-spot 의 highlight. 언제 X: 매 architectural boundary 의 final decision (domain expert + business context 필요).

❌ 안티패턴

• God Module: low cohesion + high fan-in (모두 의 import).
• Anemic Module: data 만, behavior 의 다른 module 의 own → procedural disguised as OOP.
• Tight ORM coupling: domain entity 의 ORM annotation 의 saturation → infra 의 leak.
• Premature abstraction: 매 single use-case 의 interface 의 introduce → 매 indirection 의 cost.
• Shared library 의 god lib: 매 service 의 shared "common" 의 god — 매 deploy lockstep.

🧪 검증 / 중복

• Verified (Constantine "Structured Design" 1979, Martin "Clean Architecture", Evans "DDD").
• 신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — cohesion/coupling principles 의 full content