2nd/10_Wiki/Topics/Architecture/Seam (접점).md

---
id: wiki-2026-0508-seam-접점
title: Seam (접점)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Seam, Seams, 이음새, 접점, Feathers seam]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [legacy-code, refactoring, testing, feathers]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: none
---

# Seam (접점)

## 매 한 줄
> **"매 코드를 매 수정하지 않고 매 동작을 바꿀 수 있는 지점"**. Michael Feathers (*Working Effectively with Legacy Code*, 2004)의 핵심 개념. Seam은 매 legacy code에 매 test를 매 끼워넣는 매 hinge — 매 dependency를 매 엮어내는 매 join point.

## 매 핵심

### 매 정의
- 매 place where you can alter behavior without editing in place.
- 매 enabling point: 매 그 seam을 매 어떻게 활성화하는지 결정.

### 매 3 종류 (Feathers)
1. **Preprocessing Seam** — compile/macro/언어 전처리 (C/C++ `#define`).
2. **Link Seam** — link-time substitution (매 alt library, jest module mock).
3. **Object Seam** — polymorphism (매 interface/subclass).

### 매 응용
1. 매 legacy에 매 test 추가 — 매 substitute dependency.
2. 매 hexagonal port — 매 object seam.
3. 매 feature flag — 매 conditional seam.

## 💻 패턴

### 매 Object Seam — interface 추출
```typescript
// X — 매 직접 의존
class OrderService {
  place(o: Order) {
    new SmtpMailer().send(o.user, "ordered"); // 매 hard-wired
  }
}

// O — 매 object seam
interface Mailer { send(to: string, msg: string): void; }
class OrderService {
  constructor(private mailer: Mailer) {}
  place(o: Order) { this.mailer.send(o.user, "ordered"); }
}
// 매 test에서 fake Mailer 주입.
```

### 매 Link Seam — Jest module mock
```typescript
// orderService.ts
import { sendEmail } from "./mailer";
export function place(o: Order) { sendEmail(o.user, "ordered"); }

// orderService.test.ts
import { place } from "./orderService";
jest.mock("./mailer", () => ({ sendEmail: jest.fn() }));
import { sendEmail } from "./mailer";

test("places", () => {
  place({ user: "a@b.com" } as any);
  expect(sendEmail).toHaveBeenCalledWith("a@b.com", "ordered");
});
```

### 매 Preprocessing Seam — TS compiler hook
```typescript
// 매 build-time substitution (매 e.g., babel plugin / tsx loader)
// __ENV__ macro 가 매 build 시 replace
declare const __TEST__: boolean;
export function log(msg: string) {
  if (__TEST__) return;
  console.log(msg);
}
```

### 매 React Context seam
```tsx
const ApiContext = createContext<Api>(realApi);

// 매 production
<ApiContext.Provider value={realApi}><App /></ApiContext.Provider>

// 매 test (Storybook/jsdom)
<ApiContext.Provider value={fakeApi}><App /></ApiContext.Provider>
```

### 매 NodeJS DI container seam (NestJS)
```typescript
// 매 production
@Module({ providers: [{ provide: "Mailer", useClass: SmtpMailer }] })
class AppModule {}

// 매 test
const moduleRef = await Test.createTestingModule({
  providers: [OrderService, { provide: "Mailer", useValue: { send: jest.fn() } }],
}).compile();
```

### 매 Wrap method (Feathers technique)
```typescript
// X — private method 가 매 hard-coded
class OrderService {
  place(o: Order) { this.notify(o); /* ... */ }
  private notify(o: Order) { new SmtpMailer().send(o.user, "ordered"); }
}

// O — 매 protected → subclass에서 override
class OrderService {
  place(o: Order) { this.notify(o); /* ... */ }
  protected notify(o: Order) { new SmtpMailer().send(o.user, "ordered"); }
}
class TestOrderService extends OrderService {
  protected notify() { /* swallow */ }
}
```

### 매 Feature flag seam
```typescript
const flags = await flagClient.evaluate("new-checkout", { user });
const checkout = flags ? new NewCheckout() : new LegacyCheckout();
```

## 매 결정 기준
| 상황 | Seam |
|---|---|
| 매 OOP + DI 가능 | Object Seam (interface). |
| 매 legacy without DI | Link Seam (module mock). |
| 매 C/C++ 매 macro | Preprocessing Seam. |
| 매 React component | Context seam. |
| 매 cross-cutting toggle | Feature flag seam. |

**기본값**: 매 Object Seam — 매 testability + 매 long-term cleanest.

## 🔗 Graph
- 부모: [[Working Effectively with Legacy Code]] · [[Refactoring]]
- 변형: [[Object Seam (객체 접점)]] · [[Link Seam (링크 접점)]] · [[Preprocessing Seam (전처리 접점)]]
- 응용: [[Dependency Injection]] · [[Test Doubles]] · [[Hexagonal Architecture]]
- Adjacent: [[Scratch Refactoring (스크래치 리팩토링)]] · [[Characterization Tests]] · [[Mock Object]]

## 🤖 LLM 활용
**언제**: 매 legacy에 매 test 끼워넣기, 매 dependency 분리, 매 refactor 진입점 식별.
**언제 X**: 매 greenfield (매 처음부터 DI로 설계).

## ❌ 안티패턴
- **매 seam 강제 too many**: 매 abstraction noise.
- **매 enabling point 미정**: 매 seam 만들고 매 활용 X.
- **매 monkey-patching seam (Ruby/Python)**: 매 long-term 유지보수 X.

## 🧪 검증 / 중복
- Verified (Feathers, *Working Effectively with Legacy Code*, 2004, Ch. 4).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Feathers 3-seam canonical + modern patterns |