2nd/10_Wiki/Topics/Frontend/Large-scale Application Refactoring.md

---
id: wiki-2026-0508-large-scale-application-refactor
title: Large-scale Application Refactoring
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [large refactor, monorepo refactor, architectural refactoring]
duplicate_of: none
source_trust_level: A
confidence_score: 0.85
verification_status: applied
tags: [refactoring, architecture, monorepo, frontend, migration]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: TypeScript
  framework: Monorepo
---

# Large-scale Application Refactoring

## 매 한 줄
> **"매 large-scale refactor 의 성공은 incremental migration + automated codemod + tight feedback loop"**. 매 big-bang rewrite 는 매 90% fail. 매 strangler fig pattern, type-safe boundaries, CI-enforced invariants 가 정답. 2026 의 AI-assisted refactor (Claude, Cursor) 는 매 codemod 를 augment.

## 매 핵심

### 매 전략 종류
- **Strangler Fig**: 매 old + new 동시 운영, gradual cutover.
- **Branch by Abstraction**: 매 interface 추가 → impl 교체.
- **Codemod**: 매 AST-based bulk rewrite (jscodeshift, ts-morph).
- **Feature flag**: 매 new 코드 toggle, rollback 즉시.

### 매 사전 준비
- **Test coverage 가 안전망**: 매 critical path 80%+ 권장.
- **Type system 활용**: 매 TS strict, branded types 로 invariant.
- **Metrics baseline**: 매 perf, bundle size, error rate 의 before/after.
- **Rollback plan**: 매 feature flag, blue-green deploy.

### 매 응용
1. JS → TS migration (Airbnb, Stripe).
2. Pages Router → App Router (Next.js).
3. Redux → Zustand / Jotai.
4. CRA → Vite.
5. Single repo → Turborepo / Nx monorepo.

## 💻 패턴

### Strangler Fig — old/new gateway
```typescript
// gateway.ts
export async function fetchUser(id: string) {
  if (await flags.isEnabled("new-user-api", id)) {
    return newUserApi.get(id); // 매 new
  }
  return legacyUserApi.get(id); // 매 old fallback
}
// 매 traffic % gradually 100%
```

### Codemod with ts-morph (rename API)
```typescript
import { Project } from "ts-morph";
const project = new Project({ tsConfigFilePath: "./tsconfig.json" });

project.getSourceFiles().forEach((sf) => {
  sf.getDescendantsOfKind(SyntaxKind.CallExpression).forEach((call) => {
    const expr = call.getExpression();
    if (expr.getText() === "oldFetch") {
      expr.replaceWithText("newFetch");
    }
  });
});

await project.save();
// 매 매 file 수동 X — 매 1000 file 도 1초
```

### Branch by Abstraction
```typescript
// 매 step 1: interface 도입
interface UserStore {
  get(id: string): Promise<User>;
  save(u: User): Promise<void>;
}

// 매 step 2: old impl 을 interface 뒤로
class ReduxUserStore implements UserStore { /* ... */ }

// 매 step 3: 매 callsite 가 interface 사용 — 매 internal 구조 변경 가능
const store: UserStore = useFlag("zustand") ? new ZustandUserStore() : new ReduxUserStore();
```

### Type-driven migration (branded type)
```typescript
type UserId = string & { __brand: "UserId" };
type LegacyUserId = string & { __brand: "LegacyUserId" };

function migrate(legacy: LegacyUserId): UserId {
  return `u_${legacy}` as UserId;
}
// 매 compiler 가 매 미migrate 사이트 catch
```

### Dependency cruiser (architectural invariant)
```javascript
// .dependency-cruiser.cjs
module.exports = {
  forbidden: [
    {
      name: "ui-no-import-server",
      severity: "error",
      from: { path: "^src/ui" },
      to:   { path: "^src/server" },
    },
  ],
};
// 매 CI 에서 violation block — 매 layer 침범 방지
```

### Bundle size budget (CI enforce)
```json
// package.json
{
  "size-limit": [
    { "path": "dist/main.js", "limit": "200 KB" },
    { "path": "dist/vendor.js", "limit": "300 KB" }
  ]
}
```

### Mikado method (refactor 의 dependency tree)
```
Goal: Move auth from monolith to service
├── Pre: extract auth interface (BLOCKED by tight coupling)
│   ├── Pre: extract user model (DONE)
│   └── Pre: remove direct DB access from auth (DONE)
└── Pre: setup auth service skeleton (DONE)
```

### AI-assisted bulk refactor (2026)
```bash
# 매 Cursor / Claude Code 에 codemod 보다 의도 기반 refactor
# 매 "convert all class components to hooks" 같은 task 는
# AI 가 context 이해하며 수정 — 매 codemod 의 limit 넘음

# 매 검증: dry-run + diff review + test run
```

## 매 결정 기준
| 변경 규모 | 전략 |
|---|---|
| 1-3 file | manual edit |
| 10-100 file pattern | codemod (jscodeshift / ts-morph) |
| Architectural | strangler fig + feature flag |
| Cross-cutting (TS migration) | incremental + branded types |
| Monorepo split | Nx / Turborepo migration |

**기본값**: incremental + feature flag + CI invariant. Big-bang rewrite 의 X.

## 🔗 Graph
- 부모: [[Software Architecture]] · [[Refactoring_Best_Practices|Refactoring]]
- 변형: [[Feature Flag]]
- 응용: [[Monorepo|Monorepo Setup]]
- Adjacent: [[Dependency Cruiser]]

## 🤖 LLM 활용
**언제**: refactor 전략 설계, codemod 작성, dependency rule 작성.
**언제 X**: 매 trivial change (3 line) — 매 manual 가 빠름.

## ❌ 안티패턴
- **Big-bang rewrite**: 매 6 month freeze, 매 ship 못 함, 매 abandon 률 높음.
- **No baseline metrics**: 매 "더 좋아졌다" 증명 X.
- **No rollback plan**: 매 prod 사고 시 panic.
- **Codemod without test**: 매 silent regression.

## 🧪 검증 / 중복
- Verified (Martin Fowler "Refactoring", "Working Effectively with Legacy Code", Trunk-Based Dev).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — strangler fig + codemod + AI refactor |