2nd/10_Wiki/Topics/DevOps_and_Security/Inversion.md

---
id: wiki-2026-0508-inversion
title: Inversion
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Inversion of Control, Invert Thinking, Negative Visualization]
duplicate_of: none
source_trust_level: A
confidence_score: 0.85
verification_status: applied
tags: [thinking-model, ioc, di, mental-model, design]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: nestjs
---

# Inversion

## 매 한 줄
> **"매 problem 의 reverse 매 stating — '매 fail 하는 방법' 의 enumerate, '매 control 의 누가 가지나' 의 flip"**. 매 Carl Jacobi "invert, always invert", 매 Charlie Munger 의 mental model, 매 software IoC/DI 의 design principle.

## 매 핵심

### 매 Inversion 3 layer
- **Cognitive**: "매 success" 대신 "매 failure 의 path" 을 enumerate.
- **Architectural (IoC)**: caller 가 control 하던 것을 framework 가 control.
- **Dependency (DI)**: hard-coded `new Foo()` 대신 inject.

### 매 IoC 의 forms
- **DI**: constructor/setter inject.
- **Service Locator**: registry lookup.
- **Events/Hooks**: publish-subscribe.
- **Template Method**: framework 의 skeleton, user 의 fill-in.

### 매 응용
1. Design review: failure mode enumeration.
2. Testability: mock injection.
3. Decision making: 매 worst case 의 list, 매 avoid.

## 💻 패턴

### NestJS DI
```ts
@Injectable()
export class UserRepo {
  findById(id: string) { /* ... */ }
}

@Injectable()
export class UserService {
  // 매 instance 매 직접 만들지 않음 — framework 의 inject
  constructor(private readonly repo: UserRepo) {}
  async profile(id: string) { return this.repo.findById(id); }
}

@Module({ providers: [UserRepo, UserService], exports: [UserService] })
export class UserModule {}
```

### Manual DI (no framework)
```ts
type Deps = { db: Database; cache: Cache; logger: Logger };
export const makeUserService = ({ db, cache, logger }: Deps) => ({
  async findById(id: string) {
    const cached = await cache.get(id);
    if (cached) return cached;
    logger.debug('cache miss', id);
    return db.users.findUnique({ where: { id } });
  }
});
```

### Premortem (failure inversion)
```ts
// scripts/premortem.ts
const failureModes = [
  { mode: 'DB connection lost', mitigation: 'retry + circuit breaker' },
  { mode: 'Cache stampede', mitigation: 'singleflight + jitter' },
  { mode: 'Memory leak in handler', mitigation: 'memray weekly + RSS alert' },
  { mode: 'Auth token expired mid-flow', mitigation: 'refresh interceptor' }
];
console.table(failureModes);
```

### Test seam via inversion
```ts
// 매 hard-coded fetch 대신 inject — testable
type Fetcher = (url: string) => Promise<Response>;
export const makeApi = (fetcher: Fetcher = fetch) => ({
  async get(path: string) {
    const r = await fetcher(`https://api.acme.com${path}`);
    return r.json();
  }
});

// test
const fakeFetch: Fetcher = async () => new Response(JSON.stringify({ ok: true }));
expect(await makeApi(fakeFetch).get('/x')).toEqual({ ok: true });
```

### Hook-based extension (template inversion)
```ts
// framework 의 lifecycle, user 의 hook 의 plug
type Plugin = {
  beforeRequest?: (req: Request) => Request;
  afterResponse?: (res: Response) => Response;
};

export class Server {
  private plugins: Plugin[] = [];
  use(p: Plugin) { this.plugins.push(p); }
  async handle(req: Request) {
    for (const p of this.plugins) req = p.beforeRequest?.(req) ?? req;
    let res = await this.process(req);
    for (const p of this.plugins) res = p.afterResponse?.(res) ?? res;
    return res;
  }
}
```

### Inverted error handling (Result type)
```ts
// 매 throw 대신 매 return 으로 invert — caller 의 forced handle
type Result<T, E = Error> = { ok: true; value: T } | { ok: false; error: E };

export async function fetchUser(id: string): Promise<Result<User>> {
  try {
    const u = await db.users.findUniqueOrThrow({ where: { id } });
    return { ok: true, value: u };
  } catch (e) {
    return { ok: false, error: e as Error };
  }
}
```

### Decision premortem prompt
```ts
// 매 launch 전 self-question
const premortem = `
1. 매 launch 6 month 후, 이 기능 매 fail 했다고 가정.
2. 매 가장 가능한 5 failure cause 매 무엇?
3. 매 매 cause 의 mitigation 매 무엇?
`;
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Module 매 testable 만들기 | DI |
| Framework 의 design | IoC + plugin hooks |
| Decision making | Premortem (failure inversion) |
| Error handling | Result type (return invert) |

**기본값**: constructor DI + premortem 매 architecture review 시.

## 🔗 Graph
- 부모: [[Mental_Models|Mental Models]] · [[Software Design Principles]]
- 변형: [[Dependency Injection]] · [[Inversion of Control]]
- 응용: [[NestJS]] · [[Result Type]]
- Adjacent: [[Encapsulation-via-Access-Modifiers]] · [[Testability]]

## 🤖 LLM 활용
**언제**: 매 design 의 failure mode 의 brainstorm, IoC refactor 의 candidate 식별.
**언제 X**: 매 trivial pure function 의 매 over-DI X — 매 simplicity 가 우선.

## ❌ 안티패턴
- **DI everywhere**: simple value 도 inject → 매 boilerplate explosion.
- **Service locator hell**: global registry 의 hidden dependency.
- **No premortem**: 매 ship 후에야 매 failure 발견.
- **Inversion theater**: interface 매 single impl 만 — 의 wrap 의 무의미.

## 🧪 검증 / 중복
- Verified (Charlie Munger "Poor Charlie's Almanack", Martin Fowler "IoC Containers").
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — cognitive + IoC + DI inversion 통합 |