2nd/10_Wiki/Topics/Frontend/Open-Closed Principle (OCP).md

---
id: wiki-2026-0508-open-closed-principle-ocp
title: Open-Closed Principle (OCP)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [OCP, Open Closed Principle, SOLID OCP]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [solid, ocp, design-principles, oop, architecture]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: TypeScript
  framework: none
---

# Open-Closed Principle (OCP)

## 매 한 줄
> **"매 extension 에 open, modification 에 closed"**. Bertrand Meyer (1988) 매 originally inheritance, Robert Martin (1996) 매 abstraction-based redefinition. 새 behavior 매 *추가* 의 way 로, 매 existing code 의 *수정* 없이 — strategy / plugin / open enum 등 매 광범위 응용.

## 매 핵심

### 매 두 가지 해석
- **Meyer's OCP**: 매 inheritance — class 매 closed for modification (매 published interface 안정), open for extension (매 subclass).
- **Polymorphic OCP** (Martin): 매 abstraction (interface) 의존, 매 implementation 추가 만 으로 extend.

### 매 왜 중요
- 매 regression 위험 줄임 (existing 안 건드림).
- 매 plugin / extension model 가능.
- 매 testability — 매 mock implementation 추가 만.

### 매 응용
1. Strategy pattern — payment processor (Stripe, Toss, PayPal).
2. Discriminated union 의 add new variant.
3. Plugin architecture (Vite plugin, ESLint rule).
4. Visitor pattern.
5. Middleware chain (Koa, Express).

## 💻 패턴

### Strategy interface (TS)
```ts
interface PaymentProcessor {
  charge(amount: number, token: string): Promise<{ id: string }>;
}

class StripeProcessor implements PaymentProcessor {
  async charge(amount: number, token: string) {
    return { id: `stripe_${Date.now()}` };
  }
}

class TossProcessor implements PaymentProcessor {
  async charge(amount: number, token: string) {
    return { id: `toss_${Date.now()}` };
  }
}

// New processor → just add new class. CheckoutService unchanged.
class CheckoutService {
  constructor(private processor: PaymentProcessor) {}
  pay(amt: number, t: string) { return this.processor.charge(amt, t); }
}
```

### Discriminated union + exhaustiveness
```ts
type Shape =
  | { kind: "circle"; r: number }
  | { kind: "square"; side: number }
  | { kind: "triangle"; base: number; height: number };

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.r ** 2;
    case "square": return s.side ** 2;
    case "triangle": return (s.base * s.height) / 2;
    default: {
      const _exhaust: never = s;
      throw new Error(_exhaust);
    }
  }
}
// Add 'pentagon' → TS error in `area`, no silent miss.
```

### Plugin / hook registry
```ts
type Hook<T> = (ctx: T) => void | Promise<void>;
class HookRegistry<T> {
  private hooks: Hook<T>[] = [];
  add(h: Hook<T>) { this.hooks.push(h); }
  async run(ctx: T) { for (const h of this.hooks) await h(ctx); }
}

const onUserCreated = new HookRegistry<{ userId: string }>();
onUserCreated.add(({ userId }) => sendWelcomeEmail(userId));
onUserCreated.add(({ userId }) => analytics.track("user_created", userId));
// New behavior → add new hook, no service change.
```

### Visitor pattern
```ts
interface NodeVisitor<R> {
  visitNumber(n: NumberNode): R;
  visitBinary(n: BinaryNode): R;
}

abstract class AstNode { abstract accept<R>(v: NodeVisitor<R>): R; }
class NumberNode extends AstNode {
  constructor(public val: number) { super(); }
  accept<R>(v: NodeVisitor<R>) { return v.visitNumber(this); }
}
class BinaryNode extends AstNode {
  constructor(public op: "+"|"-", public l: AstNode, public r: AstNode) { super(); }
  accept<R>(v: NodeVisitor<R>) { return v.visitBinary(this); }
}

// New traversal (printer, evaluator, optimizer) → new visitor class.
```

### Middleware chain
```ts
type Middleware<C> = (ctx: C, next: () => Promise<void>) => Promise<void>;

function compose<C>(mws: Middleware<C>[]): (ctx: C) => Promise<void> {
  return async (ctx) => {
    let i = -1;
    const dispatch = async (idx: number): Promise<void> => {
      if (idx <= i) throw new Error("next() called multiple times");
      i = idx;
      const fn = mws[idx];
      if (!fn) return;
      await fn(ctx, () => dispatch(idx + 1));
    };
    await dispatch(0);
  };
}

const app = compose<{ req: Request; res?: Response }>([
  async (c, next) => { console.log("log"); await next(); },
  async (c, next) => { c.res = new Response("ok"); await next(); },
]);
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Multiple swappable algos | Strategy / interface |
| Closed set of variants | Discriminated union + exhaustive switch |
| Open set of behaviors | Plugin / hook registry |
| Tree traversal, multiple ops | Visitor |
| Pipeline | Middleware chain |
| Just one impl, no plans | Don't pre-abstract (YAGNI) |

**기본값**: 매 second variation 발견 시 abstract — 매 first time 의 over-abstraction X.

## 🔗 Graph
- 부모: [[SOLID Principles]] · [[Design-Principles]]
- 변형: [[Visitor-Pattern]]
- 응용: [[Middleware]] · [[API Response & State Modeling|Discriminated-Unions]] · [[Dependency_Injection_(DI)|Dependency-Injection]]
- Adjacent: [[DIP]] · [[Composition-over-Inheritance]]

## 🤖 LLM 활용
**언제**: 매 add-only domain (payment processor, plugin, parser AST), variant 매 자주 추가.
**언제 X**: 매 small / unchanging code — 매 abstraction 의 cost > benefit.

## ❌ 안티패턴
- **Premature abstraction**: 매 single impl 의 interface — 매 indirection 만 추가, value 0.
- **Speculative generality**: 매 "혹시 나중에" — YAGNI.
- **Inheritance for code reuse**: 매 LSP 위반 risk. Composition 우선.
- **Open everything**: 매 모든 method virtual / hook — 매 cognitive load 폭증.
- **Ignore exhaustiveness**: 매 switch default fallback — 매 new variant 의 silent skip.

## 🧪 검증 / 중복
- Verified (Meyer "Object-Oriented Software Construction", Martin "Clean Architecture", refactoring.guru).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — strategy, discriminated union, plugin/visitor/middleware 패턴 |