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10_Wiki/Topics/Topic_Programming/Architecture/Cross-Cutting Concerns.md

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+---
+id: wiki-2026-0508-cross-cutting-concerns
+title: Cross-Cutting Concerns
+category: 10_Wiki/Topics
+status: verified
+canonical_id: self
+aliases: [Cross Cutting Concerns, AOP Concerns, 횡단 관심사]
+duplicate_of: none
+source_trust_level: A
+confidence_score: 0.9
+verification_status: applied
+tags: [aop, architecture, separation-of-concerns]
+raw_sources: []
+last_reinforced: 2026-05-10
+github_commit: pending
+tech_stack:
+  language: typescript
+  framework: nestjs
+---
+
+# Cross-Cutting Concerns
+
+## 매 한 줄
+> **"매 module 매 흩어지는 functionality (logging, auth, tx) 의 횡단"**. AOP (Aspect-Oriented Programming, Kiczales 1997) 의 motivation. 2026 현재 NestJS interceptors, Spring AOP, OpenTelemetry auto-instrumentation, Sidecar (Istio) 매 modern realization.
+
+## 매 핵심
+
+### 매 정의
+- 단일 module 의 boundary 를 넘어 매 application 매 widely-scattered concern.
+- Tangled with business logic → SoC (Separation of Concerns) 위반.
+- Solution: 매 aspect, decorator, middleware, sidecar, interceptor 로 추출.
+
+### 매 typical concerns
+- **Logging / tracing** — 매 method entry/exit log.
+- **Authentication / authorization** — 매 endpoint 매 token check.
+- **Transaction management** — DB tx begin/commit/rollback.
+- **Caching** — function result memoization.
+- **Validation** — input schema check.
+- **Rate limiting** — request throttling.
+- **Metrics / observability** — counter, histogram.
+- **Error handling** — global exception mapper.
+- **i18n** — locale-aware response.
+- **Audit** — who/when/what.
+
+### 매 응용
+1. Web framework middleware (Express, NestJS).
+2. Spring `@Transactional` / `@Cacheable`.
+3. Service mesh (Istio) — mTLS, retry, circuit-break 매 sidecar.
+4. OpenTelemetry auto-instrumentation 매 tracing 매 zero code change.
+
+## 💻 패턴
+
+### NestJS interceptor — logging + timing
+```typescript
+@Injectable()
+export class LoggingInterceptor implements NestInterceptor {
+  intercept(ctx: ExecutionContext, next: CallHandler) {
+    const start = Date.now();
+    const req = ctx.switchToHttp().getRequest();
+    return next.handle().pipe(
+      tap(() => log.info({ url: req.url, ms: Date.now() - start }))
+    );
+  }
+}
+```
+
+### NestJS guard — authz
+```typescript
+@Injectable()
+export class RolesGuard implements CanActivate {
+  canActivate(ctx: ExecutionContext): boolean {
+    const required = Reflect.getMetadata('roles', ctx.getHandler());
+    const user = ctx.switchToHttp().getRequest().user;
+    return required.some(r => user.roles.includes(r));
+  }
+}
+```
+
+### Spring AOP — `@Transactional`
+```java
+@Service
+public class OrderService {
+    @Transactional
+    public Order place(Order o) {
+        repository.save(o);
+        inventory.decrement(o.itemId);
+        // throw → rollback
+    }
+}
+```
+
+### Python decorator — caching
+```python
+from functools import lru_cache
+
+@lru_cache(maxsize=1024)
+def expensive_lookup(key: str) -> Result:
+    return db.query(key)
+```
+
+### TypeScript decorator (TC39 stage 3) — metrics
+```typescript
+function timed<This, Args extends any[], Return>(
+  target: (this: This, ...args: Args) => Return,
+  ctx: ClassMethodDecoratorContext
+) {
+  return function (this: This, ...args: Args): Return {
+    const start = performance.now();
+    try { return target.apply(this, args); }
+    finally { metrics.histogram(ctx.name, performance.now() - start); }
+  };
+}
+
+class API {
+  @timed
+  fetchUser(id: string) { /* ... */ }
+}
+```
+
+### OpenTelemetry — zero-code tracing
+```python
+# auto-instrumentation, no code changes
+# pip install opentelemetry-distro opentelemetry-instrumentation-fastapi
+# opentelemetry-instrument --traces_exporter otlp uvicorn app:app
+```
+
+### Service mesh (Istio) — mTLS via sidecar
+```yaml
+apiVersion: security.istio.io/v1
+kind: PeerAuthentication
+metadata: { name: default, namespace: prod }
+spec:
+  mtls: { mode: STRICT }
+```
+
+## 매 결정 기준
+| 상황 | Approach |
+|---|---|
+| Single service, JS/TS | NestJS interceptor / Express middleware |
+| JVM, mature DI | Spring AOP / aspectj |
+| Polyglot microservices | Service mesh (Istio / Linkerd) |
+| Tracing only | OpenTelemetry auto-instrumentation |
+| Function-level caching | language-native decorator |
+
+**기본값**: framework-native (interceptor/middleware/decorator) for in-process; service mesh for network-level.
+
+## 🔗 Graph
+- 부모: [[Separation of Concerns]] · [[Software Architecture]]
+- 변형: [[Aspect-Oriented Programming (AOP)]]
+- 응용: [[NestJS]] · [[Istio]] · [[OpenTelemetry]]
+- Adjacent: [[Dependency Injection (의존성 주입)]]
+
+## 🤖 LLM 활용
+**언제**: identify cross-cutting concern in legacy code, refactor to aspect/middleware.
+**언제 X**: design-time SoC decision (need human architectural taste).
+
+## ❌ 안티패턴
+- **Concern duplication**: 매 controller 매 try/catch + log + auth 매 copy-paste.
+- **Aspect overuse**: business logic 까지 aspect 로 → magic, debugging hell.
+- **Order dependency 무시**: interceptor chain 의 순서 의존 → fragile.
+- **Side-effect in interceptor**: DB write 매 logging interceptor → tx boundary 깨짐.
+
+## 🧪 검증 / 중복
+- Verified (Kiczales et al. 1997 *AOP*, Spring/NestJS docs 2026).
+- 신뢰도 A.
+
+## 🕓 Changelog
+| 날짜 | 변경 |
+|---|---|
+| 2026-05-08 | Phase 1 |
+| 2026-05-10 | Manual cleanup — full content covering interceptors, decorators, service mesh |