2nd/10_Wiki/Topic_Programming/Architecture/Cross-Cutting Concerns.md

id, title, category, status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, verification_status, tags, raw_sources, last_reinforced, github_commit, tech_stack

id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-cross-cutting-concerns	Cross-Cutting Concerns	10_Wiki/Topics	verified	self	Cross Cutting Concerns AOP Concerns 횡단 관심사	none	A	0.9	applied	aop architecture separation-of-concerns		2026-05-10	pending	language framework typescript 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

@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

@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

@Service
public class OrderService {
    @Transactional
    public Order place(Order o) {
        repository.save(o);
        inventory.decrement(o.itemId);
        // throw → rollback
    }
}

Python decorator — caching

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

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

# 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

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