Spring Cloud Gateway: API 게이트웨이

[Client] → [Gateway] → [User Service]
                    → [Product Service]
                    → [Order Service]

기능:
- 라우팅 (path 기반 → 서비스)
- 인증 (JWT 검증)
- 로드 밸런싱
- Rate Limiting
- 요청/응답 변환
- 로깅·모니터링

// build.gradle
implementation 'org.springframework.cloud:spring-cloud-starter-gateway'
implementation 'org.springframework.cloud:spring-cloud-starter-netflix-eureka-client'

# application.yml
spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          uri: lb://USER-SERVICE
          predicates:
            - Path=/api/users/**
          filters:
            - StripPrefix=1
            - AddRequestHeader=X-Gateway, ohs

        - id: product-service
          uri: lb://PRODUCT-SERVICE
          predicates:
            - Path=/api/products/**
          filters:
            - StripPrefix=1

        - id: order-service
          uri: lb://ORDER-SERVICE
          predicates:
            - Path=/api/orders/**
            - Method=GET,POST
          filters:
            - StripPrefix=1
            - name: RequestRateLimiter
              args:
                redis-rate-limiter.replenishRate: 10
                redis-rate-limiter.burstCapacity: 20
                key-resolver: "#{@userKeyResolver}"

@Component
@RequiredArgsConstructor
public class JwtAuthGatewayFilter implements GlobalFilter, Ordered {
    private final JwtUtil jwtUtil;

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        ServerHttpRequest req = exchange.getRequest();
        String path = req.getURI().getPath();

        // 인증 불필요 경로
        if (path.startsWith("/api/auth/") || path.startsWith("/api/public/")) {
            return chain.filter(exchange);
        }

        String header = req.getHeaders().getFirst("Authorization");
        if (header == null || !header.startsWith("Bearer ")) {
            exchange.getResponse().setStatusCode(HttpStatus.UNAUTHORIZED);
            return exchange.getResponse().setComplete();
        }

        String token = header.substring(7);
        try {
            Claims claims = jwtUtil.parse(token);
            // 사용자 정보를 헤더로 추가 — 서비스에 전달
            ServerHttpRequest mutated = req.mutate()
                .header("X-User-Id", claims.getSubject())
                .header("X-User-Role", claims.get("role", String.class))
                .build();
            return chain.filter(exchange.mutate().request(mutated).build());
        } catch (JwtException e) {
            exchange.getResponse().setStatusCode(HttpStatus.UNAUTHORIZED);
            return exchange.getResponse().setComplete();
        }
    }

    @Override
    public int getOrder() { return -100; }
}

@Configuration
public class RateLimitConfig {

    // IP 기반
    @Bean
    KeyResolver ipKeyResolver() {
        return exchange -> Mono.just(
            exchange.getRequest().getRemoteAddress().getHostString()
        );
    }

    // 사용자 기반
    @Bean
    KeyResolver userKeyResolver() {
        return exchange -> {
            String userId = exchange.getRequest().getHeaders().getFirst("X-User-Id");
            return Mono.just(userId != null ? userId : "anonymous");
        };
    }
}

spring:
  cloud:
    gateway:
      routes:
        - id: order-service
          uri: lb://ORDER-SERVICE
          predicates:
            - Path=/api/orders/**
          filters:
            - StripPrefix=1
            - name: CircuitBreaker
              args:
                name: orderCircuitBreaker
                fallbackUri: forward:/fallback/orders


resilience4j:
  circuitbreaker:
    instances:
      orderCircuitBreaker:
        slidingWindowSize: 10
        failureRateThreshold: 50
        waitDurationInOpenState: 30s