Feign Client: 서비스 간 통신

implementation 'org.springframework.cloud:spring-cloud-starter-openfeign'
implementation 'org.springframework.cloud:spring-cloud-starter-circuitbreaker-resilience4j'

@SpringBootApplication
@EnableFeignClients
public class OrderServiceApplication { }


@FeignClient(name = "product-service", path = "/products")
public interface ProductClient {

    @GetMapping("/{id}")
    ProductDto getProduct(@PathVariable Long id);

    @GetMapping
    List<ProductDto> getProducts(@RequestParam List<Long> ids);

    @PostMapping("/{id}/decrement")
    void decrementStock(@PathVariable Long id, @RequestBody StockRequest req);
}


@Service
@RequiredArgsConstructor
public class OrderService {
    private final ProductClient productClient;

    public Order placeOrder(OrderRequest req) {
        ProductDto product = productClient.getProduct(req.productId());
        // ... Eureka가 product-service URL 자동 해결
    }
}

resilience4j:
  circuitbreaker:
    configs:
      default:
        slidingWindowSize: 10
        failureRateThreshold: 50
        waitDurationInOpenState: 30s
        permittedNumberOfCallsInHalfOpenState: 3
    instances:
      productService:
        baseConfig: default

  retry:
    configs:
      default:
        maxAttempts: 3
        waitDuration: 1s
    instances:
      productService:
        baseConfig: default

  timelimiter:
    configs:
      default:
        timeoutDuration: 3s

@Service
@RequiredArgsConstructor
public class OrderService {
    private final ProductClient productClient;

    @CircuitBreaker(name = "productService", fallbackMethod = "fallbackProduct")
    @Retry(name = "productService")
    @TimeLimiter(name = "productService")
    public CompletableFuture<ProductDto> getProductWithResilience(Long id) {
        return CompletableFuture.supplyAsync(() -> productClient.getProduct(id));
    }

    public CompletableFuture<ProductDto> fallbackProduct(Long id, Throwable t) {
        log.warn("Product service down: {}", t.getMessage());
        return CompletableFuture.completedFuture(
            new ProductDto(id, "Unavailable", BigDecimal.ZERO, 0)
        );
    }
}

@Component
public class FeignAuthInterceptor implements RequestInterceptor {

    @Override
    public void apply(RequestTemplate template) {
        ServletRequestAttributes attrs = (ServletRequestAttributes)
            RequestContextHolder.getRequestAttributes();
        if (attrs != null) {
            String token = attrs.getRequest().getHeader("Authorization");
            if (token != null) {
                template.header("Authorization", token);
            }
        }
    }
}

logging:
  level:
    com.example.client.ProductClient: DEBUG

# 요청·응답 본문 모두 로깅 (개발만)
feign:
  client:
    config:
      productService:
        loggerLevel: FULL

@Component
public class CustomErrorDecoder implements ErrorDecoder {

    @Override
    public Exception decode(String methodKey, Response response) {
        return switch (response.status()) {
            case 404 -> new ProductNotFoundException();
            case 500 -> new ProductServiceUnavailableException();
            default -> new ErrorDecoder.Default().decode(methodKey, response);
        };
    }
}