Spring Cloud Basics

Architecture Overview

┌─────────────────────────────────────────────────────────────────┐
│                        API Gateway                               │
│                    (Spring Cloud Gateway)                        │
└───────────────────────────┬─────────────────────────────────────┘
                            │
┌───────────────────────────▼─────────────────────────────────────┐
│                     Service Discovery                            │
│                    (Eureka / Consul)                             │
└──────────┬─────────────────┬─────────────────┬──────────────────┘
           │                 │                 │
    ┌──────▼──────┐   ┌──────▼──────┐   ┌──────▼──────┐
    │  Service A  │   │  Service B  │   │  Service C  │
    │  (3 inst.)  │   │  (2 inst.)  │   │  (1 inst.)  │
    └─────────────┘   └─────────────┘   └─────────────┘
           │                 │                 │
           └─────────────────┼─────────────────┘
                             ▼
                    ┌────────────────┐
                    │  Config Server │
                    └────────────────┘

---

Quick Start - Eureka

Server

@SpringBootApplication
@EnableEurekaServer
public class EurekaServerApplication {
    public static void main(String[] args) {
        SpringApplication.run(EurekaServerApplication.class, args);
    }
}

server:
  port: 8761
eureka:
  client:
    register-with-eureka: false
    fetch-registry: false

Client

spring:
  application:
    name: product-service
eureka:
  client:
    service-url:
      defaultZone: http://localhost:8761/eureka
  instance:
    prefer-ip-address: true

Full Reference: See service-discovery.md for Eureka HA and Config Server.

---

Quick Start - API Gateway

spring:
  cloud:
    gateway:
      routes:
        - id: product-service
          uri: lb://product-service
          predicates:
            - Path=/api/products/**
          filters:
            - StripPrefix=0

Full Reference: See gateway.md for custom filters and programmatic routes.

---

Quick Start - Circuit Breaker

@Service
public class ProductClient {

    @CircuitBreaker(name = "productService", fallbackMethod = "fallback")
    @Retry(name = "productService")
    public List<Product> getProducts() {
        return restClient.get()
            .uri("http://product-service/api/products")
            .retrieve()
            .body(new ParameterizedTypeReference<>() {});
    }

    private List<Product> fallback(Exception e) {
        return List.of();
    }
}

resilience4j:
  circuitbreaker:
    instances:
      productService:
        sliding-window-size: 10
        failure-rate-threshold: 50
        wait-duration-in-open-state: 10s

Full Reference: See resilience.md for Retry, Bulkhead, Rate Limiter, Feign.

---

Service Communication Pattern

@Service
@RequiredArgsConstructor
public class OrderService {

    private final ProductClient productClient;
    private final InventoryClient inventoryClient;
    private final PaymentClient paymentClient;

    @Transactional
    public Order createOrder(CreateOrderRequest request) {
        // 1. Verifica prodotti
        List<Product> products = request.items().stream()
            .map(item -> productClient.getProductById(item.productId()))
            .toList();

        // 2. Verifica inventario
        boolean available = inventoryClient.checkAvailability(request.items());
        if (!available) {
            throw new InsufficientInventoryException("Items not available");
        }

        // 3. Crea ordine
        Order order = Order.create(request.customerId(), products, request.items());
        order = orderRepository.save(order);

        // 4. Riserva inventario
        inventoryClient.reserveItems(order.getId(), request.items());

        // 5. Processa pagamento (con rollback)
        try {
            PaymentResult payment = paymentClient.processPayment(
                new PaymentRequest(order.getId(), order.getTotal())
            );
            order.setPaymentId(payment.paymentId());
            order.setStatus(OrderStatus.PAID);
        } catch (PaymentFailedException e) {
            inventoryClient.releaseReservation(order.getId());
            order.setStatus(OrderStatus.PAYMENT_FAILED);
            throw e;
        }

        return orderRepository.save(order);
    }
}

---

Load Balancer

@Configuration
public class LoadBalancerConfig {

    @Bean
    @LoadBalanced
    public RestClient.Builder loadBalancedRestClientBuilder() {
        return RestClient.builder();
    }
}

// Usage: use service name instead of host
restClient.get()
    .uri("http://product-service/api/products")
    .retrieve()
    .body(new ParameterizedTypeReference<>() {});

---

Distributed Tracing

management:
  tracing:
    sampling:
      probability: 1.0
  zipkin:
    tracing:
      endpoint: http://localhost:9411/api/v2/spans

logging:
  pattern:
    level: "%5p [${spring.application.name:},%X{traceId:-},%X{spanId:-}]"

---

Best Practices

Do	Don't
Use Service Discovery for all services	Hardcode service URLs
Implement Circuit Breaker with fallback	Ignore failures
Centralize config with Config Server	Duplicate configuration
Add distributed tracing	Miss observability
Use API Gateway as single entry point	Expose services directly

---

When NOT to Use This Skill

• Single service - Spring Cloud adds unnecessary complexity
• Kubernetes native - Use K8s service discovery, ConfigMaps
• Simple deployments - Overhead not justified
• Specific components - Use dedicated skills for deep dives

---

Common Pitfalls

Error	Cause	Solution
No instances available	Service not registered	Verify Eureka registration
Connection refused	Service down	Implement Circuit Breaker
Timeout	Service slow	Configure appropriate timeouts
Config not loading	Config server unreachable	Use fail-fast: false or fallback
Load balancing not working	Missing @LoadBalanced	Annotate RestClient builder

---

Anti-Patterns

Anti-Pattern	Problem	Solution
Hardcoding service URLs	No discovery benefit	Use service names
No circuit breaker	Cascading failures	Add Resilience4j
Missing retry	Transient failures	Configure retry with backoff
No config refresh	Changes need redeploy	Use @RefreshScope
Synchronous everywhere	Tight coupling	Use async where appropriate

---

Quick Troubleshooting

Problem	Diagnostic	Fix
Service not found	Check Eureka dashboard	Verify registration
Config not loading	Check config server logs	Verify path, profile
Circuit always open	Check failure threshold	Tune thresholds
Gateway routing fails	Check predicates	Verify route config
Load balancing not working	Check @LoadBalanced	Add annotation

---

Reference Files

File	Content
service-discovery.md	Eureka Server/Client, Config Server
gateway.md	API Gateway, Filters, Routes
resilience.md	Circuit Breaker, Retry, Feign, Testing

---

External Documentation

• Spring Cloud
• Spring Cloud Gateway
• Resilience4j