Client-Side Load Balancing

// Configuration
@Configuration
public class WebConfig {
    @Bean
    @LoadBalanced   // enables client-side LB on this RestTemplate
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

// Usage — use logical service name, not a real host
@Service
public class OrderClient {
    private final RestTemplate restTemplate;

    public ProductDTO getProduct(Long id) {
        // "product-service" is the service ID in Eureka/Consul
        return restTemplate.getForObject(
            "http://product-service/api/products/{id}",
            ProductDTO.class, id);
        // SCL resolves → e.g. http://10.0.1.15:8081/api/products/42
    }
}

@Configuration
public class WebClientConfig {
    @Bean
    @LoadBalanced
    public WebClient.Builder loadBalancedWebClientBuilder() {
        return WebClient.builder();
    }
}

@Service
public class InventoryClient {
    private final WebClient webClient;

    public InventoryClient(WebClient.Builder builder) {
        this.webClient = builder.baseUrl("http://inventory-service").build();
    }

    public Mono<StockDTO> getStock(String sku) {
        return webClient.get()
            .uri("/api/stock/{sku}", sku)
            .retrieve()
            .bodyToMono(StockDTO.class);
    }
}

// application.yml — no extra LB config needed; round-robin is default
spring:
  cloud:
    loadbalancer:
      ribbon:
        enabled: false   # explicitly disable Ribbon (Spring Boot 2.x)

// Custom random load balancer
public class RandomLoadBalancer implements ReactorServiceInstanceLoadBalancer {
    private final ServiceInstanceListSupplier supplier;

    public RandomLoadBalancer(ServiceInstanceListSupplier supplier) {
        this.supplier = supplier;
    }

    @Override
    public Mono<Response<ServiceInstance>> choose(Request request) {
        return supplier.get(request)
            .next()
            .map(instances -> {
                if (instances.isEmpty()) return new EmptyResponse();
                int idx = ThreadLocalRandom.current().nextInt(instances.size());
                return new DefaultResponse(instances.get(idx));
            });
    }
}

// Register for a specific service
@Configuration
@LoadBalancerClient(name = "product-service",
                    configuration = ProductServiceLBConfig.class)
public class ProductServiceLBConfig {
    @Bean
    public ReactorServiceInstanceLoadBalancer randomLB(
            ServiceInstanceListSupplier supplier) {
        return new RandomLoadBalancer(supplier);
    }
}