7.4 Inter-Service Communication: OpenFeign & WebClient

← Back to Chapter 7: Microservices & Spring Cloud

Synchronous communication between microservices can be achieved via declarative HTTP clients (Spring Cloud OpenFeign) or non-blocking reactive clients (Spring WebClient).

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1. Spring Cloud OpenFeign (Declarative REST Client)

OpenFeign allows you to define an interface annotated with standard Spring MVC annotations. Spring automatically generates the underlying HTTP client implementation at runtime!

1. Enable Feign Clients

@SpringBootApplication
@EnableFeignClients
public class OrderApplication { ... }

2. Define the Feign Interface

// 'name' maps to the registered Eureka service name!
@FeignClient(name = "payment-service", path = "/api/v1/payments", configuration = FeignClientConfig.class)
public interface PaymentServiceClient {

    @PostMapping("/process")
    PaymentResponse processPayment(@RequestBody PaymentRequest request);

    @GetMapping("/{transactionId}")
    PaymentDetails getPaymentDetails(@PathVariable("transactionId") UUID transactionId);
}

3. Propagating Headers via RequestInterceptor

Pass auth tokens and correlation IDs to downstream services automatically:

@Configuration
public class FeignClientConfig {

    @Bean
    public RequestInterceptor bearerAuthRequestInterceptor() {
        return requestTemplate -> {
            String token = SecurityContextHolder.getContext().getAuthentication().getCredentials().toString();
            requestTemplate.header("Authorization", "Bearer " + token);
            requestTemplate.header("X-Correlation-Id", MDC.get("correlationId"));
        };
    }
}

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2. Spring WebClient (Non-Blocking & Reactive)

WebClient (from spring-boot-starter-webflux) is the modern replacement for legacy RestTemplate, supporting both asynchronous reactive streams and synchronous .block() execution.

@Service
public class InventoryClient {

    private final WebClient webClient;

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

    public Mono<InventoryResponse> checkStock(String sku) {
        return webClient.get()
                .uri("/api/v1/inventory/{sku}", sku)
                .retrieve()
                .bodyToMono(InventoryResponse.class)
                .timeout(Duration.ofMillis(1500));
    }
}

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3. Comparison: RestTemplate vs OpenFeign vs WebClient

Feature	RestTemplate	OpenFeign	WebClient
Paradigm	Blocking (Synchronous)	Blocking (Synchronous)	Non-blocking (Reactive) & Blocking
API Style	Programmatic template	Declarative Interface	Fluent Functional Builder
Recommendation	Maintenance mode	Excellent for standard microservices	Modern default for high throughput