What Happens When One Microservice Calls Another?.

// Service A (Order Service) Calling Service B (Payment Service)
package com.example.orderservice.client;

import org.springframework.stereotype.Service;
import org.springframework.web.client.RestClient;
import io.github.resilience4j.circuitbreaker.annotation.CircuitBreaker;

@Service
public class PaymentClient {

    private final RestClient restClient;

    public PaymentClient(RestClient.Builder builder) {
        // "http://payment-service" is resolved by Kubernetes DNS or Eureka!
        this.restClient = builder
            .baseUrl("http://payment-service")
            .build();
    }

    // Circuit breaker stops requests if Payment Service fails repeatedly
    @CircuitBreaker(name = "paymentService", fallbackMethod = "fallbackPayment")
    public PaymentResponse processPayment(PaymentRequest request) {
        // Behind this single line, 8 distinct network & OS steps happen:
        return restClient.post()
            .uri("/api/v1/payments")
            .header("X-Request-Source", "order-service")
            .body(request) // Jackson serializes Java object -> JSON bytes
            .retrieve()
            .body(PaymentResponse.class);
    }

    public PaymentResponse fallbackPayment(PaymentRequest request, Throwable ex) {
        // Graceful degradation when network or target service is down
        return new PaymentResponse("FAILED", "Payment service unavailable, retrying in background");
    }
}