Microservices solve organizational and scalability problems — but they add operational complexity. Most applications don’t need that complexity. A modular monolith gives you clean boundaries and loose coupling without the distributed systems overhead. Spring Modulith enforces those boundaries. The Problem with Unstructured Monoliths Without explicit boundaries, every part of the codebase can talk to every other part: // OrderService calling PaymentRepository directly — skipping the Payment module @Service public class OrderService { @Autowired PaymentRepository paymentRepository; // ← wrong @Autowired NotificationService notificationService; // ← wrong @Autowired AnalyticsService analyticsService; // ← wrong } This creates hidden coupling.