DDD Module Boundaries & Decoupling Modular Monoliths
Answer-first: A Modular Monolith prevents code degradation (“Big Ball of Mud”) by applying Domain-Driven Design (DDD) Bounded Contexts, isolating database schema namespaces (e.g. billing.payments, inventory.stock), enforcing compile-time import boundaries via Go internal packages and arch-go, and using an in-memory transactional outbox pattern for asynchronous event communication. Prerequisite: Before reading this part, please review Part 2: FinOps Cost Reality. What You’ll Learn: Go Package & Arch-Go Enforcement: How to use Go’s internal folder structure and arch-go static rules to block illegal cross-module imports at compile time. Aggregate Roots & Anti-Corruption Layers (ACL): How to encapsulate domain logic and translate external DTOs without leaking module internals. Database Schema Isolation (billing.payments, inventory.stock): How PostgreSQL schema permissions restrict SQL JOINs across modules within a shared database instance. In-Memory Transactional Outbox: How to achieve reliable event publishing without network overhead or Kafka infrastructure. The biggest reason engineering teams fear the Monolith architecture is due to past experiences with “Spaghetti Monoliths” or the “Big Ball of Mud” — where the code for the Billing function calls directly into the database of the Cart function, creating an inextricable web of cross-dependencies. ...