When Your Hexagonal Architecture Becomes a Dependency Hellscape

When Your Hexagonal Architecture Becomes a Dependency Hellscape

Navigating cross-module communication in hexagonal monoliths without descending into dependency chaos
SOFTWARE ARCHITECTURE
October 18, 2025

The promise of Hexagonal Architecture is seductive: clean domain logic, interchangeable infrastructure, and testability that makes QA teams weep with joy. Then reality hits, you’ve got User, Post, and Category modules that need to talk to each other, and suddenly your beautifully hexagonal design looks more like a dependency spider web. The core dilemma emerges: how do modules communicate without violating domain boundaries or creating that soul-crushing cyclic dependency nightmare?

The Architecture Identity Crisis

Hexagonal Architecture fundamentally cares about one thing: protecting your domain from external concerns. As developers on architecture forums note, when your problem shifts from “how do I isolate from databases and APIs” to “how do my modules interact”, you’ve crossed into modular monolith territory. This distinction is crucial, the architectural pattern isn’t solving your internal module communication problem, and pretending otherwise leads to architectural debt.

Cover image for A Quick Note On Hexagonal Architecture

The fundamental misunderstanding comes when teams apply hexagonal principles to what is essentially a bounded context communication problem ↗. Your Post module needing user existence validation isn’t a hexagonal architecture problem, it’s a domain interaction challenge that requires strategic thinking about coupling and cohesion.

Communication Patterns That Won’t Burn Your Architecture

Dependency Inversion: The First Line of Defense

The purest hexagonal approach suggests defining interfaces (ports) in the consuming module. If your Post module needs user operations, it defines a UserRepository interface with exactly what it needs, no more, no less. The User module then provides the implementation.

1// In Post module 2interface UserRepository { 3 userExists(userId: string): Promise<boolean>; 4 getUserInfo(userId: string): Promise<UserInfo>; 5} 6 7// In User module adapter 8class UserRepositoryImpl implements UserRepository { 9 async userExists(userId: string): Promise<boolean> { 10 // Implementation details hidden from Post module 11 return await this.userDatabase.exists(userId); 12 } 13 14 async getUserInfo(userId: string): Promise<UserInfo> { 15 return await this.userService.getUserById(userId); 16 } 17}

This approach maintains clean boundaries but introduces interface proliferation. When multiple modules need user operations, you’ll have similar interfaces scattered throughout your codebase. The question becomes: who owns the interface definition?

Domain Events: The Loose Coupling Lifeline

For truly decoupled communication, domain events shine. When a user registers, publish a UserRegistered event. When posts need user validation, they listen for user lifecycle events and maintain their own read models.

1// In User module 2class UserService { 3 constructor(private eventBus: EventBus) {} 4 5 async registerUser(registration: UserRegistration): Promise<User> { 6 const user = await this.userRepository.create(registration); 7 await this.eventBus.publish(new UserRegistered(user.id, user.email)); 8 return user; 9 } 10} 11 12// In Post module 13class PostCreationPolicy { 14 constructor(private userReadModel: UserReadModel) {} 15 16 async canCreatePost(userId: string): Promise<boolean> { 17 return this.userReadModel.userExists(userId); 18 } 19}

The key insight from experienced architects is treating internal module communication like external API calls. Require DTOs for requests and responses, no “reach into my tables” shortcuts allowed. This discipline pays dividends when the inevitable microservices migration happens.

Anti-Corruption Layers: When Domains Collide

Sometimes, your modules speak different languages. The Category module might model hierarchies differently than how User module tracks preferences. An anti-corruption layer translates between these domain languages without contaminating either.

1class CategoryUserACL { 2 translateUserPreferencesToCategoryStructure( 3 userPrefs: UserPreferences 4 ): CategoryStructure { 5 // Isolate translation logic here 6 // Neither module knows about the other's internal model 7 } 8}

This pattern becomes essential when integrating legacy systems or third-party services into your hexagonal architecture, preventing domain model pollution.

The Testing Trade-Offs

Each communication pattern carries testing implications that many teams underestimate. Dependency inversion makes mocking straightforward but can lead to interface explosion. Domain events enable clean unit testing but require sophisticated integration test strategies.

Consider this testing matrix:

Communication PatternUnit Test ComplexityIntegration Test ComplexityMock Burden
Direct DependencyLowHighHigh
Dependency InversionMediumMediumMedium
Domain EventsLowHighLow
Anti-Corruption LayerMediumMediumMedium

The testing overhead often determines which pattern works best for your team’s maturity and testing culture.

When to Break the Rules

Architecture purity can be its own enemy. Sometimes, a carefully considered shared kernel, a small, well-defined shared module, saves you from architectural over-engineering. The key is intentionality: document why you’re breaking the pattern, establish clear ownership boundaries, and ensure the shared code represents truly cross-cutting concerns.

Many development teams fall into the trap of creating “utils” modules that become dependency magnets. Instead, consider if the shared functionality truly belongs to a separate bounded context or if it should be duplicated with module-specific variations.

The Modular Monolith Mindset

Perhaps the most important realization is that successful hexagonal architecture in monoliths requires thinking like you’re building microservices. Define clear module APIs, use DTOs for cross-module communication, and treat each module as if it might need to live on a different server someday.

As one architecture discussion highlights, exposing module APIs inside the monolith as if they were network calls forces the right discipline. This approach naturally prevents the tight coupling that makes monoliths difficult to evolve and ultimately split.

Practical Implementation Checklist

Before you commit to a communication pattern, ask these questions:

  1. Ownership: Who owns the interface definition, consumer or provider?
  2. Data Exposure: Are you exposing only what’s necessary through well-defined DTOs?
  3. Failure Modes: How does this communication pattern handle failures?
  4. Testing Strategy: Can you test modules in isolation effectively?
  5. Evolution Cost: How expensive is it to change this communication pattern later?

The answers will guide you toward the right balance between architectural purity and practical development velocity.

Hexagonal Architecture gives you the tools to build maintainable systems, but it doesn’t absolve you from making hard decisions about module boundaries. The real art lies in knowing when to apply strict ports-and-adapters purity versus when to embrace pragmatic coupling.

The most successful teams recognize that architecture is about enabling change, not preventing it. Your communication patterns should serve your domain logic, not the other way around. And sometimes, that means bending the hexagonal rules just enough to keep your team productive while maintaining the architectural integrity that makes evolution possible.

The next time you find yourself wrestling with cross-module dependencies, remember: the goal isn’t perfect hexagonal purity, it’s building software that can adapt to tomorrow’s requirements without requiring a complete rewrite today.

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