architecture-paradigm-modular-monolith

The Modular Monolith Paradigm

When To Use

• Organizing large codebases into well-bounded modules
• Teams wanting microservice boundaries without distributed complexity

When NOT To Use

• Already distributed as microservices
• Tiny applications where module boundaries add unnecessary complexity

When to Employ This Paradigm

• When you desire team autonomy similar to that of microservices, but without the operational overhead of a distributed system.
• When release velocity is slowed by tangled dependencies between internal modules.
• When a monolithic architecture is simpler to operate today, but there is a clear need to evolve toward a service-based model in the future.

Adoption Steps

1. Identify Modules: Define module boundaries that align with distinct business capabilities or Bounded Contexts from Domain-Driven Design.
2. Encapsulate Internals: Use language-level visibility modifiers (e.g., public/private), separate packages, or namespaces to hide the implementation details of each module.
3. Expose Public Contracts: Each module should expose its functionality through well-defined facades, APIs, or events. Forbid direct database table access or direct implementation calls between modules.
4. Enforce Architectural Fitness: Implement automated tests that fail the build if forbidden dependencies or package references are introduced between modules.
5. Plan for Evolution: Continuously track metrics such as change coupling and deployment scope to make informed decisions about if and when to split a module into a separate service.

Key Deliverables

• An Architecture Decision Record (ADR) that maps module boundaries and defines the rules for any shared code.
• Formal contract documentation (e.g., OpenAPI specs, event schemas) for every interaction point between modules.
• Automated dependency checks and dedicated CI/CD jobs for each module to enforce boundaries.

Risks & Mitigations

• Regression to a "Big Ball of Mud":
  • Mitigation: Without strict enforcement, module boundaries will inevitably erode. Treat any boundary violation as a build-breaking error and maintain a disciplined approach to code reviews.
• Shared Database Hotspots:
  • Mitigation: High contention on a shared database can become a bottleneck. Introduce clear schema ownership, use view-based access to restrict data visibility, or implement data replication strategies to reduce coupling.

Troubleshooting

Common Issues

Skill not loading Check YAML frontmatter syntax and required fields

Token limits exceeded Use progressive disclosure - move details to modules

Modules not found Verify module paths in SKILL.md are correct

Concrete Components

Vocabulary for the tools and abstractions an implementation of this paradigm tends to carry. Not dependencies, and not tools: frontmatter.

• dependency-analyzer: module dependency graph builder for spotting forbidden edges
• module-boundary-enforcer: fails the build when a module imports across a boundary
• refactoring-planner: ranks modules by extraction-readiness for a future split

Exit Criteria

• An ADR maps module boundaries to business capabilities, defines rules for shared code, and states the conditions under which a module would be extracted into a separate service.
• Each module exposes functionality only through documented facades, APIs, or events; direct database table access between modules is absent (verified by schema ownership review).
• Automated dependency checks fail the CI build on any forbidden cross-module import, and at least one such violation test case is documented.
• Change-coupling metrics (modules that always change together) are tracked and any module pair with > 50% coupling is flagged as a candidate for boundary review.