Architecture Paradigm Modular Monolith

---
name: architecture-paradigm-modular-monolith
description: Applies modular monolith with enforced internal boundaries. Use when teams want service-level autonomy without distributed system overhead.
alwaysApply: false
category: architectural-pattern
tags:
- architecture
- modular-monolith
- monolith
- internal-boundaries
- team-autonomy
dependencies: []
tools: []
usage_patterns:
- paradigm-implementation
- monolith-modernization
- team-scaling
complexity: medium
model_hint: standard
estimated_tokens: 700
---
# 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.