Modularization Skill

Overview

This skill teaches architects how to decompose systems into well-defined modules with clear boundaries, minimal coupling, and high cohesion. Proper modularization is the foundation of maintainable, scalable, and testable software systems.

What is a Module?

A module is a self-contained unit of code that:

•Has a single, well-defined responsibility
•Exposes a clear public interface (API)
•Hides its internal implementation details
•Can be developed, tested, and deployed independently
•Has explicit dependencies on other modules

Why Modularization Matters:

Benefit	Explanation
Maintainability	Changes to one module do not ripple through the entire system
Testability	Modules can be tested in isolation with mocked dependencies
Scalability	Teams can work on different modules in parallel
Reusability	Well-designed modules can be reused across projects
Understandability	Smaller modules are easier to comprehend than monolithic code

Prerequisites

•Understanding of the system's functional requirements
•Access to existing codebase (if refactoring)
•Knowledge of domain concepts and bounded contexts
•Familiarity with dependency analysis tools

Instructions

Use this skill when:

•Starting a new project and need to plan its structure
•Refactoring a monolithic application into modules
•Analyzing existing code for coupling problems
•Designing APIs between system components
•Deciding where to place shared functionality
•Evaluating module boundaries for a microservices migration

Execute these phases in order:

•Principle Application - Apply SOLID principles to guide decomposition
•Boundary Identification - Find natural module boundaries
•API Design - Define interfaces between modules
•Dependency Management - Manage module dependencies and shared code
•Validation - Verify modularization quality

Checklist

Copy this checklist and track your progress:

Output

Phase	Output Artifact	Contents
Principle Application	Principle analysis document	SOLID compliance assessment for each module
Boundary Identification	Module boundary map	Modules, responsibilities, bounded contexts
API Design	API specification	Interface definitions, contracts, DTOs
Dependency Management	Dependency graph	Module relationships, shared code strategy
Validation	Quality report	Coupling metrics, cohesion scores, test strategy

Core Modularization Principles

For detailed principles, see references/solid-principles.md:

•
1. •Single Responsibility Principle (SRP) - One reason to change
•
1. •Interface Segregation Principle (ISP) - Small, focused interfaces
•
1. •Dependency Inversion Principle (DIP) - Depend on abstractions
•
1. •High Cohesion - Related elements together
•
1. •Low Coupling - Minimal inter-module dependencies

Module Boundary Identification

For detailed boundary identification, see references/boundary-patterns.md:

•Step 1: Map Domain Concepts
•Step 2: Identify Change Vectors
•Step 3: Find Natural Seams
•Step 4: Define Bounded Contexts
•Step 5: Validate Boundaries

API Design Between Modules

For detailed API design, see references/api-design-guide.md:

•API Design Principles (Minimal Surface, Versioning, Compatibility)
•API Contract Templates
•Versioning Strategy
•Error Handling Contracts

Shared Code Management

For shared code strategies, see references/dependency-analysis.md:

•The Shared Code Problem
•Shared Module Guidelines
•Dependency Direction Rules
•Circular Dependency Detection

Breaking Up Monoliths

For monolith decomposition, see references/strangler-pattern.md:

•The Strangler Fig Pattern
•Step-by-Step Monolith Decomposition
•Module Dependency Graphs
•Detecting Circular Dependencies

Module Testing

For testing strategies, see references/module-testing.md:

•Testing Strategies by Scope
•Module Testing Pattern
•Mocking Module Dependencies
•Contract Testing

Error Handling

Problem	Cause	Solution
Module too large	Multiple responsibilities	Apply SRP - split into focused modules
Circular dependency detected	Modules depend on each other	Extract common code or invert dependency
Cannot test module in isolation	Hidden dependencies	Apply DIP - inject dependencies
API breaks consumers	Missing versioning	Implement semantic versioning
Shared module causes ripple changes	Too much shared code	Reduce shared code surface, consider duplication
Unclear module boundaries	No bounded context analysis	Map domain concepts, identify change vectors
High coupling detected	Direct concrete dependencies	Introduce interfaces, reduce API surface
Low cohesion in module	Unrelated functionality	Move unrelated code to appropriate modules

Examples

Example 1: E-Commerce Modularization

code

Domain Analysis:
- Products, Orders, Customers, Payments, Shipping

Module Breakdown:
1. ProductCatalog Module
   - Product CRUD
   - Category management
   - Search/filtering

2. OrderManagement Module
   - Order creation/cancellation
   - Order status tracking
   - Order history

3. CustomerModule
   - Customer profiles
   - Authentication
   - Address management

4. PaymentModule
   - Payment processing
   - Refunds
   - Payment methods

5. ShippingModule
   - Shipping rate calculation
   - Label generation
   - Tracking

6. InventoryModule
   - Stock levels
   - Reservations
   - Reorder alerts

Dependency Graph:
  WebAPI
    |
    +-> ProductCatalog
    +-> OrderManagement --> PaymentModule
    |                  |-> ShippingModule
    |                  +-> InventoryModule
    +-> CustomerModule

Example 2: Breaking Up User Monolith

code

Before (Monolith):
UserService (2000 lines)
  - registerUser()
  - authenticateUser()
  - resetPassword()
  - updateProfile()
  - uploadAvatar()
  - sendVerificationEmail()
  - checkPermissions()
  - generateUserReport()

After (Modules):
1. AuthenticationModule
   - authenticateUser()
   - resetPassword()

2. UserProfileModule
   - updateProfile()
   - uploadAvatar()

3. RegistrationModule
   - registerUser()
   - sendVerificationEmail()

4. PermissionsModule
   - checkPermissions()

5. UserReportingModule
   - generateUserReport()

Resources

Resource	Use When
SOLID Principles Reference	Applying design principles
Boundary Patterns	Identifying module boundaries
API Design Guide	Designing module interfaces
Dependency Analysis	Analyzing and visualizing dependencies
Strangler Pattern	Breaking up monoliths gradually
Module Testing Guide	Testing modular systems

Summary

Phase	Purpose	Key Output
Principle Application	Apply SOLID to guide design	Principle compliance checklist
Boundary Identification	Find natural module boundaries	Module boundary map
API Design	Define module interfaces	API contracts
Dependency Management	Manage relationships	Dependency graph
Validation	Verify quality	Coupling/cohesion metrics

Key Principles:

•Single Responsibility - One reason to change per module
•Interface Segregation - Small, focused interfaces
•Dependency Inversion - Depend on abstractions
•High Cohesion - Related elements together
•Low Coupling - Minimal inter-module dependencies

Troubleshooting

Issue	Cause	Solution
Circular dependency detected	Modules depend on each other bidirectionally	Use `tldr arch` to visualize layers, extract shared interface to utility module, apply Dependency Inversion Principle
Module boundaries unclear	Insufficient domain analysis or overlapping responsibilities	Review `boundary-patterns.md`, map domain concepts again, identify change vectors, redefine bounded contexts
Cannot test module in isolation	Hidden dependencies or tight coupling	Apply Dependency Injection, create interface abstractions, use mocking framework for dependencies
API breaking consumer modules	Missing versioning strategy or backward compatibility	Implement semantic versioning, add deprecation warnings before removal, maintain parallel versions during transition
High coupling metrics	Direct concrete dependencies between modules	Introduce interface abstractions, reduce API surface area, review `api-design-guide.md` for minimal surface pattern
Module too large (>500 LOC)	Multiple responsibilities violating SRP	Split by responsibility using `solid-principles.md`, create sub-modules with focused interfaces
Shared module causes ripple changes	Over-shared code between modules	Review `dependency-analysis.md`, reduce shared surface, consider strategic duplication over coupling
Deployment dependencies blocking independence	Runtime coupling or database sharing	Plan strangler pattern migration using `strangler-pattern.md`, introduce service boundaries, separate databases

Next Steps

•Read core principles section thoroughly
•Inventory your current system or planned system
•Map domain concepts to candidate modules
•Identify change vectors and bounded contexts
•Validate boundaries with stakeholders
•Design APIs between modules
•Create dependency graph
•Plan extraction sequence (for monoliths)
•Implement module tests
•Iterate and refine boundaries as needed

Final Note: Perfect modularization is achieved iteratively. Start with clear principles, make reasonable boundary decisions, and refine as you learn more about the system's actual behavior and change patterns.