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From Monolith to Modules: Refactoring Payment Providers in NestJS

A Real-World Guide to Self-Registering Architectures and Cleaner Code

Oluwakemi Awosile

Tired of god factories and 200-parameter constructors? In one intense week, I turned a payment provider monolith into modular, self-registering NestJS heaven. This book shows you how without losing your mind (mostly).

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About

About

About the Book

Learn how to escape monolithic factories in NestJS payment systems.
Based on my real 1-week refactor that turned crushing tech debt into clean, maintainable, modular code.
Includes ready-to-use patterns, templates, and hard-won lessons; plus the personal cost of that sprint (yes, eye twitch included 😅).

Whether you're maintaining a fintech backend or scaling a NestJS app, this book delivers battle-tested strategies to make your code cleaner, more extensible, and truly team-friendly.

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TypeScriptAPI DesignSystem IntegrationEnterprise ArchitectureSoftware Architecture

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Author

About the Author

Oluwakemi Awosile

Oluwakemi Awosile is a Senior Backend Engineer with over 10 years of experience building scalable fintech systems. Former Tech Lead at Interswitch (Nigeria’s leading payment platform), she now works in a fintech company, specializing in NestJS architecture and refactoring legacy code into maintainable, modular designs.

Her 1-week refactor of a monolithic payment provider system - born from a failing CI/CD pipeline - transformed tech debt into clean, extensible code and inspired this book.

Oluwakemi is passionate about real-world engineering, clean architecture, and sharing lessons from the trenches (including the occasional eye twitch 😅).

Contents

Table of Contents

From Monolith to Modules

  1. Refactoring Payment Providers in NestJS
  2. Refund Policy

The Refactor That Saved Our CI/CD Pipeline

  1. A Story of Technical Debt, Memory Limits, and One Intense Week
  2. The Breaking Point
  3. The Diagnosis
  4. The Architecture Before
  5. The Solution: Self-Registering Modules
  6. The Sprint
  7. The Results
  8. The Lessons
  9. What’s Next

Anatomy of a God-Module

  1. How Good Intentions Create Architectural Nightmares
  2. What Is a God-Module?
  3. Case Study: The Hundred-Plus Registrations
  4. The Constructor Injection Trap
  5. The Hidden Cost of Duplicate Instances
  6. Why Payment Systems Attract This Debt
  7. Signs Your Codebase Is Suffering
  8. The NestJS Way: Module Encapsulation
  9. What’s Next

The Registry Pattern

  1. Zero-Dependency Factories That Scale to Infinity
  2. The Core Transformation
  3. Why This Works with NestJS
  4. The Module Hierarchy: Core, Shared, and Providers
  5. The Dependency Flow: Why No Circular Dependencies
  6. The Orchestrator Pattern
  7. What’s Next

Self-Registering Modules with useFactory

  1. Complete Templates You Can Copy and Paste
  2. Anatomy of a Provider Module
  3. The Module Declaration
  4. Breaking Down the Pattern
  5. Multi-Provider Modules
  6. Factory Registration Reference
  7. Common Mistakes to Avoid
  8. What’s Next

Edge Cases and Troubleshooting

  1. When Things Go Wrong (And How to Fix Them)
  2. The “Provider Not Registered” Error
  3. Providers with Partial Capabilities
  4. Type Casting for Provider-Specific Methods
  5. Architectural Violations: When Providers Depend on Domain Logic
  6. Testing Provider Modules in Isolation
  7. The Accidental Singleton Problem
  8. What’s Next

Migration Strategy

  1. Zero Downtime, Continuous Delivery, No Big Bang
  2. The Hybrid Factory: Old and New Together
  3. The Migration Sequence
  4. Batching Strategy: How Many at Once?
  5. The Messy Middle: When Performance Gets Worse
  6. Handling Test Failures
  7. Continuing Feature Development During Migration
  8. Measuring Progress
  9. The Final Push: Removing Legacy Code
  10. Timeline Expectations
  11. What’s Next

Scaling to Hundreds of Providers

  1. Organization, Discoverability, and Long-Term Maintainability
  2. Folder Structure: Organizing Provider Modules
  3. My Recommendation
  4. Naming Conventions That Scale
  5. The Aggregator Hierarchy
  6. Selective Imports: Not Everything Needs Everything
  7. Adding New Providers: The Checklist
  8. Documentation for Discoverability

Payment Providers Registry

  1. BNPL Providers
  2. Card Processors
  3. Performance Considerations at Scale
  4. What’s Next

Sustainable Engineering

  1. The Human Element of Technical Transformation
  2. Getting Buy-In for Refactoring Work
  3. Communicating Progress
  4. Handling Setbacks
  5. Documenting Your Work

ADR: Payment Provider Module Architecture

  1. Context
  2. Decision
  3. Consequences
  4. The Career Impact of Architectural Work
  5. Preventing Regression
  6. Final Thoughts

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