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Refactoring Journey 🔄

Overview

This section documents our refactoring journey as we evolved from a simple layered architecture to a sophisticated event-driven, multi-currency financial system. Each refactor was driven by real business needs and technical challenges.

Key Refactoring Milestones

1. Event-Driven Architecture 🔄

Status: ✅ Completed

Our final architecture that decouples currency conversion from payment processing and business validation.

Key Design Decisions:

  • Decoupled Currency Conversion: Pure, reusable service with no side effects
  • Business Validation After Conversion: All checks performed in account's native currency
  • Payment Triggered by Business Validation: Not by conversion completion
  • Event Chaining Pattern: Clear flow of dependent operations

Benefits Achieved:

  • ✅ Correct multi-currency validation
  • ✅ Reusable conversion logic
  • ✅ No if-statements for control flow
  • ✅ Clear separation of concerns
  • ✅ Flexible payment triggers

View Final Architecture →

2. Lessons Learned 📚

Status: ✅ Completed

Documentation of key problems solved and design decisions that led to our final architecture.

Key Insights:

  • Currency conversion should be pure and reusable
  • Business validation must happen after conversion
  • Event chaining eliminates if-statements
  • Payment should be triggered by business validation

View Lessons Learned →

3. Account Service Refactor 🏦

Status: ✅ Completed

Refactored account service to follow clean architecture principles with proper separation of concerns.

Key Changes:

  • Separated domain logic from infrastructure
  • Introduced repository pattern
  • Added proper error handling
  • Improved testability

View Account Service Refactor →

4. Decorator Pattern Implementation 🎨

Status: ✅ Completed

Implemented decorator pattern for cross-cutting concerns like logging, caching, and validation.

Key Benefits:

  • Clean separation of concerns
  • Reusable cross-cutting logic
  • Easy to add new decorators
  • Improved testability

View Decorator Pattern →

5. Event-Driven Deposit Flow 💰

Status: ✅ Completed

Implemented event-driven architecture for deposit operations with currency conversion.

Key Features:

  • Event-driven validation
  • Currency conversion handling
  • Payment integration
  • Transactional persistence

View Deposit Flow →

6. Event-Driven Withdraw Flow 💸

Status: ✅ Completed

Implemented event-driven architecture for withdraw operations with proper validation.

Key Features:

  • Sufficient funds validation
  • Currency conversion
  • Payment processing
  • Transaction recording

View Withdraw Flow →

7. Event-Driven Transfer Flow 🔄

Status: ✅ Completed

Implemented event-driven architecture for internal transfers between accounts.

Key Features:

  • Source and target validation
  • Currency conversion
  • Domain transfer operations
  • Transactional consistency

View Transfer Flow →

8. Unit of Work Pattern Implementation 🏗️

Status: ✅ Completed

Implemented Unit of Work pattern for transactional consistency across repositories.

Key Benefits:

  • Transactional consistency
  • Clean repository interfaces
  • Easy to test
  • Proper error handling

View Unit of Work Pattern →

9. Transaction CQRS Refactor 📊

Status: ✅ Completed

Refactored transaction handling to follow CQRS (Command Query Responsibility Segregation) pattern.

Key Changes:

  • Separated read and write operations
  • Optimized queries
  • Improved performance
  • Better scalability

View CQRS Refactor →

10. Transaction HandleProcessed Refactor 💾

Status: ✅ Completed

Refactored transaction persistence to use proper repository pattern with Unit of Work.

Key Improvements:

  • Consistent persistence logic
  • Transactional operations
  • Better error handling
  • Improved testability

View HandleProcessed Refactor →

11. Layered Dependency Injection 🏛️

Status: ✅ Completed

Implemented proper dependency injection across all layers of the application.

Key Benefits:

  • Loose coupling
  • Easy testing
  • Clear dependencies
  • Maintainable code

View Dependency Injection →

Architecture Evolution

Before Refactoring

Simple Layered Architecture
├── Controllers
├── Services
├── Repositories
└── Database

After Refactoring

Event-Driven Clean Architecture
├── Event Bus
├── Event Handlers
├── Domain Services
├── Repositories (with UoW)
├── Infrastructure
└── Cross-cutting Concerns (Decorators)

Key Principles Established

1. Event-Driven Design

  • Events as First-Class Citizens: All business operations emit events
  • Event Chaining: Dependent operations flow through events
  • Decoupled Handlers: Each handler has a single responsibility

2. Clean Architecture

  • Domain-Driven Design: Business logic in domain layer
  • Dependency Inversion: Depend on abstractions, not concretions
  • Separation of Concerns: Clear boundaries between layers

3. Multi-Currency Support

  • Currency Conversion: Pure, reusable service
  • Business Validation: Always in account's native currency
  • Flexible Payment: Triggered by business validation

4. Testing Excellence

  • Unit Tests: Each component tested in isolation
  • Integration Tests: End-to-end event flow testing
  • Mock Dependencies: Easy to mock external services

Benefits Achieved

1. Maintainability

  • Clear separation of concerns
  • Easy to modify individual components
  • Well-documented architecture
  • Consistent patterns

2. Testability

  • Each component can be tested in isolation
  • Mock dependencies easily injected
  • Event-driven testing patterns
  • Clear test boundaries

3. Scalability

  • Handlers can be scaled independently
  • Event bus can be distributed
  • Easy to add new business operations
  • No tight coupling

4. Flexibility

  • New currencies can be added without changing business logic
  • New payment providers can be integrated easily
  • Business rules can be modified independently
  • Clear extension points

5. Observability

  • Clear event flow for debugging
  • Structured logging at each step
  • Audit trail of all operations
  • Easy to monitor and alert

Migration Strategy

Phase 1: Foundation

  1. Implement clean architecture principles
  2. Add dependency injection
  3. Introduce repository pattern with Unit of Work

Phase 2: Event-Driven

  1. Implement event bus
  2. Create event-driven handlers
  3. Add event chaining patterns

Phase 3: Multi-Currency

  1. Implement currency conversion service
  2. Add multi-currency validation
  3. Decouple conversion from payment

Phase 4: Optimization

  1. Implement CQRS for transactions
  2. Add caching and performance optimizations
  3. Improve error handling and observability

Final Architecture Summary

Our final architecture is a sophisticated event-driven, multi-currency financial system that:

  • ✅ Decouples concerns through event-driven design
  • ✅ Handles multi-currency operations correctly
  • ✅ Maintains consistency through Unit of Work pattern
  • ✅ Scales effectively through clean separation
  • ✅ Tests thoroughly through dependency injection
  • ✅ Observes clearly through structured logging

This architecture provides a solid foundation for future growth while maintaining clean separation of concerns and enabling easy extension of business operations.

Next Steps

  1. Monitor Performance: Track system performance and optimize as needed
  2. Add New Features: Use established patterns for new business operations
  3. Improve Observability: Add metrics, tracing, and alerting
  4. Scale Infrastructure: Consider distributed event bus for high load
  5. Document Patterns: Continue documenting best practices and patterns

The refactoring journey has transformed our codebase from a simple layered application to a sophisticated, event-driven, multi-currency financial system that is maintainable, testable, scalable, and flexible.