🧹 Clean Event-Driven Architecture Refactoring

🎯 Goals Achieved

This refactoring successfully addressed the messy event handler registration and achieved the following goals:

• ✅ No misleading events - Clear event flow and responsibilities
• ✅ DRY (Don't Repeat Yourself) - Generic payment handlers instead of operation-specific ones
• ✅ SRP (Single Responsibility Principle) - Each handler has one clear responsibility
• ✅ No payment handlers per operation - Single generic payment handlers that subscribe to multiple validation events

🔄 Before vs After

🧨 Before (Messy)

// Operation-specific payment handlers (violates DRY)
bus.Subscribe("Deposit.CurrencyConverted", deposithandler.ConversionDoneHandler(bus, deps.Uow, deps.PaymentProvider, deps.Logger))
bus.Subscribe("Withdraw.CurrencyConverted", withdrawhandler.ConversionDoneHandler(bus, deps.Uow, deps.PaymentProvider, deps.Logger))

// Duplicate payment persistence handlers (violates DRY)
bus.Subscribe("Payment.Initiated", deposithandler.PaymentPersistenceHandler(bus, deps.Uow, deps.Logger))
bus.Subscribe("Payment.Initiated", withdrawhandler.PaymentPersistenceHandler(bus, deps.Uow, deps.Logger))

// Conversion handlers doing payment initiation (violates SRP)
// ConversionDoneHandler was doing both business validation AND payment initiation

✨ After (Clean)

// Generic payment handlers (DRY)
bus.Subscribe("Deposit.Validated", paymenthandler.PaymentInitiationHandler(bus, deps.PaymentProvider, deps.Logger))
bus.Subscribe("Withdraw.Validated", paymenthandler.PaymentInitiationHandler(bus, deps.PaymentProvider, deps.Logger))

// Single generic payment persistence handler (DRY)
bus.Subscribe("Payment.Initiated", paymenthandler.PaymentPersistenceHandler(bus, deps.Uow, deps.Logger))

// Conversion handlers focus only on business validation (SRP)
bus.Subscribe("Deposit.CurrencyConverted", deposithandler.ConversionDoneHandler(bus, deps.Uow, deps.Logger))
bus.Subscribe("Withdraw.CurrencyConverted", withdrawhandler.ConversionDoneHandler(bus, deps.Uow, deps.Logger))

🏗️ Architecture Changes

💳 1. Generic Payment Initiation Handler

Location: pkg/handler/payment/initiation_handler.go

Responsibilities:

• Handles Deposit.Validated and Withdraw.Validated
• Initiates payment with the payment provider
• Emits Payment.Initiated

Benefits:

• ✅ DRY: Single handler for all payment initiation
• ✅ SRP: Only handles payment initiation, not business validation
• ✅ Extensible: Easy to add new validation events (e.g., Transfer.Validated)

💾 2. Generic Payment HandleProcessed Handler

Location: pkg/handler/payment/persistence_handler.go

Responsibilities:

• Handles Payment.Initiated for all operations
• Updates transaction with payment ID
• Emits PaymentIdPersistedEvent

Benefits:

• ✅ DRY: Single handler for all payment persistence
• ✅ SRP: Only handles payment persistence, not business logic
• ✅ Consistent: Same persistence logic for all operations

✅ 3. Clean Conversion Done Handlers

Location: pkg/handler/account/deposit/conversion_done.go and pkg/handler/account/withdraw/conversion_done.go

Responsibilities:

• Handle *CurrencyConverted events (e.g., Deposit.CurrencyConverted, Withdraw.CurrencyConverted)
• Perform business validation after conversion
• Emit validation events to trigger payment initiation

Benefits:

• ✅ SRP: Only handles business validation, not payment initiation
• ✅ Clear Flow: Validation → Payment Initiation (not Validation + Payment)
• ✅ Testable: Easy to test business validation logic separately

🔄 Event Flow

📥 Deposit Flow

Deposit.Requested → ValidationHandler → Deposit.Validated → PaymentInitiationHandler → Payment.Initiated → PaymentPersistenceHandler

📤 Withdraw Flow

Withdraw.Requested → ValidationHandler → Withdraw.Validated → PaymentInitiationHandler → Payment.Initiated → PaymentPersistenceHandler

🔄 Transfer Flow (No Payment)

Transfer.Requested → ValidationHandler → Transfer.Validated → DomainOpHandler → Transfer.Completed → PersistenceHandler

🧪 Testing

🧪 Updated Tests

• ✅ Payment Initiation Handler: Tests for Deposit.Validated and Withdraw.Validated
• ✅ Payment HandleProcessed Handler: Tests for Payment.Initiated
• ✅ Conversion Done Handlers: Tests for business validation only

📈 Test Coverage

go test ./pkg/handler/payment/... -v  # ✅ All passing
go test ./pkg/handler/... -v          # ✅ All passing

🗂️ File Changes

➕ Added/Modified

• ✅ pkg/handler/payment/initiation_handler.go - Generic payment initiation
• ✅ pkg/handler/payment/persistence_handler.go - Generic payment persistence
• ✅ pkg/handler/account/deposit/conversion_done.go - Clean business validation
• ✅ pkg/handler/account/withdraw/conversion_done.go - Clean business validation
• ✅ app/app.go - Updated event handler registration

🗑️ Removed

• ❌ pkg/handler/account/deposit/payment_persistence.go - Duplicate code
• ❌ pkg/handler/account/withdraw/payment_persistence.go - Duplicate code

🎯 Key Principles Applied

📣 1. Event-Driven Design

• Events trigger next steps, not conditional logic
• Clear event flow: Validation → Payment → HandleProcessed
• No if-else statements for control flow

🎯 2. Single Responsibility Principle

• Each handler has one clear responsibility
• Conversion handlers: Business validation only
• Payment handlers: Payment operations only
• HandleProcessed handlers: Database operations only

♻️ 3. Don't Repeat Yourself

• Generic payment handlers for all operations
• Single payment persistence logic
• Reusable event structures

🧩 4. Separation of Concerns

• Business validation separated from payment initiation
• Payment logic separated from conversion logic
• HandleProcessed logic separated from business logic

🚀 Benefits

🛠️ 1. Maintainability

• Clear separation of concerns
• Easy to understand and modify individual components
• Consistent patterns across all handlers

🧪 2. Testability

• Each handler can be tested independently
• Clear input/output expectations
• Easy to mock dependencies

🧱 3. Extensibility

• Easy to add new operations (e.g., loan payments)
• Easy to add new payment providers
• Easy to add new validation rules

🛡️ 4. Reliability

• No duplicate code to maintain
• Clear error handling paths
• Consistent event flow

🔮 Future Enhancements

➕ 1. Add Transfer Payment Support

// Easy to extend - just add new event subscription
bus.Subscribe("Transfer.Validated", paymenthandler.PaymentInitiationHandler(bus, deps.PaymentProvider, deps.Logger))

✅ 2. Add Payment Completion Handlers

// Generic payment completion handling
bus.Subscribe("Payment.Completed", paymenthandler.PaymentCompletionHandler(bus, deps.Uow, deps.Logger))

🚨 3. Add Payment Failure Handlers

// Generic payment failure handling
bus.Subscribe("Payment.Failed", paymenthandler.PaymentFailureHandler(bus, deps.Uow, deps.Logger))

📚 Related Documentation

• Event-Driven Architecture
• Event-Driven Deposit Flow
• Event-Driven Withdraw Flow
• Event-Driven Transfer Flow
• Domain Events