Implement Event Sourcing for Audit Trail

[Dev-Vik-Tor]

Implement Event Sourcing for Audit Trail

Overview

This PR implements a complete event sourcing system for immutable audit trails to support regulatory compliance and fraud investigation. The implementation provides cryptographically-linked event chains with tamper detection capabilities.

Closes #1146

🚀 Changes Implemented

Database Schema

• Added Event model with immutable event log structure
  • eventId: Global unique event identifier
  • streamId: Reference to related stream
  • eventType: Event classification (CREATE, WITHDRAW, CANCEL, etc.)
  • payload: Complete event data as JSON
  • timestamp: Event occurrence time
  • hash: SHA-256 hash of event data
  • previousHash: Hash chain linking for integrity

Core Services

EventSourceService

• Purpose: Append events to immutable log with hash chaining
• Location: src/services/event-source.service.ts
• Features:
  • Atomic event appending with hash chain verification
  • Stream-specific event retrieval
  • Event type filtering
  • Recent events query support

HashChainVerificationService

• Purpose: Verify cryptographic integrity of event chain
• Location: src/services/hash-chain-verification.service.ts
• Features:
  • Full chain integrity verification
  • Range-based verification for performance
  • Tampering detection and reporting
  • Comprehensive error reporting

StreamMigrationService

• Purpose: Migrate existing data to event sourcing format
• Location: src/services/stream-migration.service.ts
• Features:
  • Existing stream data migration
  • Audit log migration with timestamp preservation
  • Backward compatibility maintenance

Utilities & Scripts

Hash Chain Utility

• Location: src/lib/event-hash-chain.ts
• Purpose: SHA-256 hash computation for event integrity
• Features: Deterministic canonical JSON serialization

Migration Script

• Location: src/scripts/migrate-to-event-sourcing.ts
• Purpose: Production-ready data migration
• Features:
  • Automated migration execution
  • Post-migration integrity verification
  • Error handling and rollback support

Testing

Comprehensive Test Suite

• Location: src/__jest__/event-sourcing.test.ts
• Coverage:
  • ✅ 100-event chain integrity verification (requirement fulfilled)
  • Hash chain tampering detection
  • Empty chain handling
  • Range verification functionality
  • Error condition testing

🔒 Security Features

• Immutable Events: Once written, events cannot be modified
• Cryptographic Chaining: SHA-256 hash chains prevent tampering
• Integrity Verification: Automated detection of broken chains
• Audit Trail: Complete history for regulatory compliance

📊 Impact

Regulatory Compliance

• Immutable audit logs for financial regulations
• Complete event history with cryptographic proof
• Fraud investigation support with tamper evidence

Performance Considerations

• Indexed queries for efficient event retrieval
• Range-based verification for large datasets
• Optimized hash computation

Backward Compatibility

• Existing audit log data preserved
• Migration maintains historical timestamps
• No breaking changes to current API

🧪 Testing

• Hash Chain Integrity: Verified with 100 sequential events
• Tampering Detection: Confirms detection of modified events
• Migration Safety: Tests data preservation during migration
• Performance: Range queries tested for large datasets

📁 Files Changed

New Files

• src/services/event-source.service.ts - Core event sourcing
• src/services/hash-chain-verification.service.ts - Integrity verification
• src/services/stream-migration.service.ts - Data migration
• src/lib/event-hash-chain.ts - Hash utilities
• src/scripts/migrate-to-event-sourcing.ts - Migration script
• src/__jest__/event-sourcing.test.ts - Test suite

Modified Files

• prisma/schema.prisma - Added Event model
• src/services/index.ts - Service exports

🚀 Deployment Notes

1. Database Migration: Run Prisma migration to create Event table
2. Data Migration: Execute migration script to populate event log
3. Verification: Run integrity check post-migration
4. Monitoring: Monitor hash chain verification in production

✅ Requirements Fulfilled

• Create events table with required schema
• Implement EventSourceService for appending events
• Write hash chain verification utility
• Migrate existing streams to event log
• Add test verifying hash chain integrity for 100 events

This implementation provides the foundation for regulatory compliance and fraud investigation through immutable, cryptographically-verified audit trails.