Event Sourcing over CRUD for Financial Transaction Audit Trails

DECISION: Adopt Event Sourcing instead of standard CRUD for financial transaction audit trails.

PROBLEM: Standard CRUD in PostgreSQL overwrites previous row states. When compliance requested the history of transaction #8821, the team could not provide who changed the status from pending to approved. Updates destroy the audit trail.

ALTERNATIVES CONSIDERED:
1. Database triggers writing to a history_logs table - REJECTED because triggers only see raw row changes (e.g., status 1→2) and lose business context, user intent, and the why behind each change.

DECISION DETAILS:
- Store an immutable sequence of domain events (e.g., TransactionCreated, TransactionApproved, StatusUpdated) instead of mutable rows
- Use Kafka as the immutable event log backing the event store
- Build a materialized view in Postgres for fast read queries on the frontend
- Follow Martin Fowlers Event Sourcing Pattern as the reference architecture (industry standard)

FILES INVOLVED:
- src/services/transactionService.ts — event producers
- src/events/eventStore.ts — consumer/store logic

TEAM: Alice (Lead), Bob (Eng), Charlie (Security)