After years of building fintech systems, here are the patterns that have served me well.
1. Idempotency is Non-Negotiable
Every financial operation must be idempotent. Duplicate charges can destroy trust.
@Transactional
public TransferResult transfer(String idempotencyKey, TransferRequest request) {
// Check if already processed
Optional<Transfer> existing = transferRepo.findByIdempotencyKey(idempotencyKey);
if (existing.isPresent()) {
return existing.get().getResult();
}
// Process transfer...
}
2. Audit Everything
Every state change needs an audit trail. Use event sourcing or at minimum, comprehensive logging.
3. Atomic Operations
Database transactions should cover entire business operations. Partial updates can lead to inconsistent states.
4. Defensive Coding
- Validate all inputs
- Handle edge cases (zero amounts, negative values)
- Use decimal types for money, never floats
5. Compliance by Design
Build regulatory requirements into your architecture from the start.