The FinTech Engineering Challenge
Financial software operates under constraints that most other software doesn't face. Every transaction must be accurate. Every record must be auditable. Every system must be secure. And it all has to scale under unpredictable load.
Immutable Financial Records
The most important architectural decision in financial software is immutability. Financial records must never be modified after posting. This isn't just good practice — it's often a regulatory requirement.
Implementation Pattern:
- Transactions are created once and marked as posted
- Corrections create reversal entries, not updates
- The audit trail is the system of record
SaleInvoice (IsPosted = true)
→ creates JournalEntry [Dr: AccountsReceivable, Cr: SalesRevenue]
→ creates StockMovement [Qty: -10, Type: Sale]
Return/Correction:
→ creates CreditNote
→ creates JournalEntry [Dr: SalesRevenue, Cr: AccountsReceivable] (reversal)
→ creates StockMovement [Qty: +10, Type: Return]Double-Entry Bookkeeping
Every financial transaction must have equal and opposite entries. Debits must equal credits. This isn't optional — it's the mathematical foundation that ensures financial reports balance.
Security Architecture for FinTech
Authentication: OAuth 2.0 with PKCE for public clients, client credentials for server-to-server
Authorization: Scope-based access control (Read, Write, Finance, Payroll)
Data Encryption: TLS 1.3 in transit, AES-256 at rest
Audit Logging: Every state change logged with timestamp, actor, and before/after values
Rate Limiting: Per-user, per-endpoint limits to prevent abuse
Compliance by Design
Build compliance into the architecture, not as an afterthought:
- Soft deletes on all records (regulatory data retention requirements)
- Immutable transaction log (audit requirements)
- Role-based access control (principle of least privilege)
- Data encryption at rest and in transit (data protection regulations)
The cost of retrofitting compliance is 10x the cost of building it in from the start.