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:
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:
The cost of retrofitting compliance is 10x the cost of building it in from the start.