Architecture & Financial Integrity:
- Goal: Process transactions with third-party payment gateways ensuring zero double-charges, strict PCI-DSS compliance, and accurate financial reconciliation.
1. Security & PCI-DSS Scope Reduction:
- Never allow raw credit card numbers to touch internal application servers.
- Use payment gateway client-side SDKs / hosted fields (Stripe/Cashfree) that tokenize card details directly in the user's browser, returning a secure payment token to the backend.
2. Idempotency & Concurrency Protection:
- Require an Idempotency-Key on all payment API requests.
- Check and record the idempotency key in a transactional database table before calling external payment APIs. If a duplicate request arrives, return the cached transaction status without re-charging.
3. Distributed Transaction Execution (Saga Pattern):
- Use the Saga Pattern (choreographed or orchestrated) for multi-step checkout: reserve inventory -> authorize payment -> capture payment -> update ledger.
- Implement compensating transactions (e.g., refund payment or release inventory) if downstream steps fail.
4. Double-Entry Ledger & Reconciliation:
- Implement immutable double-entry bookkeeping (debits = credits) in a relational datastore (PostgreSQL).
- Run nightly automated reconciliation jobs comparing internal ledger records against payment gateway settlement CSVs.