Building FinGreat: Multi-Currency Transaction Engines with Django & GoLang

from django.db import transaction
from decimal import Decimal

@transaction.atomic
def process_settlement_transfer(sender_acc_id, receiver_acc_id, amount_usd, fx_rate):
    # 1. Pessimistic Row Lock on Accounts
    sender = Account.objects.select_for_update().get(id=sender_acc_id)
    receiver = Account.objects.select_for_update().get(id=receiver_acc_id)

    if sender.balance < amount_usd:
        raise InsufficientFundsException("Debit exceeds available balance")

    amount_ngn = amount_usd * fx_rate

    # 2. Immutable Journal Lines
    JournalEntry.objects.create(account=sender, debit=amount_usd, credit=Decimal("0.00"))
    JournalEntry.objects.create(account=receiver, debit=Decimal("0.00"), credit=amount_ngn)

    # 3. Update Materialized Balances
    sender.balance -= amount_usd
    receiver.balance += amount_ngn
    sender.save()
    receiver.save()