Double-entry ledger: what it is and why fintech uses it

# ======================================================================
# Q: What is a double-entry ledger and why do fintech companies use it?
#
# Every movement is recorded as at least two entries (debit and credit) that sum
# to zero. Entries are append-only: we never UPDATE a balance, we add a
# reversing entry instead. The balance is the sum of entries (or a cached
# balance updated in the same transaction). It gives a full audit trail and easy
# reconciliation.
# ======================================================================

def transfer(src_id, dst_id, amount: Decimal, ref: str):
    with transaction.atomic():
        # lock in a fixed order to avoid deadlock
        accts = {a.pk: a for a in Account.objects.select_for_update()
                 .filter(pk__in=[src_id, dst_id]).order_by('pk')}
        if accts[src_id].balance < amount:
            raise InsufficientFunds
        tx = LedgerTransaction.objects.create(reference=ref)  # unique ref = idempotency
        Entry.objects.bulk_create([
            Entry(tx=tx, account_id=src_id, amount=-amount),
            Entry(tx=tx, account_id=dst_id, amount=amount)])
        Account.objects.filter(pk=src_id).update(balance=F('balance') - amount)
        Account.objects.filter(pk=dst_id).update(balance=F('balance') + amount)