Why should you care?

NPCI-style transfers need exact paise matching and no duplicate settlement when clients retry HTTP.

See it live — copy this example

Run in MySQL Workbench or the mysql CLI.

START TRANSACTION;
INSERT INTO ledger_entries (account_id, txn_ref, amount_inr, entry_type)
VALUES (1, 'UPI-7788', -500.00, 'DEBIT'),
       (2, 'UPI-7788',  500.00, 'CREDIT');
UPDATE accounts SET balance_inr = balance_inr - 500 WHERE account_id = 1;
UPDATE accounts SET balance_inr = balance_inr + 500 WHERE account_id = 2;
COMMIT;

What happened?

• ledger_entries records both legs with shared txn_ref.
• Account balances update in same txn.
• Duplicate txn_ref blocked by UNIQUE — retry safe.

Practice next

1. CREATE ledger_entries with UNIQUE(txn_ref, account_id, entry_type).
2. Run transfer txn; verify sums.
3. Retry same txn_ref — should fail or no-op via app logic.
4. Add CHECK that debit amounts are negative in ledger.
5. Nightly job: find txn_ref where SUM(amount_inr) <> 0.

Remember

Double-entry: debits + credits balance. One txn_ref ties paired entries. UNIQUE keys enforce idempotent retries.

DataFlow wallet ledger

Fintech stores every UPI move in ledger_entries before push notification.

Outcome: Support can trace any balance dispute to exact entries.