Transactions

ORM transactions group multiple reads and writes under one database transaction. Use them to protect business invariants—not as a default wrapper around every HTTP request.

					await db.transaction(async (tx) => {
  const balance = await tx.account.findForUpdate(id);
  if (balance.amount < transfer) throw new Error('insufficient');
  await tx.account.debit(id, transfer);
  await tx.account.credit(targetId, transfer);
});
				

On interviews: discuss scope, isolation anomalies, retries, outbox patterns for messages, and avoiding network I/O while holding locks.

Common pitfalls: long transactions increasing contention; payment or message calls inside a DB transaction; retrying without idempotency keys.

The trade-off is correctness guarantees versus lock time and connection pressure.

Checklist:

• Define the invariant being protected.
• Keep lock time short.
• Plan retries and side effects.
• Separate DB commit from external calls.