Code Quiz

Isolation Level Set Too Late

A money-transfer transaction tries to raise its isolation level, but places the command in the wrong spot.

Codejavascript

// node-postgres: transfer funds with SERIALIZABLE isolation
async function transfer(client, from, to, amount) {
  await client.query('BEGIN');
  const { rows } = await client.query(
    'SELECT balance FROM accounts WHERE id = $1', [from]
  );
  await client.query('SET TRANSACTION ISOLATION LEVEL SERIALIZABLE');
  if (rows[0].balance < amount) throw new Error('insufficient funds');
  await client.query('UPDATE accounts SET balance = balance - $1 WHERE id = $2', [amount, from]);
  await client.query('UPDATE accounts SET balance = balance + $1 WHERE id = $2', [amount, to]);
  await client.query('COMMIT');
}

What is the bug in this transaction?

Next →Isolation Levels and Phantom Reads