How do transactions work, and how do you use them in Node?

Detailed Answer

Answer: A transaction makes several operations succeed or fail together, preserving data integrity. It provides ACID guarantees: Atomicity, Consistency, Isolation, Durability.

Classic example — money transfer (both updates must apply or neither):

const client = await pool.connect();
try {
  await client.query('BEGIN');
  await client.query('UPDATE accounts SET balance = balance - $1 WHERE id = $2', [amt, from]);
  await client.query('UPDATE accounts SET balance = balance + $1 WHERE id = $2', [amt, to]);
  await client.query('COMMIT');
} catch (err) {
  await client.query('ROLLBACK');   // undo everything on any failure
  throw err;
} finally {
  client.release();                 // always return the connection
}

Key points:

Use one connection for the whole transaction — you can't spread BEGIN/COMMIT across pooled connections.
Always ROLLBACK on error and release() in finally.

With an ORM (Prisma):

await prisma.$transaction([
  prisma.account.update({ where: { id: from }, data: { balance: { decrement: amt } } }),
  prisma.account.update({ where: { id: to },   data: { balance: { increment: amt } } }),
]);

Isolation levels (READ COMMITTED, REPEATABLE READ, SERIALIZABLE) control what concurrent transactions can see, trading strictness for concurrency. Higher levels prevent anomalies (dirty/non-repeatable/phantom reads) but can cause serialization failures you must retry.

When you need them: any multi-step write that must be consistent — orders + inventory, transfers, "create user + profile + audit log." Skipping transactions here leads to partial writes and corrupted state.

How do transactions work, and how do you use them in Node?

Quick Answer

Detailed Answer