Transaction handling in Node.js

Introduction

Transaction handling helps keep your data safe by making sure a group of actions all succeed or all fail together.

When saving multiple related pieces of data that must all be correct together, like an order and its items.

When updating several tables in a database and you want to avoid partial changes.

When transferring money between accounts to prevent losing or creating money by mistake.

When you want to undo all changes if any step in a process fails.

When working with databases that support transactions to keep data consistent.

Syntax

Node.js

async function doTransaction() {
  const client = await pool.connect();
  try {
    await client.query('BEGIN');
    // your queries here
    await client.query('COMMIT');
  } catch (e) {
    await client.query('ROLLBACK');
    throw e;
  } finally {
    client.release();
  }
}

Use BEGIN to start a transaction, COMMIT to save changes, and ROLLBACK to undo if something goes wrong.

Always release the database client after finishing to avoid connection leaks.

Examples

Start a transaction, insert a user, then save the change.

Node.js

await client.query('BEGIN');
await client.query('INSERT INTO users(name) VALUES($1)', ['Alice']);
await client.query('COMMIT');

Transfer money between accounts safely by updating both balances inside a transaction.

Node.js

try {
  await client.query('BEGIN');
  await client.query('UPDATE accounts SET balance = balance - 100 WHERE id = $1', [fromId]);
  await client.query('UPDATE accounts SET balance = balance + 100 WHERE id = $1', [toId]);
  await client.query('COMMIT');
} catch (e) {
  await client.query('ROLLBACK');
  throw e;
}

Sample Program

This program safely transfers money between two accounts. It checks the balance, updates both accounts, and uses a transaction to keep data correct.

Node.js

import pkg from 'pg';
const { Pool } = pkg;

const pool = new Pool({
  user: 'user',
  host: 'localhost',
  database: 'testdb',
  password: 'password',
  port: 5432,
});

async function transferMoney(fromId, toId, amount) {
  const client = await pool.connect();
  try {
    await client.query('BEGIN');
    const res1 = await client.query('SELECT balance FROM accounts WHERE id = $1', [fromId]);
    if (res1.rows.length === 0) throw new Error('From account not found');
    if (res1.rows[0].balance < amount) throw new Error('Insufficient funds');

    await client.query('UPDATE accounts SET balance = balance - $1 WHERE id = $2', [amount, fromId]);
    await client.query('UPDATE accounts SET balance = balance + $1 WHERE id = $2', [amount, toId]);

    await client.query('COMMIT');
    console.log('Transfer completed');
  } catch (e) {
    await client.query('ROLLBACK');
    console.error('Transfer failed:', e.message);
  } finally {
    client.release();
  }
}

// Example usage
transferMoney(1, 2, 50);

Important Notes

Always handle errors to rollback transactions and avoid corrupt data.

Transactions can lock rows, so keep them short to avoid slowing down other users.

Not all databases support transactions the same way; check your database docs.

Summary

Transactions group multiple database actions to succeed or fail together.

Use BEGIN, COMMIT, and ROLLBACK commands to control transactions.

Always release database connections and handle errors carefully.

Practice

1. What is the main purpose of using transactions in Node.js database operations?

easy

A. To ensure multiple database actions succeed or fail together

B. To speed up database queries

C. To create multiple database connections

D. To log database errors automatically

Solution

1. Step 1: Understand the role of transactions

   Transactions group several database actions so they either all succeed or all fail, maintaining data integrity.

2. Step 2: Compare options

   Only To ensure multiple database actions succeed or fail together correctly describes this purpose. Other options describe unrelated features.

3. Final Answer:

   To ensure multiple database actions succeed or fail together -> Option A

4. Quick Check:

   Transaction purpose = group actions for success/failure [OK]

Hint: Transactions keep multiple DB actions atomic and consistent [OK]

Common Mistakes:

• Thinking transactions speed up queries
• Confusing transactions with connection pooling
• Assuming transactions auto-log errors

2. Which of the following is the correct way to start a transaction using a Node.js database client?

easy

A. client.query('START TRANSACTION')

B. client.query('OPEN TRANSACTION')

C. client.query('BEGIN')

D. client.query('INIT TRANSACTION')

Solution

1. Step 1: Recall standard SQL transaction commands

   The standard command to start a transaction is 'BEGIN'.

2. Step 2: Validate options

   Only client.query('BEGIN') uses the correct SQL command 'BEGIN'. Other options are invalid SQL commands.

3. Final Answer:

   client.query('BEGIN') -> Option C

4. Quick Check:

   Start transaction command = BEGIN [OK]

Hint: Use 'BEGIN' to start transactions in SQL [OK]

Common Mistakes:

• Using non-standard commands like 'START TRANSACTION'
• Confusing transaction start with connection open
• Using commands that don't exist in SQL

3. Consider this Node.js code snippet using a PostgreSQL client:

await client.query('BEGIN');
await client.query('INSERT INTO users(name) VALUES($1)', ['Alice']);
await client.query('COMMIT');
console.log('Transaction complete');

What will be printed if all queries succeed?

medium

A. Transaction complete

B. Error: Transaction failed

C. No output

D. Transaction rolled back

Solution

1. Step 1: Analyze the transaction flow

   The code starts a transaction, inserts a user, then commits the transaction.

2. Step 2: Check output after successful commit

   Since all queries succeed, the commit happens and the console logs 'Transaction complete'.

3. Final Answer:

   Transaction complete -> Option A

4. Quick Check:

   Successful commit logs message [OK]

Hint: Successful commit prints confirmation message [OK]

Common Mistakes:

• Assuming errors print without exceptions
• Confusing commit with rollback output
• Ignoring async/await behavior

4. Identify the error in this Node.js transaction code snippet:

try {
  await client.query('BEGIN');
  await client.query('INSERT INTO orders(id) VALUES(1)');
  await client.query('COMMIT');
} catch (e) {
  await client.query('ROLLBACK');
  console.error('Transaction failed');
}

medium

A. Missing await before ROLLBACK query

B. No error handling for ROLLBACK failure

C. BEGIN should be after INSERT query

D. COMMIT should be inside catch block

Solution

1. Step 1: Review transaction error handling

   The code correctly uses try-catch to rollback on error.

2. Step 2: Identify missing error handling for rollback

   If the ROLLBACK query itself fails, this code does not catch that error, which can cause unhandled exceptions.

3. Final Answer:

   No error handling for ROLLBACK failure -> Option B

4. Quick Check:

   Rollback errors need separate handling [OK]

Hint: Always handle errors from ROLLBACK queries too [OK]

Common Mistakes:

• Assuming ROLLBACK never fails
• Placing BEGIN after queries
• Putting COMMIT inside catch block

5. You want to perform two related database inserts in Node.js using transactions. Which pattern correctly ensures both inserts succeed or both fail, releasing the client connection properly?

const client = await pool.connect();
try {
  await client.query('BEGIN');
  await client.query('INSERT INTO accounts(id) VALUES(1)');
  await client.query('INSERT INTO profiles(account_id) VALUES(1)');
  await client.query('COMMIT');
} catch (e) {
  await client.query('ROLLBACK');
  throw e;
} finally {
  client.release();
}

hard

A. Throwing error in catch block breaks transaction

B. client.release() should be called before COMMIT

C. ROLLBACK should be inside finally block

D. This pattern is correct and safely handles transactions and connection release

Solution

1. Step 1: Check transaction flow and error handling

   The code starts a transaction, performs two inserts, commits if successful, rolls back on error, and rethrows the error.

2. Step 2: Verify connection release

   The client connection is released in the finally block, ensuring it always happens regardless of success or failure.

3. Final Answer:

   This pattern is correct and safely handles transactions and connection release -> Option D

4. Quick Check:

   Try-catch-finally with commit, rollback, release = safe pattern [OK]

Hint: Use try-catch-finally with commit, rollback, and release [OK]

Common Mistakes:

• Releasing connection too early
• Placing rollback in finally block
• Not rethrowing errors after rollback