Transaction handling in Node.js - Mini Project: Build & Apply

Transaction Handling in Node.js

📖 Scenario: You are building a simple banking system where users can transfer money between accounts. To keep data safe, you need to handle transactions properly so that either the whole transfer happens or nothing happens at all.

🎯 Goal: Build a Node.js script that uses a transaction to transfer money between two accounts safely.

📋 What You'll Learn

Create an initial accounts object with two accounts and their balances

Add a variable to hold the transfer amount

Write a function that performs the transfer inside a transaction

Complete the transaction handling with commit and rollback logic

💡 Why This Matters

🌍 Real World

Handling transactions is crucial in banking and e-commerce to keep data consistent and avoid errors during money transfers or purchases.

💼 Career

Understanding transaction handling is important for backend developers working with databases and APIs to ensure data integrity.

1

Create initial accounts data

Create a constant called accounts as an object with two keys: alice with value 500 and bob with value 300.

Node.js

// Create the accounts object with alice and bob balances
// Your code here

Hint

Use const accounts = { alice: 500, bob: 300 }; to create the object.

2

Add transfer amount variable

Add a constant called transferAmount and set it to 200.

Node.js

const accounts = { alice: 500, bob: 300 };
// Define the transfer amount
// Your code here

Hint

Use const transferAmount = 200; to set the amount to transfer.

3

Write transfer function with transaction logic

Write an async function called transfer that takes no parameters. Inside it, create a variable transactionActive set to true. Then subtract transferAmount from accounts.alice and add it to accounts.bob. If accounts.alice becomes negative, set transactionActive to false.

Node.js

const accounts = { alice: 500, bob: 300 };
const transferAmount = 200;

// Write async function transfer
// Your code here

Hint

Define the function with async function transfer(). Use let transactionActive = true; and update balances. Check if accounts.alice is negative to set transactionActive to false.

4

Complete transaction with commit or rollback

Inside the transfer function, after the balance changes, add an if statement that checks transactionActive. If true, add a comment // Commit transaction. Else, add a comment // Rollback transaction and reset accounts.alice to 500 and accounts.bob to 300.

Node.js

const accounts = { alice: 500, bob: 300 };
const transferAmount = 200;

async function transfer() {
    let transactionActive = true;
    accounts.alice -= transferAmount;
    accounts.bob += transferAmount;
    if (accounts.alice < 0) {
        transactionActive = false;
    }
    // Add commit or rollback logic here
// Your code here
}

Hint

Use if (transactionActive) { /* commit */ } else { /* rollback */ } and reset balances on rollback.

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