Transaction handling in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a transaction using a Node.js database client.

Node.js

const client = await pool.connect();
try {
  await client.query('[1]');
  // perform queries
} finally {
  client.release();
}

Drag options to blanks, or click blank then click option'

AbeginTransaction

BBEGIN

CstartTransaction

DinitTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'beginTransaction' which is not the correct command name in this client.

Using 'startTransaction' or 'initTransaction' which do not exist.

In Node.js database clients like 'pg', the SQL command to start a transaction is BEGIN.

2fill in blank

medium

Complete the code to commit the transaction after successful queries.

Node.js

try {
  await client.query('INSERT INTO users(name) VALUES($1)', ['Alice']);
  await client.query('[1]');
} catch (e) {
  await client.query('ROLLBACK');
}

Drag options to blanks, or click blank then click option'

ACOMMIT

Bfinalize

Csave

DcommitTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'commitTransaction' which is not a command in this context.

Using 'save' or 'finalize' which are not valid transaction commands.

The SQL command to save changes in a transaction is COMMIT.

3fill in blank

hard

Fix the error in the code to rollback the transaction on error.

Node.js

try {
  await client.query('UPDATE accounts SET balance = balance - 100 WHERE id = $1', [accountId]);
  await client.query('UPDATE accounts SET balance = balance + 100 WHERE id = $2', [recipientId]);
  await client.query('COMMIT');
} catch (e) {
  await client.query('[1]');
}

Drag options to blanks, or click blank then click option'

Acancel

BrollbackTransaction

CROLLBACK

Dundo

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'rollbackTransaction' which is not a command in this client.

Using 'cancel' or 'undo' which are not valid transaction commands.

The correct SQL command to undo changes in a transaction is ROLLBACK.

4fill in blank

hard

Fill both blanks to correctly handle transaction with async/await.

Node.js

async function transferFunds(client, fromId, toId, amount) {
  try {
    await client.query('[1]');
    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('[2]');
  } catch (error) {
    await client.query('ROLLBACK');
    throw error;
  }
}

Drag options to blanks, or click blank then click option'

ABEGIN

BCOMMIT

Cstart

Dend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' or 'end' which are not valid transaction commands.

Not committing after beginning the transaction.

Transactions start with BEGIN and end with COMMIT to save changes.

5fill in blank

hard

Fill all three blanks to create a transaction that rolls back on error and releases the client.

Node.js

const client = await pool.connect();
try {
  await client.query('[1]');
  await client.query('DELETE FROM orders WHERE id = $1', [orderId]);
  await client.query('[2]');
} catch (err) {
  await client.query('[3]');
  throw err;
} finally {
  client.release();
}

Drag options to blanks, or click blank then click option'

ABEGIN

BCOMMIT

CROLLBACK

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' instead of 'BEGIN' to start the transaction.

Not rolling back on error.

Not committing after successful queries.

Start the transaction with BEGIN, save changes with COMMIT, and undo changes with ROLLBACK on error.

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