Multi-document transactions in MongoDB - Mini Project: Build & Apply

Using Multi-Document Transactions in MongoDB

📖 Scenario: You are managing a simple banking system where users can transfer money between accounts. Each account is stored as a document in a MongoDB collection. To keep the data consistent, you need to update both the sender's and receiver's account balances in a single transaction.

🎯 Goal: Build a multi-document transaction in MongoDB that safely transfers money from one account to another, ensuring both accounts are updated together or not at all.

📋 What You'll Learn

Create two account documents with specific balances

Set up a MongoDB session for transactions

Write a transaction that deducts money from one account and adds it to another

Commit the transaction to save changes atomically

💡 Why This Matters

🌍 Real World

Banking and financial applications require multi-document transactions to keep account balances consistent during transfers.

💼 Career

Understanding MongoDB transactions is important for backend developers working with NoSQL databases to ensure data integrity.

1

Create the accounts collection with two account documents

Create a MongoDB collection called accounts and insert two documents with these exact fields and values: { _id: 1, name: "Alice", balance: 500 } and { _id: 2, name: "Bob", balance: 300 }.

MongoDB

// Insert two account documents into the accounts collection
// Your code here

Hint

Use insertMany on the accounts collection to add both documents at once.

2

Start a client session for the transaction

Create a variable called session by starting a client session using client.startSession().

MongoDB

db.accounts.insertMany([
  { _id: 1, name: "Alice", balance: 500 },
  { _id: 2, name: "Bob", balance: 300 }
])

// Start a client session and assign it to session
// Your code here

Hint

Use client.startSession() to create a session for transactions.

3

Write the transaction to transfer money

Use session.withTransaction() to run a transaction function that deducts 100 from Alice's balance and adds 100 to Bob's balance using updateOne with { $inc: { balance: -100 } } and { $inc: { balance: 100 } } respectively. Use session as an option in both updates.

MongoDB

db.accounts.insertMany([
  { _id: 1, name: "Alice", balance: 500 },
  { _id: 2, name: "Bob", balance: 300 }
])

const session = client.startSession()

// Use session.withTransaction() to transfer 100 from Alice to Bob
// Your code here

Hint

Inside withTransaction, use updateOne twice with the session option to update both accounts.

4

End the session after the transaction

Call session.endSession() to properly close the client session after the transaction.

MongoDB

db.accounts.insertMany([
  { _id: 1, name: "Alice", balance: 500 },
  { _id: 2, name: "Bob", balance: 300 }
])

const session = client.startSession()

await session.withTransaction(async () => {
  await db.accounts.updateOne({ _id: 1 }, { $inc: { balance: -100 } }, { session })
  await db.accounts.updateOne({ _id: 2 }, { $inc: { balance: 100 } }, { session })
})

// End the session
// Your code here

Hint

Always call endSession() to close the session after finishing the transaction.

Practice

1. What is the main purpose of using multi-document transactions in MongoDB?

easy

A. To create indexes automatically

B. To speed up single document queries

C. To ensure multiple document changes succeed or fail together

D. To backup the database

Solution

1. Step 1: Understand transaction purpose

   Transactions group multiple operations so they either all succeed or all fail, ensuring data consistency.

2. Step 2: Compare options

   Only To ensure multiple document changes succeed or fail together describes this behavior. Other options describe unrelated features.

3. Final Answer:

   To ensure multiple document changes succeed or fail together -> Option C

4. Quick Check:

   Transaction purpose = group changes atomically [OK]

Hint: Transactions group multiple changes to keep data consistent [OK]

Common Mistakes:

• Confusing transactions with indexing
• Thinking transactions speed up single queries
• Assuming transactions create backups

2. Which of the following is the correct way to start a transaction in a MongoDB session using the Node.js driver?

easy

A. db.transactionStart()

B. session.startTransaction()

C. session.begin()

D. start.sessionTransaction()

Solution

1. Step 1: Recall MongoDB transaction syntax

   The correct method to start a transaction on a session is startTransaction().

2. Step 2: Eliminate incorrect options

   Options A, B, and D are not valid MongoDB session methods.

3. Final Answer:

   session.startTransaction() -> Option B

4. Quick Check:

   Start transaction method = startTransaction() [OK]

Hint: Use session.startTransaction() to begin a transaction [OK]

Common Mistakes:

• Using incorrect method names like begin()
• Calling transaction methods on db instead of session
• Mixing method order or syntax

3. Consider this MongoDB transaction code snippet in Node.js:

const session = client.startSession();
session.startTransaction();
await collection1.insertOne({item: 'apple'}, {session});
await collection2.updateOne({item: 'banana'}, {$inc: {qty: 1}}, {session});
await session.commitTransaction();
session.endSession();
console.log('Transaction complete');

What will be printed if both operations succeed?

medium

A. Transaction complete

B. Transaction aborted

C. Error: session not started

D. No output

Solution

1. Step 1: Analyze transaction flow

   The code starts a session, begins a transaction, performs two operations, commits the transaction, ends the session, then logs a message.

2. Step 2: Determine output on success

   If both operations succeed and commitTransaction() completes without error, the console.log will print 'Transaction complete'.

3. Final Answer:

   Transaction complete -> Option A

4. Quick Check:

   Successful commit logs 'Transaction complete' [OK]

Hint: If commitTransaction succeeds, console logs 'Transaction complete' [OK]

Common Mistakes:

• Assuming commitTransaction logs automatically
• Confusing session start errors with commit success
• Ignoring async/await behavior

4. You wrote this MongoDB transaction code but it throws an error:

const session = client.startSession();
session.startTransaction;
await collection.insertOne({name: 'John'}, {session});
await session.commitTransaction();
session.endSession();

What is the error and how to fix it?

medium

A. insertOne does not accept session option

B. session.endSession() should be called before commitTransaction()

C. session.startSession() should be awaited

D. Missing parentheses on startTransaction; use session.startTransaction()

Solution

1. Step 1: Identify the syntax error

   The code calls session.startTransaction without parentheses, so the function is not executed.

2. Step 2: Correct the method call

   Add parentheses to call the function: session.startTransaction().

3. Final Answer:

   Missing parentheses on startTransaction; use session.startTransaction() -> Option D

4. Quick Check:

   Method calls need parentheses () [OK]

Hint: Always add () to call methods like startTransaction() [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Calling endSession before commitTransaction
• Thinking startSession is async and needs await

5. You want to update two collections atomically: deduct 10 from accounts.balance and add 10 to transactions.amount. Which transaction code snippet correctly ensures both updates succeed or fail together?

hard

A. const session = client.startSession();
session.startTransaction();
await accounts.updateOne({user: 'A'}, {$inc: {balance: -10}}, {session});
await transactions.updateOne({user: 'A'}, {$inc: {amount: 10}}, {session});
await session.commitTransaction();
session.endSession();

B. await accounts.updateOne({user: 'A'}, {$inc: {balance: -10}});
await transactions.updateOne({user: 'A'}, {$inc: {amount: 10}});

C. const session = client.startSession();
await accounts.updateOne({user: 'A'}, {$inc: {balance: -10}}, {session});
await transactions.updateOne({user: 'A'}, {$inc: {amount: 10}}, {session});
session.commitTransaction();
session.endSession();

D. const session = client.startSession();
session.startTransaction();
accounts.updateOne({user: 'A'}, {$inc: {balance: -10}}, {session});
transactions.updateOne({user: 'A'}, {$inc: {amount: 10}}, {session});
session.commitTransaction();
session.endSession();

Solution

1. Step 1: Check transaction start and commit

   const session = client.startSession(); session.startTransaction(); await accounts.updateOne({user: 'A'}, {$inc: {balance: -10}}, {session}); await transactions.updateOne({user: 'A'}, {$inc: {amount: 10}}, {session}); await session.commitTransaction(); session.endSession(); correctly calls startTransaction() and commitTransaction() on the session.

2. Step 2: Verify operations use session and await

   const session = client.startSession(); session.startTransaction(); await accounts.updateOne({user: 'A'}, {$inc: {balance: -10}}, {session}); await transactions.updateOne({user: 'A'}, {$inc: {amount: 10}}, {session}); await session.commitTransaction(); session.endSession(); uses await on both update operations and passes the session option, ensuring atomicity.

3. Step 3: Identify errors in other options

   await accounts.updateOne({user: 'A'}, {$inc: {balance: -10}}); await transactions.updateOne({user: 'A'}, {$inc: {amount: 10}}); lacks transaction usage. const session = client.startSession(); await accounts.updateOne({user: 'A'}, {$inc: {balance: -10}}, {session}); await transactions.updateOne({user: 'A'}, {$inc: {amount: 10}}, {session}); session.commitTransaction(); session.endSession(); misses startTransaction(). const session = client.startSession(); session.startTransaction(); accounts.updateOne({user: 'A'}, {$inc: {balance: -10}}, {session}); transactions.updateOne({user: 'A'}, {$inc: {amount: 10}}, {session}); session.commitTransaction(); session.endSession(); misses await on operations.

4. Final Answer:

   Option A code snippet -> Option A

5. Quick Check:

   Start, await ops with session, commit = correct transaction [OK]

Hint: Start transaction, await ops with session, then commit [OK]

Common Mistakes:

• Not awaiting async operations inside transaction
• Forgetting to start the transaction
• Running operations outside session context