Multi-document transactions in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a transaction session in MongoDB.

MongoDB

const session = client.startSession();
await session.[1]();

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

AstartTransaction

BcommitTransaction

CendSession

DabortTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using commitTransaction instead of startTransaction.

Calling endSession before starting the transaction.

The startTransaction() method begins a transaction in the session.

2fill in blank

medium

Complete the code to commit the transaction after operations.

MongoDB

await session.[1]();

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

AstartTransaction

BabortTransaction

CcommitTransaction

DendSession

Attempts:

3 left

💡 Hint

Common Mistakes

Using abortTransaction instead of commitTransaction.

Calling startTransaction again instead of committing.

The commitTransaction() method saves all changes made during the transaction.

3fill in blank

hard

Fix the error in the code to abort the transaction on failure.

MongoDB

try {
  await session.startTransaction();
  // operations
  await session.commitTransaction();
} catch (error) {
  await session.[1]();
}

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

AstartTransaction

BcommitTransaction

CendSession

DabortTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Calling commitTransaction in the catch block.

Calling startTransaction again inside catch.

On error, abortTransaction() cancels all changes made in the transaction.

4fill in blank

hard

Fill both blanks to run operations inside a transaction session.

MongoDB

const session = client.startSession();
try {
  await session.[1]();
  await collection.[2](doc, { session });
  await session.commitTransaction();
} finally {
  await session.endSession();
}

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

AstartTransaction

BinsertOne

CupdateOne

DabortTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using updateOne instead of insertOne for inserting.

Not starting the transaction before operations.

Start the transaction with startTransaction() and perform the insert with insertOne() inside the session.

5fill in blank

hard

Fill all three blanks to update two documents atomically in a transaction.

MongoDB

const session = client.startSession();
try {
  await session.[1]();
  await collection1.[2]({ _id: 1 }, { $set: { status: 'done' } }, { session });
  await collection2.[3]({ _id: 2 }, { $inc: { count: 1 } }, { session });
  await session.commitTransaction();
} catch (e) {
  await session.abortTransaction();
} finally {
  await session.endSession();
}

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

AstartTransaction

BupdateOne

DinsertOne

Attempts:

3 left

💡 Hint

Common Mistakes

Using insertOne instead of updateOne for updates.

Not starting the transaction before updates.

Start the transaction, then update documents in both collections using updateOne() inside the session.

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