When transactions are necessary vs unnecessary in MongoDB - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a transaction in MongoDB.

MongoDB

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

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

AstartTransaction

BcommitTransaction

CabortTransaction

DendSession

Attempts:

3 left

💡 Hint

Common Mistakes

Using commitTransaction instead of startTransaction

Calling endSession before starting a transaction

To begin a transaction in MongoDB, you use startTransaction() on the session.

2fill in blank

medium

Complete the code to commit a 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

Calling startTransaction again instead of commitTransaction

Using abortTransaction when you want to save changes

After completing operations in a transaction, you commit it with commitTransaction().

3fill in blank

hard

Fix the error in the code to abort a transaction properly.

MongoDB

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

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

AstartTransaction

BabortTransaction

CcommitTransaction

DendSession

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to start a new transaction inside the catch block

Calling commitTransaction again instead of abortTransaction

If committing a transaction fails, you should abort it using abortTransaction() to undo changes.

4fill in blank

hard

Fill both blanks to create a transaction that updates two collections atomically.

MongoDB

const session = client.startSession();
try {
  session.[1]();
  await collection1.updateOne(filter1, update1, { session });
  await collection2.updateOne(filter2, update2, { session });
  await session.[2]();
} finally {
  await session.endSession();
}

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

AstartTransaction

BcommitTransaction

CabortTransaction

DendSession

Attempts:

3 left

💡 Hint

Common Mistakes

Using abortTransaction instead of commitTransaction

Not starting the transaction before updates

Start the transaction with startTransaction() and commit it with commitTransaction() after updates.

5fill in blank

hard

Fill all three blanks to handle a transaction with error rollback and session cleanup.

MongoDB

const session = client.startSession();
try {
  session.[1]();
  await collection.updateOne(filter, update, { session });
  await session.[2]();
} catch (error) {
  await session.[3]();
} finally {
  await session.endSession();
}

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

AstartTransaction

BcommitTransaction

CabortTransaction

DendSession

Attempts:

3 left

💡 Hint

Common Mistakes

Not aborting the transaction on error

Not starting the transaction before operations

Start the transaction, commit if successful, and abort if there is an error.

Practice

1. When should you use transactions in MongoDB?

easy

A. For every single document update to improve speed

B. When multiple related changes must all succeed or fail together

C. Only when reading data from the database

D. When you want to skip committing changes

Solution

1. Step 1: Understand the purpose of transactions

   Transactions ensure that multiple related operations either all succeed or all fail, keeping data consistent.

2. Step 2: Identify when transactions are necessary

   They are necessary when you have multiple changes that depend on each other and must be atomic.

3. Final Answer:

   When multiple related changes must all succeed or fail together -> Option B

4. Quick Check:

   Use transactions for atomic multi-step changes [OK]

Hint: Use transactions only for multiple related changes [OK]

Common Mistakes:

• Using transactions for single document updates
• Thinking transactions speed up simple operations
• Not committing or aborting transactions properly

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

easy

A. transaction.start();

B. db.beginTransaction();

C. session.startTransaction();

D. session.begin();

Solution

1. Step 1: Recall MongoDB transaction syntax

   In MongoDB Node.js driver, transactions start with session.startTransaction().

2. Step 2: Eliminate incorrect options

   db.beginTransaction() and transaction.start() are not valid MongoDB commands; session.begin() is also incorrect.

3. Final Answer:

   session.startTransaction(); -> Option C

4. Quick Check:

   Use session.startTransaction() to begin [OK]

Hint: Start transactions with session.startTransaction() [OK]

Common Mistakes:

• Using SQL-like transaction commands
• Confusing session methods with db methods
• Missing parentheses in method calls

3. Consider this MongoDB transaction code snippet:

const session = client.startSession();
session.startTransaction();
await collection.updateOne({ _id: 1 }, { $set: { status: 'pending' } }, { session });
await collection.updateOne({ _id: 2 }, { $set: { status: 'pending' } }, { session });
await session.commitTransaction();
session.endSession();

What is the result if the second update fails?

medium

A. Transaction commits partially with warnings

B. First update is saved, second update fails

C. Both updates succeed regardless of failure

D. Both updates are rolled back, no changes saved

Solution

1. Step 1: Understand transaction atomicity

   In MongoDB, transactions ensure all operations succeed or none do.

2. Step 2: Analyze failure impact

   If the second update fails, the transaction aborts and rolls back all changes, including the first update.

3. Final Answer:

   Both updates are rolled back, no changes saved -> Option D

4. Quick Check:

   Transaction failure rolls back all changes [OK]

Hint: If one operation fails, all transaction changes rollback [OK]

Common Mistakes:

• Assuming partial commits happen in transactions
• Thinking updates outside transaction are affected
• Ignoring the need to abort on failure

4. You wrote this MongoDB transaction code but the update is not part of the transaction:

const session = client.startSession();
session.startTransaction();
await collection.updateOne({ _id: 1 }, { $set: { status: 'done' } });
await session.commitTransaction();
session.endSession();

What is the likely cause?

medium

A. Missing session option in updateOne calls

B. Calling commitTransaction before startTransaction

C. Not ending the session after commit

D. Using updateOne instead of updateMany

Solution

1. Step 1: Check transaction usage in updateOne

   When using transactions, each operation must include the session option to be part of the transaction.

2. Step 2: Identify missing session option

   The updateOne call lacks { session }, so it runs outside the transaction.

3. Final Answer:

   Missing session option in updateOne calls -> Option A

4. Quick Check:

   Include session option in all transaction operations [OK]

Hint: Always pass session option in transaction operations [OK]

Common Mistakes:

• Forgetting to pass session in queries
• Calling commitTransaction too early
• Not ending session after commit

5. You have a MongoDB collection where you update a single document's status field frequently. Should you use transactions for these updates? Why or why not?

hard

A. No, because single-document updates are atomic by default

B. No, because transactions cannot be used on single documents

C. Yes, to improve update speed

D. Yes, because transactions always ensure data safety

Solution

1. Step 1: Understand atomicity of single-document updates

   MongoDB guarantees atomicity for single-document writes without needing transactions.

2. Step 2: Decide on transaction necessity

   Since single-document updates are atomic, using transactions adds unnecessary overhead and slows performance.

3. Final Answer:

   No, because single-document updates are atomic by default -> Option A

4. Quick Check:

   Single-document updates don't need transactions [OK]

Hint: Skip transactions for single-document updates to stay fast [OK]

Common Mistakes:

• Thinking transactions speed up single updates
• Believing transactions are required for all writes
• Assuming transactions can't be used on single documents