Transactions vs atomic document writes in MongoDB - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update a single field atomically in a MongoDB document.

MongoDB

db.collection.updateOne({ _id: 1 }, { $set: { status: [1] } })

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

A"active"

Bactive

Cstatus

D{ active }

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around string values.

Using variable names without quotes.

In MongoDB, string values must be in quotes. So to set the field status to the string "active", you need quotes.

2fill in blank

medium

Complete the code to start a transaction session in MongoDB.

MongoDB

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

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

AbeginTransaction

BinitTransaction

CopenTransaction

DstartTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using beginTransaction() which is not a MongoDB method.

Using incorrect method names that do not exist.

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

3fill in blank

hard

Fix the error in the transaction commit code.

MongoDB

await session.[1]();

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

AcommitTransaction

Bcommit

CcommitTxn

DendTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using commit() which does not exist in MongoDB sessions.

Using incorrect method names like commitTxn().

The correct method to commit a transaction in MongoDB is commitTransaction().

4fill in blank

hard

Fill both blanks to update two fields atomically inside a transaction.

MongoDB

await collection.updateOne({ _id: 1 }, { $set: { status: [1], count: [2] } }, { session });

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

A"completed"

B42

C"pending"

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Putting numbers in quotes.

Forgetting quotes around string values.

Inside a transaction, you can update multiple fields atomically. Here, status is set to the string "completed" and count to the number 42.

5fill in blank

hard

Fill all three blanks to correctly start a transaction, update a document, and commit the transaction.

MongoDB

const session = client.startSession();
try {
  session.[1]();
  await collection.updateOne({ _id: 2 }, { $set: { processed: [2] } }, { session });
  await session.[3]();
} finally {
  await session.endSession();
}

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

AstartTransaction

Btrue

CcommitTransaction

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using false instead of true for the processed field.

Using wrong method names for starting or committing transactions.

This code starts a transaction with startTransaction(), updates the processed field to true, and commits the transaction with commitTransaction().

Practice

1. Which statement best describes atomic document writes in MongoDB?

easy

A. They require manual rollback on failure.

B. They group multiple documents to update together.

C. They allow partial updates on multiple documents.

D. They update a single document completely or not at all.

Solution

1. Step 1: Understand atomic writes scope

   Atomic writes in MongoDB apply only to single documents, ensuring full update or no update.

2. Step 2: Compare with multi-document operations

   Multi-document updates require transactions, not atomic writes.

3. Final Answer:

   They update a single document completely or not at all. -> Option D

4. Quick Check:

   Atomic writes = single document update [OK]

Hint: Atomic writes affect one document fully or not at all [OK]

Common Mistakes:

• Thinking atomic writes cover multiple documents
• Confusing atomic writes with transactions
• Assuming partial updates are atomic

2. Which of the following is the correct way to start a transaction in MongoDB using the shell?

easy

A. session.startTransaction()

B. db.startTransaction()

C. transaction.begin()

D. start.transaction()

Solution

1. Step 1: Recall MongoDB transaction syntax

   Transactions start on a session object using startTransaction() method.

2. Step 2: Verify options

   Only session.startTransaction() matches the correct syntax.

3. Final Answer:

   session.startTransaction() -> Option A

4. Quick Check:

   Start transaction = session.startTransaction() [OK]

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

Common Mistakes:

• Using db.startTransaction() which is invalid
• Confusing method names like transaction.begin()
• Incorrect method call syntax

3. Given the following code snippet, what will be the result if one update fails inside the transaction?

const session = client.startSession();
session.startTransaction();
try {
  await collection.updateOne({ _id: 1 }, { $set: { value: 10 } }, { session });
  await collection.updateOne({ _id: 2 }, { $set: { value: 20 } }, { session });
  await session.commitTransaction();
} catch (e) {
  await session.abortTransaction();
}

medium

A. Both updates are applied or none if any fails.

B. Only the first update is applied if the second fails.

C. Both updates are applied regardless of errors.

D. Updates are applied partially without rollback.

Solution

1. Step 1: Understand transaction behavior

   Transactions ensure all operations succeed or all fail together.

2. Step 2: Analyze code flow

   If any update fails, catch block aborts transaction, rolling back changes.

3. Final Answer:

   Both updates are applied or none if any fails. -> Option A

4. Quick Check:

   Transaction = all or nothing [OK]

Hint: Transactions commit all or rollback all changes [OK]

Common Mistakes:

• Assuming partial updates apply on failure
• Ignoring abortTransaction() effect
• Thinking updates apply independently

4. Identify the error in this MongoDB transaction code snippet:

const session = client.startSession();
session.startTransaction();
await collection.updateOne({ _id: 1 }, { $set: { value: 10 } });
await session.commitTransaction();
session.endSession();

medium

A. updateOne cannot be used inside transactions.

B. Missing session option in updateOne call.

C. startTransaction() must be called after updateOne.

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

Solution

1. Step 1: Check transaction usage in updateOne

   Operations inside a transaction must include the session option to link them.

2. Step 2: Verify code calls

   updateOne lacks { session } option, so it runs outside transaction.

3. Final Answer:

   Missing session option in updateOne call. -> Option B

4. Quick Check:

   Include session in operations inside transactions [OK]

Hint: Always pass { session } to operations inside transactions [OK]

Common Mistakes:

• Forgetting to pass session option in operations
• Calling endSession before commitTransaction
• Misordering startTransaction call

5. You need to update a user's profile document and also add a log entry in a separate collection. Which approach is best to ensure both updates succeed or fail together?

hard

A. Perform two separate atomic document writes without transactions.

B. Update the profile document atomically and ignore the log entry.

C. Use a transaction to update both collections atomically.

D. Update the log entry first, then the profile document without transactions.

Solution

1. Step 1: Identify multi-document update requirement

   Updating two collections means multiple documents involved.

2. Step 2: Choose appropriate method

   Transactions ensure both updates succeed or fail together, preserving consistency.

3. Final Answer:

   Use a transaction to update both collections atomically. -> Option C

4. Quick Check:

   Multi-document update = use transactions [OK]

Hint: Use transactions for multi-document atomic updates [OK]

Common Mistakes:

• Using atomic writes for multiple collections
• Ignoring failure possibility in one update
• Assuming separate writes are automatically atomic