Transaction performance considerations in MongoDB - Interactive Code 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'

AinitTransaction

BbeginTransaction

CstartTransaction

DopenTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not exist like 'beginTransaction'.

Confusing session start with client start.

In MongoDB, startTransaction() is the correct method to begin a transaction within a session.

2fill in blank

medium

Complete the code to commit a transaction in MongoDB.

MongoDB

await session.[1]();

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

Acommit

BsaveTransaction

CfinalizeTransaction

DcommitTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using just 'commit' which is not a method.

Using 'saveTransaction' which does not exist.

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

3fill in blank

hard

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

MongoDB

await session.[1]();

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

AabortTransaction

BrollbackTransaction

CstopTransaction

DcancelTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'rollbackTransaction' which is not a MongoDB method.

Using 'cancelTransaction' which does not exist.

The correct method to abort a transaction in MongoDB is abortTransaction().

4fill in blank

hard

Fill both blanks to set transaction options for read concern and write concern.

MongoDB

const transactionOptions = {
  readConcern: { level: '[1]' },
  writeConcern: { w: '[2]' }
};

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

Amajority

Blocal

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'majority' for read concern which can slow performance.

Using write concern '0' which does not wait for acknowledgment.

Setting readConcern to 'local' and writeConcern to '1' is a common way to balance performance and consistency.

5fill in blank

hard

Fill all three blanks to create a transaction with options and start it.

MongoDB

const session = client.startSession();
const options = { readConcern: { level: '[1]' }, writeConcern: { w: '[2]' } };
session.[3](options);

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

Amajority

B1

CstartTransaction

Dlocal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'local' instead of 'majority' for read concern here.

Forgetting to pass options to startTransaction.

This code sets readConcern to 'majority', writeConcern to '1', and starts the transaction with startTransaction().

Practice

1. What is a key reason to use transactions in MongoDB?

easy

A. To keep data changes safe and consistent

B. To speed up query execution

C. To reduce disk space usage

D. To avoid indexing

Solution

1. Step 1: Understand the purpose of transactions

   Transactions ensure that multiple related data changes happen together or not at all, keeping data consistent.

2. Step 2: Identify the correct purpose

   Speeding up queries, reducing disk space, or avoiding indexing are not the main reasons for transactions.

3. Final Answer:

   To keep data changes safe and consistent -> Option A

4. Quick Check:

   Transactions = Data safety and consistency [OK]

Hint: Transactions ensure data safety, not speed [OK]

Common Mistakes:

• Thinking transactions speed up queries
• Confusing transactions with indexing
• Believing transactions reduce disk space

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

easy

A. transaction.begin()

B. db.startTransaction()

C. startTransaction()

D. session.startTransaction()

Solution

1. Step 1: Recall MongoDB transaction syntax

   Transactions are started on a session object using session.startTransaction().

2. Step 2: Check other options

   db.startTransaction() and startTransaction() are invalid commands. transaction.begin() is not a MongoDB method.

3. Final Answer:

   session.startTransaction() -> Option D

4. Quick Check:

   Start transaction = session.startTransaction() [OK]

Hint: Transactions start on session objects with startTransaction() [OK]

Common Mistakes:

• Trying to start transaction directly on db object
• Using incomplete or wrong method names
• Confusing transaction start with commit or abort

3. Consider this MongoDB transaction code snippet:

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

What is the expected result of this code?

medium

A. The document is inserted but not in a transaction

B. The document {name: 'Alice'} is inserted safely in a transaction

C. The code throws a syntax error

D. The transaction is aborted automatically

Solution

1. Step 1: Analyze transaction steps

   The code starts a session, begins a transaction, inserts a document with the session, commits the transaction, and ends the session.

2. Step 2: Understand transaction effect

   Because the insert is done inside the transaction and committed, the document is safely inserted as part of the transaction.

3. Final Answer:

   The document {name: 'Alice'} is inserted safely in a transaction -> Option B

4. Quick Check:

   Transaction insert + commit = safe insert [OK]

Hint: Insert inside transaction + commit = safe insert [OK]

Common Mistakes:

• Ignoring the session option in insertOne
• Assuming transaction auto-aborts without error
• Confusing syntax errors with runtime behavior

4. You wrote this code but the transaction does not commit:

const session = client.startSession();
session.startTransaction();
await collection.insertOne({item: 'book'});
await session.commitTransaction();
session.endSession();

What is the main issue causing the transaction to fail?

medium

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

B. commitTransaction() must be called before startTransaction()

C. The insertOne operation is missing the session option

D. Transactions cannot include insertOne operations

Solution

1. Step 1: Check transaction usage in code

   The insertOne call does not include the session option, so it runs outside the transaction.

2. Step 2: Understand transaction requirements

   All operations inside a transaction must specify the session to be part of it. Without it, commitTransaction has no effect on that operation.

3. Final Answer:

   The insertOne operation is missing the session option -> Option C

4. Quick Check:

   Missing session in operation = transaction fails [OK]

Hint: Include session option in all operations inside transactions [OK]

Common Mistakes:

• Calling commitTransaction before startTransaction
• Ending session before committing
• Thinking insertOne can't be in transactions

5. Which practice helps improve transaction performance in MongoDB when working with many documents?

hard

A. Keep transactions short and avoid long-running operations

B. Use transactions for every single write operation

C. Avoid using indexes inside transactions

D. Run multiple transactions in parallel on the same documents

Solution

1. Step 1: Understand transaction performance impact

   Long transactions hold locks and resources longer, slowing performance and increasing conflicts.

2. Step 2: Identify best practice

   Keeping transactions short reduces lock time and improves throughput. Using transactions for every write or running parallel conflicting transactions harms performance. Indexes improve query speed and are not avoided.

3. Final Answer:

   Keep transactions short and avoid long-running operations -> Option A

4. Quick Check:

   Short transactions = better performance [OK]

Hint: Keep transactions short to boost speed and reduce conflicts [OK]

Common Mistakes:

• Using transactions for every write unnecessarily
• Avoiding indexes inside transactions
• Running conflicting transactions in parallel