Transaction isolation 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'

AstartTransaction

BinitTransaction

CbeginTransaction

DopenTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'beginTransaction' instead of 'startTransaction'.

Trying to call 'openTransaction' which does not exist.

In MongoDB, the correct method to start a transaction on a session is startTransaction().

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'

AfinalizeTransaction

BcommitTransaction

CendTransaction

Dcommit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'commit' which is not a method on the session.

Using 'endTransaction' 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 in MongoDB.

MongoDB

await session.[1]();

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

AabortTransaction

Babort

CrollbackTransaction

DcancelTransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'rollbackTransaction' which is not a MongoDB method.

Using 'abort' without 'Transaction'.

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'

Asnapshot

Bmajority

Clocal

Dlinearizable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'local' for readConcern in transactions which is weaker.

Using 'linearizable' which is not a valid readConcern level.

For transaction isolation, readConcern is often set to snapshot and writeConcern to majority to ensure strong consistency.

5fill in blank

hard

Fill all three blanks to run a transaction with a callback function.

MongoDB

await session.withTransaction(async () => {
  await collection.[1](doc, { session });
  await collection.[2]({ _id: id }, { session });
}, { readConcern: { level: '[3]' } });

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

AinsertOne

BdeleteOne

Csnapshot

DupdateOne

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'updateOne' instead of 'deleteOne' for deletion.

Using 'local' instead of 'snapshot' for readConcern.

Inside a transaction callback, you can perform operations like insertOne and deleteOne. The transaction's read concern is set to snapshot for isolation.

Practice

1. What does transaction isolation in MongoDB primarily ensure?

easy

A. Data consistency when multiple operations happen at the same time

B. Faster query execution

C. Automatic data backup

D. Unlimited storage capacity

Solution

1. Step 1: Understand transaction isolation purpose

   Transaction isolation ensures that concurrent operations do not interfere and cause inconsistent data.

2. Step 2: Relate to MongoDB transactions

   MongoDB uses transaction isolation to keep data consistent when many changes happen simultaneously.

3. Final Answer:

   Data consistency when multiple operations happen at the same time -> Option A

4. Quick Check:

   Transaction isolation = Data consistency [OK]

Hint: Transaction isolation means safe, consistent data changes [OK]

Common Mistakes:

• Confusing isolation with performance improvements
• Thinking it automatically backs up data
• Assuming it increases storage

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

easy

A. session.startTransaction()

B. db.startTransaction()

C. transaction.begin()

D. session.beginTransaction()

Solution

1. Step 1: Recall MongoDB transaction syntax

   In MongoDB, transactions start on a session object using startTransaction().

2. Step 2: Check each option

   Only session.startTransaction() is the correct method; others are invalid or do not exist.

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()
• Using session.beginTransaction() which does not exist

3. Consider this MongoDB transaction code snippet:

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

What is the transaction isolation level used by MongoDB in this case?

medium

A. Read Uncommitted

B. Serializable

C. Read Committed

D. Snapshot Isolation

Solution

1. Step 1: Identify MongoDB default isolation level

   MongoDB uses snapshot isolation for transactions to provide a consistent view of data.

2. Step 2: Match options with MongoDB behavior

   Snapshot Isolation matches MongoDB's transaction isolation; others like Read Uncommitted or Serializable do not apply.

3. Final Answer:

   Snapshot Isolation -> Option D

4. Quick Check:

   MongoDB transactions = Snapshot Isolation [OK]

Hint: MongoDB transactions use snapshot isolation by default [OK]

Common Mistakes:

• Confusing with Read Committed or Serializable isolation
• Assuming MongoDB uses Read Uncommitted
• Not knowing MongoDB's snapshot isolation feature

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

const session = client.startSession();
session.beginTransaction();
await collection.updateOne({name: 'John'}, {$set: {age: 30}}, {session});
await session.commitTransaction();
session.endSession();

What is the main issue causing the error?

medium

A. Missing await before session.commitTransaction()

B. Using session.beginTransaction() instead of session.startTransaction()

C. Not passing session to updateOne()

D. Calling session.endSession() too early

Solution

1. Step 1: Check transaction start method

   MongoDB sessions use startTransaction(), not beginTransaction(). Using beginTransaction() causes an error.

2. Step 2: Verify other parts

   Await is correctly used before commitTransaction(), session is passed to updateOne(), and endSession() is called after commit.

3. Final Answer:

   Using session.beginTransaction() instead of session.startTransaction() -> Option B

4. Quick Check:

   Correct method is startTransaction() [OK]

Hint: Use startTransaction(), not beginTransaction() [OK]

Common Mistakes:

• Using beginTransaction() which does not exist
• Forgetting to pass session in operations
• Calling endSession() before commit

5. You want to ensure that two related updates in MongoDB either both succeed or both fail to keep data consistent. Which approach best achieves this using transaction isolation?

hard

A. Perform both updates separately without a transaction

B. Use findOneAndUpdate without transactions

C. Use a session and start a transaction to group both updates, then commit or abort together

D. Run updates in parallel without session

Solution

1. Step 1: Understand transaction purpose

   Transactions group multiple operations so they succeed or fail as one unit, ensuring consistency.

2. Step 2: Apply MongoDB transaction usage

   Using a session with startTransaction(), performing updates, then commit or abort ensures atomicity and isolation.

3. Step 3: Evaluate other options

   Performing updates separately or in parallel without transactions risks partial success and inconsistent data.

4. Final Answer:

   Use a session and start a transaction to group both updates, then commit or abort together -> Option C

5. Quick Check:

   Group updates in transaction = consistency [OK]

Hint: Group related changes in a transaction session [OK]

Common Mistakes:

• Updating separately risking partial failure
• Not using sessions for transactions
• Assuming findOneAndUpdate is atomic across multiple operations