Commit and abort behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after a successful commit in a MongoDB transaction?

Consider a MongoDB transaction that inserts a document into the users collection and then commits successfully. What will be the result of querying users after the commit?

MongoDB

session.startTransaction();
db.users.insertOne({name: 'Alice'}, {session});
session.commitTransaction();
db.users.find({name: 'Alice'}).toArray();

A [{_id: ObjectId(...), name: 'Alice'}]

B null

C TransactionError

D []

💡 Hint

Think about what happens to data after a commit in a transaction.

After a successful commit, the inserted document becomes visible outside the transaction, so querying returns the new document.

❓ query_result

intermediate

What happens to data after aborting a MongoDB transaction?

A transaction inserts a document into the orders collection but then aborts. What will be the result of querying orders for that document?

MongoDB

session.startTransaction();
db.orders.insertOne({orderId: 123}, {session});
session.abortTransaction();
db.orders.find({orderId: 123}).toArray();

A TransactionAbortedError

B [{orderId: 123}]

C []

D null

💡 Hint

Consider what aborting a transaction does to the changes made inside it.

Aborting a transaction discards all changes made during it, so the inserted document is not saved.

📝 Syntax

advanced

Which option correctly starts and commits a MongoDB transaction?

Identify the option that uses correct syntax to start a session, begin a transaction, insert a document, and commit the transaction.

A

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

B

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

C

const session = client.startSession();
session.beginTransaction();
db.collection('products').insertOne({item: 'book'}, {session});
session.commit();
session.endSession();

D

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

💡 Hint

Check the correct method names and usage of session in operations.

Option B correctly uses startTransaction(), passes the session to the insert operation, commits, and ends the session.

🔧 Debug

advanced

Why does this MongoDB transaction fail to commit?

Given the code below, why does the transaction fail to commit?

MongoDB

const session = client.startSession();
session.startTransaction();
db.collection('inventory').updateOne({sku: 'abc'}, {$inc: {qty: -1}}, {session});
session.commitTransaction();
session.endSession();

A The update operation is missing the session option, so it is not part of the transaction.

B The transaction cannot update documents, only insert.

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

D endSession() must be called before commitTransaction().

💡 Hint

Think about how MongoDB knows which operations belong to the transaction.

Operations must include the session option to be part of the transaction. Without it, the update runs outside the transaction, causing commit failure.

🧠 Conceptual

expert

What is the effect of a multi-document transaction abort in MongoDB?

In a multi-document transaction that modifies several collections, what happens if the transaction aborts?

A Changes are partially saved depending on the order of operations.

B Only changes in the first collection are rolled back; others remain.

C The transaction abort causes a partial commit of changes.

D All changes made in all collections during the transaction are rolled back, leaving the database unchanged.

💡 Hint

Recall the atomicity property of transactions.

MongoDB transactions are atomic; aborting rolls back all changes made during the transaction across all collections.