When transactions are necessary vs unnecessary in MongoDB - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

When are transactions necessary in MongoDB?

Which scenario below best describes when you should use transactions in MongoDB?

A Updating multiple documents in different collections that must all succeed or fail together

B Inserting a single document into a collection

C Reading a single document from a collection

D Creating an index on a collection

💡 Hint

Think about when you need all changes to happen together or not at all.

Transactions are necessary when multiple operations across one or more collections must be atomic, meaning all succeed or all fail together. Simple single-document operations do not require transactions.

🧠 Conceptual

intermediate

When are transactions unnecessary in MongoDB?

Which of the following operations does NOT require a transaction in MongoDB?

A Updating multiple documents in different collections atomically

B Inserting a single document into a collection

C Updating related documents in multiple collections

D Performing multiple writes that must all succeed or fail together

💡 Hint

Think about operations that affect only one document.

Single-document operations in MongoDB are atomic by default and do not require explicit transactions.

❓ query_result

advanced

What is the output of this transaction example?

Consider this MongoDB transaction code snippet. What will be the final value of the 'balance' field in the 'accounts' collection for accountId 123 after the transaction commits successfully?

MongoDB

const session = client.startSession();
let finalBalance;
await session.withTransaction(async () => {
  const account = await accounts.findOne({ accountId: 123 }, { session });
  await accounts.updateOne({ accountId: 123 }, { $inc: { balance: -100 } }, { session });
  finalBalance = account.balance - 100;
});
console.log(finalBalance);

A Undefined, because finalBalance is not set

B Original balance

C Original balance minus 100

D Original balance plus 100

💡 Hint

Look at how finalBalance is calculated inside the transaction.

The transaction reads the original balance, subtracts 100, updates the document, and sets finalBalance accordingly. After commit, finalBalance reflects the updated balance.

📝 Syntax

advanced

Which option correctly starts a MongoDB transaction session?

Select the correct syntax to start a transaction session in MongoDB using the Node.js driver.

A const session = client.transaction();

B const session = client.startTransaction();

C const session = client.beginSession(); session.beginTransaction();

D const session = client.startSession(); session.startTransaction();

💡 Hint

Check the official MongoDB Node.js driver method names.

The correct way is to call startSession() on the client, then call startTransaction() on the session object.

🔧 Debug

expert

Why does this MongoDB transaction fail to commit?

Given this code snippet, which operation is missing the session option, so it is outside the transaction? const session = client.startSession(); await session.withTransaction(async () => { await orders.insertOne({ orderId: 1, item: 'book' }, { session }); await inventory.updateOne({ item: 'book' }, { $inc: { stock: -1 } }); }); session.endSession();

A The updateOne operation is missing the session option, so it is outside the transaction

B The insertOne operation is missing the session option, so it is outside the transaction

C session.endSession() must be called before withTransaction

D Transactions are not supported on single-document operations

💡 Hint

Check if all operations inside the transaction use the session.

All operations inside a transaction must include the session option to be part of the transaction. Missing it means the operation executes outside the transaction.