Read concern and write concern in transactions in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the effect of setting readConcern: 'majority' in a MongoDB transaction?

Consider a MongoDB transaction started with readConcern: 'majority'. What does this setting guarantee about the data read inside the transaction?

MongoDB

session.startTransaction({ readConcern: { level: 'majority' } });

A The transaction reads uncommitted data from other transactions.

B The transaction reads data that has been acknowledged by a majority of replica set members.

C The transaction reads only data from the primary node, ignoring secondaries.

D The transaction reads data without any consistency guarantees.

💡 Hint

Think about how MongoDB ensures data consistency across replica sets.

Setting readConcern: 'majority' ensures that the transaction reads data that has been acknowledged by a majority of replica set members, providing strong consistency.

❓ query_result

intermediate

What does writeConcern: { w: 'majority' } ensure in a MongoDB transaction?

When a transaction is committed with writeConcern: { w: 'majority' }, what does this guarantee about the write operation?

MongoDB

session.commitTransaction({ writeConcern: { w: 'majority' } });

A The write is acknowledged immediately without waiting for replication.

B The write is acknowledged only by the primary node.

C The write is not acknowledged and may be lost if the primary fails.

D The write is acknowledged by a majority of replica set members before the commit returns success.

💡 Hint

Consider how MongoDB ensures durability of writes in a replica set.

Using writeConcern: { w: 'majority' } ensures that the write operation is confirmed by most replica set members before the commit is considered successful, increasing durability.

📝 Syntax

advanced

Identify the correct syntax to start a MongoDB transaction with readConcern: 'snapshot' and writeConcern: { w: 'majority' }.

Which of the following code snippets correctly starts a transaction with readConcern: 'snapshot' and writeConcern: { w: 'majority' }?

A session.startTransaction({ readConcern: { level: 'snapshot' }, writeConcern: { w: 'majority' } });

B session.startTransaction({ readConcern: { level: snapshot }, writeConcern: { w: majority } });

C session.startTransaction({ readConcern: 'snapshot', writeConcern: 'majority' });

D session.startTransaction({ readConcern: { level: 'majority' }, writeConcern: { w: 'snapshot' } });

💡 Hint

Remember the correct structure for specifying readConcern and writeConcern options.

The correct syntax uses objects with keys and string values. readConcern requires a level key with a string value, and writeConcern requires a w key with a string value.

🧠 Conceptual

advanced

Why is readConcern: 'snapshot' recommended for multi-document transactions?

In MongoDB, what is the main advantage of using readConcern: 'snapshot' inside a multi-document transaction?

A It provides a consistent view of data as it existed at the start of the transaction.

B It allows reading uncommitted changes from other transactions.

C It disables replication during the transaction.

D It forces the transaction to read only from secondary nodes.

💡 Hint

Think about how transactions maintain data consistency during multiple operations.

readConcern: 'snapshot' ensures that all reads within the transaction see a stable snapshot of the data as it was at the start, preventing inconsistencies.

🔧 Debug

expert

What error occurs when committing a transaction with an invalid writeConcern value?

Consider the following commit code:

session.commitTransaction({ writeConcern: { w: 'invalidLevel' } });

What error will MongoDB raise?

MongoDB

session.commitTransaction({ writeConcern: { w: 'invalidLevel' } });

A TypeError: writeConcern must be a string

B SyntaxError: Invalid writeConcern syntax

C MongoServerError: Unknown write concern level 'invalidLevel'

D No error; commit succeeds with default writeConcern

💡 Hint

Invalid write concern levels are not accepted by MongoDB.

MongoDB validates the write concern level and raises a MongoServerError if the level is unknown or invalid.