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.

Practice

1. What does readConcern control in a MongoDB transaction?

easy

A. How fresh or confirmed the data you read is during the transaction

B. How many servers confirm your data changes before commit

C. The speed of the transaction execution

D. The size of the data being written

Solution

1. Step 1: Understand the role of readConcern

   Read concern defines the level of isolation and freshness of data read during a transaction.

2. Step 2: Differentiate from writeConcern

   Write concern controls confirmation of writes, not reads.

3. Final Answer:

   How fresh or confirmed the data you read is during the transaction -> Option A

4. Quick Check:

   readConcern = data freshness [OK]

Hint: Read concern is about data freshness, write concern is about confirmation [OK]

Common Mistakes:

• Confusing readConcern with writeConcern
• Thinking readConcern controls write confirmations
• Assuming readConcern affects transaction speed

2. Which of the following is the correct way to specify writeConcern in a MongoDB transaction?

easy

A. { writeConcern: { w: 'majority' } }

B. { readConcern: { w: 'majority' } }

C. { writeConcern: { r: 'majority' } }

D. { writeConcern: 'readConcern' }

Solution

1. Step 1: Recall writeConcern syntax

   Write concern uses the key w to specify acknowledgement level, e.g., 'majority'.

2. Step 2: Check options for correctness

   { writeConcern: { w: 'majority' } } correctly uses writeConcern: { w: 'majority' }. Others misuse keys or mix readConcern and writeConcern.

3. Final Answer:

   { writeConcern: { w: 'majority' } } -> Option A

4. Quick Check:

   writeConcern uses w key [OK]

Hint: writeConcern uses w, readConcern uses level [OK]

Common Mistakes:

• Using 'r' instead of 'w' for writeConcern
• Mixing readConcern and writeConcern keys
• Passing writeConcern as a string instead of object

3. Given this transaction code snippet, what is the effective read concern level?

const session = client.startSession();
session.startTransaction({ readConcern: { level: 'snapshot' }, writeConcern: { w: 'majority' } });
// transaction operations
await session.commitTransaction();

medium

A. local

B. majority

C. linearizable

D. snapshot

Solution

1. Step 1: Identify readConcern in transaction options

   The transaction is started with readConcern: { level: 'snapshot' }.

2. Step 2: Understand readConcern levels

   'snapshot' means the transaction reads a consistent snapshot of data at start time.

3. Final Answer:

   snapshot -> Option D

4. Quick Check:

   readConcern level = snapshot [OK]

Hint: Check readConcern.level in startTransaction options [OK]

Common Mistakes:

• Confusing writeConcern level with readConcern level
• Assuming default readConcern instead of specified
• Mixing up 'majority' with readConcern levels

4. Identify the error in this transaction code snippet:

const session = client.startSession();
session.startTransaction({ readConcern: { level: 'majority' }, writeConcern: { w: 1 } });
// transaction operations
await session.commitTransaction();

medium

A. writeConcern w: 1 is invalid

B. Missing session.endSession() call

C. readConcern level 'majority' is invalid

D. startTransaction syntax is incorrect

Solution

1. Step 1: Check for session lifecycle management

   The code does not call session.endSession() after commitTransaction(). This is necessary to properly release session resources and avoid leaks.

2. Step 2: Verify other components

   readConcern level 'majority' is valid; writeConcern { w: 1 } is valid (acknowledgement from primary); syntax is correct.

3. Final Answer:

   Missing session.endSession() call -> Option B

4. Quick Check:

   endSession after commit/abort [OK]

Hint: Always call session.endSession() after commitTransaction or abortTransaction [OK]

Common Mistakes:

• Thinking readConcern 'majority' is invalid for transactions
• Assuming writeConcern w:1 is invalid
• Believing session.endSession() is optional

5. You want a transaction that reads the most recent confirmed data and ensures all writes are acknowledged by a majority of servers. Which combination of readConcern and writeConcern should you use?

hard

A. readConcern: 'snapshot', writeConcern: { w: 0 }

B. readConcern: 'local', writeConcern: { w: 1 }

C. readConcern: 'majority', writeConcern: { w: 'majority' }

D. readConcern: 'linearizable', writeConcern: { w: 'majority' }

Solution

1. Step 1: Understand readConcern 'majority'

   This level reads data confirmed by a majority of servers, ensuring recent confirmed data.

2. Step 2: Understand writeConcern { w: 'majority' }

   This ensures writes are acknowledged by a majority of servers before commit.

3. Step 3: Evaluate other options

   readConcern: 'local', writeConcern: { w: 1 } uses weaker writeConcern; C uses no write acknowledgement; D uses linearizable readConcern which is stricter than needed.

4. Final Answer:

   readConcern: 'majority', writeConcern: { w: 'majority' } -> Option C

5. Quick Check:

   Majority read and write concern = readConcern: 'majority', writeConcern: { w: 'majority' } [OK]

Hint: Use 'majority' for both read and write for confirmed data [OK]

Common Mistakes:

• Choosing writeConcern w:1 for majority confirmation
• Confusing 'snapshot' with 'majority' readConcern
• Ignoring writeConcern w:0 means no acknowledgement