Write concern and data durability in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Write Concern Levels

Which write concern level ensures that the write operation is acknowledged by the primary and at least one secondary replica before returning success?

A "w: 2" (Acknowledgment from primary and at least one secondary)

B "w: 1" (Acknowledgment from primary only)

C "w: 0" (No acknowledgment required)

D "w: majority" (Acknowledgment from majority of replica set members)

💡 Hint

Think about how many nodes must confirm the write for durability.

Write concern "w: 2" requires acknowledgment from the primary and at least one secondary, ensuring data durability across multiple nodes.

❓ query_result

intermediate

Result of Write Concern with w:0

What is the result of a MongoDB insert operation with write concern set to { w: 0 }?

MongoDB

db.collection.insertOne({name: "Alice"}, {writeConcern: {w: 0}})

A The operation returns immediately without any acknowledgment.

B The operation waits for acknowledgment from the primary node.

C The operation waits for acknowledgment from all replica set members.

D The operation throws an error due to invalid write concern.

💡 Hint

Consider what w: 0 means for acknowledgment.

Write concern w: 0 means the client does not wait for any acknowledgment, so the operation returns immediately.

📝 Syntax

advanced

Correct Write Concern Syntax for Majority

Which of the following MongoDB write concern options correctly requests acknowledgment from the majority of replica set members?

A { writeConcern: { w: "majority", j: true } }

B { writeConcern: { w: majority } }

C { writeConcern: { w: 'majority' } }

D { writeConcern: { w: "majority" } }

💡 Hint

Check the correct way to specify string values in JavaScript objects.

Option D correctly uses double quotes around the string "majority". Option D uses single quotes which is valid in JavaScript but MongoDB shell prefers double quotes. Option D is invalid because majority is not a variable. Option D is invalid because majority is unquoted.

❓ optimization

advanced

Optimizing Write Concern for Performance and Durability

You want to optimize a MongoDB write operation to balance durability and performance. Which write concern setting is the best choice?

A { w: 0, j: false }

B { w: 1, j: true }

C { w: 1, j: false }

D { w: "majority", j: true }

💡 Hint

Consider acknowledgment from primary and journaling for durability.

Option B waits for acknowledgment from the primary and ensures the write is journaled, providing durability with reasonable performance.

🔧 Debug

expert

Diagnosing Write Concern Timeout Error

A MongoDB write operation with { w: 3, wtimeout: 1000 } fails with a write concern timeout error. What is the most likely cause?

A The write operation syntax is incorrect and causes a timeout.

B The primary node is down, so no acknowledgment is possible.

C The replica set has fewer than 3 members available to acknowledge the write.

D The write concern timeout value is too high, causing delay.

💡 Hint

Think about how many nodes must confirm the write and the replica set size.

If the replica set does not have at least 3 members available, the write concern cannot be satisfied within the timeout, causing the error.

Practice

1. What does the writeConcern option in MongoDB control?

easy

A. How MongoDB confirms that data is saved to the database

B. The format of data stored in MongoDB

C. The speed of data retrieval from MongoDB

D. The size limit of documents in MongoDB

Solution

1. Step 1: Understand the role of write concern

   Write concern tells MongoDB how to confirm that a write operation is saved and durable.

2. Step 2: Differentiate from other options

   It does not control data format, retrieval speed, or document size.

3. Final Answer:

   How MongoDB confirms that data is saved to the database -> Option A

4. Quick Check:

   Write concern = data save confirmation [OK]

Hint: Write concern = confirmation of data saved [OK]

Common Mistakes:

• Confusing write concern with data format
• Thinking it controls read speed
• Assuming it limits document size

2. Which of the following is the correct way to specify a write concern that waits for acknowledgment from the primary in MongoDB?

easy

A. { writeConcern: { w: -1 } }

B. { writeConcern: { w: 0 } }

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

D. { writeConcern: { w: 1 } }

Solution

1. Step 1: Recall write concern values

   w:1 means wait for acknowledgment from the primary node only.

2. Step 2: Check other options

   w:0 means no acknowledgment; w:'majority' waits for majority nodes; w:-1 is invalid.

3. Final Answer:

   { writeConcern: { w: 1 } } -> Option D

4. Quick Check:

   w:1 = primary ack wait [OK]

Hint: w:1 waits for primary acknowledgment [OK]

Common Mistakes:

• Using w:0 which means no acknowledgment
• Confusing 'majority' with primary only
• Using invalid negative values

3. Consider this MongoDB insert command with write concern:

db.collection.insertOne({name: 'Alice'}, {writeConcern: {w: 'majority', wtimeout: 5000}})

What happens if the majority of nodes do not acknowledge the write within 5 seconds?

medium

A. The write operation succeeds immediately without waiting

B. The write operation retries indefinitely until acknowledged

C. The write operation fails with a timeout error

D. The write operation ignores the write concern and writes anyway

Solution

1. Step 1: Understand w:'majority' and wtimeout

   The write waits for majority nodes to confirm within 5000 ms (5 seconds).

2. Step 2: Outcome if timeout exceeded

   If majority nodes don't respond in time, MongoDB returns a timeout error and the write is considered failed.

3. Final Answer:

   The write operation fails with a timeout error -> Option C

4. Quick Check:

   w:'majority' + wtimeout = fail if no timely ack [OK]

Hint: wtimeout limits wait; timeout causes failure [OK]

Common Mistakes:

• Assuming write succeeds immediately
• Thinking MongoDB retries forever
• Ignoring write concern on timeout

4. You wrote this code to insert a document with write concern:

db.collection.insertOne({item: 'book'}, {writeConcern: {w: 2, wtimeout: '3000'}})

But it throws an error. What is the problem?

medium

A. wtimeout must be a number, not a string

B. w:2 is invalid; only 0,1, or 'majority' allowed

C. insertOne does not accept writeConcern option

D. The document field 'item' is not allowed

Solution

1. Step 1: Check writeConcern syntax

   wtimeout expects a number (milliseconds), not a string.

2. Step 2: Validate other parts

   w:2 is valid if the replica set has at least 2 nodes; insertOne accepts writeConcern; 'item' field is valid.

3. Final Answer:

   wtimeout must be a number, not a string -> Option A

4. Quick Check:

   wtimeout type must be number [OK]

Hint: wtimeout needs number, not quotes [OK]

Common Mistakes:

• Using string for wtimeout
• Thinking w:2 is invalid always
• Assuming insertOne disallows writeConcern

5. You want to ensure that a write is saved to disk on the primary and replicated to at least two secondary nodes before confirming success. Which write concern setting achieves this?

hard

A. { writeConcern: { w: 1, j: true } }

B. { writeConcern: { w: 3, j: true } }

C. { writeConcern: { w: 'majority', j: false } }

D. { writeConcern: { w: 2, j: false } }

Solution

1. Step 1: Understand w and j options

   w:3 means wait for acknowledgment from primary plus two secondaries (total 3 nodes). j:true means wait for write to be committed to journal (disk) on primary.

2. Step 2: Check other options

   B waits only for primary with journaling; C waits for majority but no journaling; D waits for 2 nodes but no journaling.

3. Final Answer:

   { writeConcern: { w: 3, j: true } } -> Option B

4. Quick Check:

   w:3 + j:true = 3 nodes + disk write [OK]

Hint: w:number = nodes ack; j:true = disk commit [OK]

Common Mistakes:

• Ignoring journaling for durability
• Using w:'majority' without knowing node count
• Assuming w:2 means two secondaries only