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.