Write concern levels (w: 1, majority) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does write concern w:1 guarantee?

In MongoDB, if you set the write concern to { w: 1 }, what does it guarantee about the write operation?

A The write is not acknowledged at all.

B The write is acknowledged by all nodes in the replica set.

C The write is acknowledged only after majority of nodes confirm.

D The write is acknowledged by the primary node only.

💡 Hint

Think about which node confirms the write first in a replica set.

Write concern { w: 1 } means the primary node acknowledges the write. It does not wait for secondaries.

❓ query_result

intermediate

What does write concern w: 'majority' guarantee?

In MongoDB, if you set the write concern to { w: 'majority' }, what does it guarantee about the write operation?

A The write is acknowledged by a majority of voting nodes in the replica set.

B The write is acknowledged by the primary node only.

C The write is acknowledged by all nodes including arbiters.

D The write is acknowledged only after all secondaries confirm.

💡 Hint

Majority means more than half of voting nodes.

Write concern { w: 'majority' } waits for acknowledgment from a majority of voting nodes, ensuring data durability.

📝 Syntax

advanced

Which write concern syntax is correct to wait for majority?

Choose the correct MongoDB write concern syntax to ensure the write is acknowledged by the majority of replica set members.

A { w: 'majority' }

B { w: majority }

C { w: 0, j: true }

D { w: 1, j: false }

💡 Hint

Remember that 'majority' is a string value in write concern.

The correct syntax uses quotes around 'majority' because it is a string. Without quotes, it is invalid.

🧠 Conceptual

advanced

Why might you choose write concern w:1 over w:'majority'?

Which reason best explains why a developer might choose { w: 1 } instead of { w: 'majority' } for write concern?

A To guarantee writes are journaled on disk before acknowledgment.

B To ensure data is written to all nodes before continuing.

C To get faster write acknowledgments with less waiting.

D To avoid any acknowledgment and improve performance.

💡 Hint

Think about speed versus durability trade-offs.

Write concern w:1 acknowledges after the primary writes, so it is faster but less durable than majority.

🔧 Debug

expert

What error occurs with invalid write concern value?

Consider this MongoDB write operation with write concern:

{ w: 2 }

Assuming the replica set has only 3 nodes, what error or behavior will occur?

A The write succeeds immediately without waiting for any nodes.

B The write waits for acknowledgment from exactly 2 nodes and succeeds if they respond.

C The write fails with <code>WriteConcernError</code> because w:2 is invalid for this replica set.

D The write causes a syntax error and does not run.

💡 Hint

Write concern w can be a number specifying how many nodes must acknowledge.

Write concern w:2 waits for acknowledgment from 2 nodes (primary + one secondary). It succeeds if those nodes respond.