Primary and secondary nodes in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Role of Primary Node in MongoDB Replica Set

In a MongoDB replica set, what is the main responsibility of the primary node?

A It balances the load by distributing read requests among all nodes.

B It only stores backup data and does not accept any client requests.

C It handles all write operations and replicates data to secondary nodes.

D It monitors network traffic but does not store any data.

💡 Hint

Think about which node accepts changes and shares them with others.

The primary node is the only node that accepts write operations. It then replicates these changes to secondary nodes to keep data consistent.

❓ query_result

intermediate

Identifying the Primary Node with rs.status()

Given the output of rs.status() command in MongoDB, which field indicates the primary node?

MongoDB

rs.status()

A The member with "stateStr" value "PRIMARY"

B The member with "health" value 0

C The member with "stateStr" value "SECONDARY"

D The member with "optimeDate" set to null

💡 Hint

Look for the status string that explicitly says which node is primary.

The stateStr field shows the role of each member. The primary node is marked with "PRIMARY".

📝 Syntax

advanced

Correct Command to Step Down Primary Node

Which MongoDB shell command correctly forces the current primary node to step down and triggers an election?

A rs.forceStepDown(60)

B rs.stepdown(60)

C rs.step_down(60)

D rs.stepDown(60)

💡 Hint

Check the exact spelling and camel case of the command.

The correct command is rs.stepDown() with capital D and no underscores.

❓ optimization

advanced

Optimizing Read Preference for Secondary Nodes

You want to distribute read operations to secondary nodes to reduce load on the primary. Which read preference setting achieves this in MongoDB?

A "primaryOnly"

B "secondaryPreferred"

C "nearest"

D "primaryPreferred"

💡 Hint

Think about a setting that prefers secondaries but falls back to primary if needed.

The "secondaryPreferred" read preference directs reads to secondary nodes but uses primary if no secondary is available.

🔧 Debug

expert

Diagnosing Replica Set Election Issue

A MongoDB replica set has three nodes: one primary and two secondaries. Suddenly, the primary becomes unreachable. However, no new primary is elected. What is the most likely cause?

A The majority of nodes are not available, so election cannot proceed.

B The secondary nodes have higher priority than the primary.

C The replica set configuration has no arbiter node.

D The primary node was manually forced to step down without election enabled.

💡 Hint

Consider how MongoDB decides to elect a new primary based on node availability.

MongoDB requires a majority of nodes to be available to elect a new primary. If the primary is down and only one secondary is reachable, no majority exists, so no election occurs.