Replica set architecture mental model in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Primary role in a MongoDB replica set

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

A It handles all write operations and replicates data to secondaries.

B It monitors network traffic but does not store data.

C It serves read requests exclusively and never accepts writes.

D It only stores backups and does not handle any client requests.

💡 Hint

Think about which node accepts changes from users.

The primary node is responsible for accepting all write operations and then replicating those changes to the secondary nodes to keep data consistent.

❓ query_result

intermediate

Output of rs.status() command

What key information does the rs.status() command provide in a MongoDB replica set?

A It displays the network configuration of the replica set members.

B It shows the current state of each member, including primary or secondary roles.

C It lists all databases and their sizes on the primary node.

D It returns the user access permissions for the replica set.

💡 Hint

Think about what status means for nodes in a set.

The rs.status() command reports the health and state of each member in the replica set, such as which is primary, secondary, or if any are down.

📝 Syntax

advanced

Correct syntax to initiate a replica set

Which of the following commands correctly initiates a MongoDB replica set named 'rs0' with one member?

MongoDB

rs.initiate({ _id: 'rs0', members: [{ _id: 0, host: 'localhost:27017' }] })

A replicaSet.initiate({ name: 'rs0', members: ['localhost:27017'] })

B rs.start({ id: 'rs0', nodes: [{ id: 0, address: 'localhost:27017' }] })

C rs.create({ _id: 'rs0', members: [{ _id: 0, host: 'localhost:27017' }] })

D rs.initiate({ _id: 'rs0', members: [{ _id: 0, host: 'localhost:27017' }] })

💡 Hint

Look for the official MongoDB command to start a replica set.

The rs.initiate() command with the correct object specifying _id and members is the proper way to start a replica set.

❓ optimization

advanced

Improving read availability in a replica set

To improve read availability and reduce load on the primary, which configuration is best in a MongoDB replica set?

A Enable read preference to 'secondary' so reads can be served by secondary nodes.

B Disable replication to secondaries to reduce network traffic.

C Set all nodes as primary to allow writes everywhere.

D Use only one node to avoid replication lag.

💡 Hint

Think about how to distribute read requests safely.

Setting read preference to 'secondary' allows clients to read from secondary nodes, reducing load on the primary and improving availability.

🔧 Debug

expert

Diagnosing replica set member state

A secondary member in a MongoDB replica set is stuck in the 'RECOVERING' state and not becoming 'SECONDARY'. What is the most likely cause?

A The secondary has network issues and cannot connect to the client.

B The primary node is down, so the secondary cannot become primary.

C The secondary is still syncing data from the primary and has not caught up yet.

D The replica set name is incorrect on the secondary configuration.

💡 Hint

Consider what 'RECOVERING' means for a secondary node.

A secondary in 'RECOVERING' state is usually syncing data from the primary and will become 'SECONDARY' once caught up.