Primary and secondary nodes in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a primary node in MongoDB?

A primary node is the main server in a MongoDB replica set that receives all write operations. It replicates data to secondary nodes.

beginner

What is a secondary node in MongoDB?

A secondary node is a server in a MongoDB replica set that copies data from the primary node and can serve read operations.

beginner

Why does MongoDB use primary and secondary nodes?

To ensure data is safe and available. The primary handles writes, and secondaries keep copies to protect against failure and allow reads.

intermediate

What happens if the primary node fails in MongoDB?

The replica set automatically elects a new primary from the secondary nodes to keep the database working without interruption.

beginner

Can secondary nodes accept write operations directly?

No, only the primary node accepts writes. Secondary nodes replicate data from the primary and can serve reads if configured.

What role does the primary node play in a MongoDB replica set?

A Elects secondary nodes

B Handles all write operations

C Only serves read operations

D Backs up data to cloud

The primary node handles all write operations and replicates data to secondary nodes.

What is the main purpose of secondary nodes?

A To elect the primary node

B To accept write operations

C To replicate data from the primary

D To delete old data

Secondary nodes replicate data from the primary node to keep copies of the data.

If the primary node fails, what happens next?

A A secondary node is elected as the new primary

B The database stops working

C All data is lost

D The primary node repairs itself automatically

MongoDB elects a new primary from the secondary nodes to maintain availability.

Can secondary nodes serve read requests in MongoDB?

A Only for write operations

B No, only primary nodes can serve reads

C Only during primary failure

D Yes, if configured to do so

Secondary nodes can serve read requests if the system is set up to allow it.

Which node type in MongoDB replica set accepts write operations?

A Primary node

B Secondary node

C Both primary and secondary

D Neither primary nor secondary

Only the primary node accepts write operations in a MongoDB replica set.

Explain the roles of primary and secondary nodes in a MongoDB replica set.

Think about who writes data and who copies it.

Describe what happens during a primary node failure in MongoDB.

Focus on failover and election process.

Practice

1. Which statement best describes the role of a primary node in a MongoDB replica set?

easy

A. It acts as a backup and does not participate in replication.

B. It only reads data and never accepts writes.

C. It accepts write operations and replicates data to secondary nodes.

D. It is used only for analytics and reporting.

Solution

1. Step 1: Understand the primary node's function

   The primary node is the main server that accepts all write operations in a replica set.

2. Step 2: Role in replication

   The primary replicates data changes to secondary nodes to keep data consistent.

3. Final Answer:

   It accepts write operations and replicates data to secondary nodes. -> Option C

4. Quick Check:

   Primary node = accepts writes and replicates [OK]

Hint: Primary node handles writes; secondaries replicate [OK]

Common Mistakes:

• Thinking primary only reads data
• Confusing secondary with primary roles
• Assuming primary is just a backup

2. Which of the following is the correct command to check the status of a MongoDB replica set?

easy

A. replica.status()

B. db.replicaStatus()

C. show replicaSet

D. rs.status()

Solution

1. Step 1: Recall MongoDB replica set commands

   The command rs.status() is used to check the status of the replica set.

2. Step 2: Verify other options

   Other options are not valid MongoDB shell commands for replica set status.

3. Final Answer:

   rs.status() -> Option D

4. Quick Check:

   Replica set status command = rs.status() [OK]

Hint: Use rs.status() to check replica set health [OK]

Common Mistakes:

• Using incorrect or non-existent commands
• Confusing database commands with replica set commands
• Typing commands with wrong syntax

3. Given a replica set with one primary and two secondary nodes, what will happen if the primary node goes down?

medium

A. The replica set will stop all read and write operations immediately.

B. One of the secondary nodes will be elected as the new primary.

C. The primary node will automatically restart without intervention.

D. All secondary nodes will become primary nodes simultaneously.

Solution

1. Step 1: Understand replica set failover

   If the primary node fails, the replica set automatically elects a new primary from the secondaries.

2. Step 2: Check other options

   The replica set does not stop all operations; it does not restart primary automatically; multiple primaries cannot exist simultaneously.

3. Final Answer:

   One of the secondary nodes will be elected as the new primary. -> Option B

4. Quick Check:

   Failover elects new primary from secondaries [OK]

Hint: Replica sets auto-elect new primary if current fails [OK]

Common Mistakes:

• Thinking all nodes become primary
• Assuming system stops all operations
• Believing primary restarts automatically

4. You run rs.stepDown() on the primary node but get an error saying "not primary". What is the likely cause?

medium

A. The node is already a secondary, not primary.

B. The command syntax is incorrect.

C. The replica set has only one node.

D. The network connection to the primary is lost.

Solution

1. Step 1: Understand rs.stepDown() usage

   This command only works on a primary node to force it to step down.

2. Step 2: Analyze the error message

   "not primary" means the node is already a secondary, so it cannot step down.

3. Final Answer:

   The node is already a secondary, not primary. -> Option A

4. Quick Check:

   rs.stepDown() error "not primary" = node is secondary [OK]

Hint: rs.stepDown() only works on primary nodes [OK]

Common Mistakes:

• Trying to step down a secondary node
• Assuming syntax error causes this message
• Ignoring the node's current role

5. You want to configure a MongoDB replica set with one primary and two secondaries. Which of the following configurations ensures data safety and availability during a primary failure?

hard

A. Set all nodes with priority 1 and enable majority write concern.

B. Set one secondary with priority 0 and disable write concern.

C. Use only one node as primary without secondaries.

D. Set all nodes with priority 0 to prevent elections.

Solution

1. Step 1: Understand priority and write concern

   Setting all nodes with priority 1 allows any secondary to become primary if needed. Majority write concern ensures writes are acknowledged by most nodes.

2. Step 2: Evaluate other options

   Priority 0 disables election for that node; disabling write concern risks data loss; single node has no failover; priority 0 on all nodes prevents elections.

3. Final Answer:

   Set all nodes with priority 1 and enable majority write concern. -> Option A

4. Quick Check:

   Priority 1 + majority write concern = safe failover [OK]

Hint: Use priority 1 and majority write concern for safety [OK]

Common Mistakes:

• Disabling write concern risking data loss
• Setting priority 0 disables elections
• Using single node without replicas