Primary and secondary nodes in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the primary node in a MongoDB replica set.

MongoDB

rs.status().members.find(member => member.[1] === 'PRIMARY')

Drag options to blanks, or click blank then click option'

AstateStr

Bname

Chealth

Duptime

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name' instead of 'stateStr' to identify the primary node.

Confusing 'health' with node role.

The 'stateStr' field indicates the node state, such as 'PRIMARY' or 'SECONDARY'.

2fill in blank

medium

Complete the command to step down the primary node in a MongoDB replica set.

MongoDB

rs.[1]()

Drag options to blanks, or click blank then click option'

AstepDown

Bfreeze

Creconfig

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reconfig' which changes the replica set configuration.

Using 'freeze' which prevents elections temporarily.

The 'stepDown()' command forces the primary to step down and triggers an election.

3fill in blank

hard

Fix the error in the command to add a secondary node to the replica set.

MongoDB

rs.add('[1]')

Drag options to blanks, or click blank then click option'

Alocalhost

Blocalhost:27018

C27018

DsecondaryNode

Attempts:

3 left

💡 Hint

Common Mistakes

Providing only hostname without port.

Providing only port number.

The 'add()' method requires the hostname and port in 'host:port' format.

4fill in blank

hard

Fill both blanks to create a query that finds all secondary nodes in the replica set.

MongoDB

rs.status().members.filter(member => member.[1] === '[2]')

Drag options to blanks, or click blank then click option'

AstateStr

Bname

CSECONDARY

DPRIMARY

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name' instead of 'stateStr' for filtering.

Filtering for 'PRIMARY' instead of 'SECONDARY'.

Filtering by 'stateStr' equal to 'SECONDARY' returns all secondary nodes.

5fill in blank

hard

Fill all three blanks to configure a member as a hidden secondary node in the replica set configuration.

MongoDB

{ _id: [1], host: '[2]', hidden: [3] }

Drag options to blanks, or click blank then click option'

A2

Blocalhost:27019

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Setting 'hidden' to false for a hidden member.

Omitting the port in the host string.

The member id is 2, host is 'localhost:27019', and 'hidden' must be true to hide the secondary.

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