Replica set configuration basics in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Check the primary member of a replica set

Given the following command run on a MongoDB replica set, what is the output field that indicates the current primary member's name?

MongoDB

rs.status()

A The field 'members' with a subfield 'stateStr' equal to 'PRIMARY' shows the primary member's name.

B The field 'config' contains the primary member's name.

C The field 'set' contains the primary member's name.

D The field 'primary' directly contains the hostname of the primary member.

💡 Hint

Look for a field that directly names the primary member in the replica set status output.

The rs.status() command returns a document with a members array where each member has a stateStr field. The member with stateStr equal to 'PRIMARY' is the current primary member. The primary field is not part of rs.status() output but is returned by rs.isMaster().

🧠 Conceptual

intermediate

Understanding voting members in a replica set

In a MongoDB replica set configuration, what is the maximum number of voting members allowed to ensure proper election and fault tolerance?

A 3 voting members maximum

B 5 voting members maximum

C 7 voting members maximum

D 9 voting members maximum

💡 Hint

MongoDB limits voting members to a certain number to maintain election stability.

MongoDB allows up to 7 voting members in a replica set to maintain a stable election process and fault tolerance. Having more than 7 voting members is not supported.

📝 Syntax

advanced

Identify the correct syntax to add a new member to a replica set

Which of the following commands correctly adds a new member with hostname 'mongo3.example.net:27017' to an existing replica set configuration?

MongoDB

var cfg = rs.conf();
cfg.members.push({ _id: 2, host: 'mongo3.example.net:27017' });
rs.reconfig(cfg);

A var cfg = rs.conf(); cfg.members.push({ _id: 2, host: 'mongo3.example.net:27017' }); rs.reconfig(cfg);

B var cfg = rs.status(); cfg.members.push({ _id: 2, host: 'mongo3.example.net:27017' }); rs.reconfig(cfg);

C var cfg = rs.conf(); cfg.members.add({ _id: 2, host: 'mongo3.example.net:27017' }); rs.reconfig(cfg);

D var cfg = rs.conf(); cfg.push({ _id: 2, host: 'mongo3.example.net:27017' }); rs.reconfig(cfg);

💡 Hint

Check the method used to get the current config and how to add a member to the members array.

The rs.conf() method returns the current replica set configuration. To add a member, you push a new object to the members array. Then you call rs.reconfig() with the updated config. Using rs.status() or incorrect methods like add or pushing directly to cfg will cause errors.

🔧 Debug

advanced

Diagnose the error when reconfiguring a replica set

A user runs the following commands to add a new member but gets an error: 'reconfig failed: new configuration must have a unique _id for each member'. What is the likely cause?

MongoDB

var cfg = rs.conf();
cfg.members.push({ _id: 1, host: 'mongo3.example.net:27017' });
rs.reconfig(cfg);

A The rs.reconfig() command must be run with force:true option.

B The host field is incorrect and must include the replica set name.

C The configuration object must be cloned before modification.

D The _id 1 is already used by an existing member, causing a duplicate _id error.

💡 Hint

Check the uniqueness of the _id field for each member in the config.

Each member in a replica set configuration must have a unique _id. Using an _id that already exists causes the error. The host field format is correct, and force is not required unless the primary is down. Cloning is not mandatory.

🧠 Conceptual

expert

Understanding priority and elections in replica sets

In a MongoDB replica set, what effect does setting a member's priority to 0 have on elections?

A The member cannot vote or become primary.

B The member cannot become primary but can vote in elections.

C The member becomes the highest priority for primary election.

D The member is removed from the replica set.

💡 Hint

Priority controls eligibility to become primary, voting controls election votes.

Setting a member's priority to 0 means it cannot become primary but still participates in voting during elections. To prevent voting, the votes field must be set to 0.

Practice

1. What is the main purpose of a MongoDB replica set?

easy

A. To keep multiple copies of data for safety and availability

B. To speed up queries by using multiple CPUs

C. To store data in different formats

D. To backup data to an external server

Solution

1. Step 1: Understand what a replica set is

   A replica set is a group of MongoDB servers that keep copies of the same data.

2. Step 2: Identify the main goal of this setup

   The main goal is to keep data safe and available by having multiple copies.

3. Final Answer:

   To keep multiple copies of data for safety and availability -> Option A

4. Quick Check:

   Replica set purpose = data safety and availability [OK]

Hint: Replica sets keep data copies for safety and uptime [OK]

Common Mistakes:

• Thinking replica sets speed up queries directly
• Confusing replica sets with backups
• Assuming replica sets change data formats

2. Which command correctly initiates a replica set with one member named 'mongo1'?

easy

A. rs.start({ _id: 'rs0', members: ['mongo1'] })

B. rs.initiate({ _id: 'rs0', members: [{ _id: 0, host: 'mongo1:27017' }] })

C. replicaSet.create({ name: 'rs0', nodes: ['mongo1'] })

D. initReplicaSet({ id: 'rs0', hosts: ['mongo1:27017'] })

Solution

1. Step 1: Recall the correct syntax for initiating a replica set

   The command is rs.initiate() with a config object including _id and members array.

2. Step 2: Check the options for correct syntax and fields

   rs.initiate({ _id: 'rs0', members: [{ _id: 0, host: 'mongo1:27017' }] }) uses rs.initiate with _id and members array with _id and host fields correctly.

3. Final Answer:

   rs.initiate({ _id: 'rs0', members: [{ _id: 0, host: 'mongo1:27017' }] }) -> Option B

4. Quick Check:

   rs.initiate with config object = correct syntax [OK]

Hint: Use rs.initiate with _id and members array for setup [OK]

Common Mistakes:

• Using wrong command names like rs.start or initReplicaSet
• Passing members as simple strings instead of objects
• Missing port number in host field

3. Given this replica set config:

rs.initiate({ _id: 'rs0', members: [ { _id: 0, host: 'mongo1:27017' }, { _id: 1, host: 'mongo2:27017' } ] })

What will be the state of the second member immediately after initiation?

medium

A. ARBITER

B. SECONDARY

C. PRIMARY

D. STARTUP

Solution

1. Step 1: Understand member states after rs.initiate()

   After initiation, members start in STARTUP state before election completes.

2. Step 2: Identify the initial state of the second member

   The second member will be in STARTUP state until it syncs and elections finish.

3. Final Answer:

   STARTUP -> Option D

4. Quick Check:

   New members start as STARTUP before becoming SECONDARY [OK]

Hint: New members start in STARTUP state before election [OK]

Common Mistakes:

• Assuming member is immediately PRIMARY or SECONDARY
• Confusing ARBITER with normal members
• Ignoring the startup phase after initiation

4. You run rs.initiate() but get an error: "members array is missing or empty." What is the likely cause?

medium

A. You did not provide a members array in the config object

B. You used the wrong command name

C. You included an invalid port number

D. You forgot to start the mongod service

Solution

1. Step 1: Analyze the error message

   The error says the members array is missing or empty, meaning the config lacks members.

2. Step 2: Match error to common causes

   Not providing the members array in rs.initiate config causes this error.

3. Final Answer:

   You did not provide a members array in the config object -> Option A

4. Quick Check:

   Missing members array = error message [OK]

Hint: Always include members array in rs.initiate config [OK]

Common Mistakes:

• Ignoring the need for members array
• Assuming wrong command name causes this error
• Thinking service not started causes this error

5. You want to add a third member as an arbiter to your replica set. Which configuration snippet correctly adds it?

hard

A. { _id: 2, host: 'mongo3:27017', votes: 0 }

B. { _id: 2, host: 'mongo3:27017', priority: 2 }

C. { _id: 2, host: 'mongo3:27017', arbiterOnly: true }

D. { _id: 2, host: 'mongo3:27017', hidden: true }

Solution

1. Step 1: Understand arbiter configuration

   An arbiter member is configured with the field arbiterOnly set to true.

2. Step 2: Identify the correct option

   { _id: 2, host: 'mongo3:27017', arbiterOnly: true } sets arbiterOnly: true, which correctly marks the member as an arbiter.

3. Final Answer:

   { _id: 2, host: 'mongo3:27017', arbiterOnly: true } -> Option C

4. Quick Check:

   ArbiterOnly true = arbiter member [OK]

Hint: Use arbiterOnly: true to add an arbiter member [OK]

Common Mistakes:

• Using priority or votes fields incorrectly for arbiters
• Confusing hidden members with arbiters
• Not setting arbiterOnly to true for arbiters