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.