Replica set architecture mental model 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.stateStr === [1])

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

A"SECONDARY"

B"PRIMARY"

C"ARBITER"

D"HIDDEN"

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing "SECONDARY" which only replicates data but does not accept writes.

Confusing "ARBITER" with primary; arbiters do not hold data.

The primary node is the one with stateStr equal to "PRIMARY" in the replica set status.

2fill in blank

medium

Complete the command to add a new member to the replica set configuration.

MongoDB

cfg.members.push({ _id: 2, host: [1] }); rs.reconfig(cfg)

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

A"localhost:27020"

B"localhost:27017"

C"localhost:27019"

D"localhost:27018"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same port as the primary node (27017), causing conflicts.

Forgetting to call rs.reconfig after modifying the configuration.

When adding a new member, specify its host and port correctly. Here, port 27018 is used as an example for the new member.

3fill in blank

hard

Fix the error in the command to check the replica set status.

MongoDB

rs.[1]()

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

Astatus

Bstat

Cstate

Dinfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'stat' or 'state' which are not valid rs commands.

Trying 'info' which does not exist in the rs object.

The correct command to check the replica set status is rs.status(). Other options are invalid commands.

4fill in blank

hard

Fill both blanks to create a query that reads from secondary members only.

MongoDB

db.getMongo().setReadPref({ mode: [1], tags: [2] })

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

A"secondary"

B"primary"

C{}

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Setting mode to "primary" which reads only from the primary node.

Using null for tags which may cause errors.

To read from secondary members, set the read preference mode to "secondary" and use an empty tags object {} to include all secondaries.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps member hosts to their states for members in the replica set.

MongoDB

const memberStates = rs.status().members.reduce((acc, [3]) => { acc[[1]] = [2]; return acc; }, {})

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

Amember.host

Bmember.stateStr

Cmember

Dmembers

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'members' instead of 'member' in the loop variable.

Swapping key and value positions.

This expression creates an object mapping each member's host to its state string by iterating over each member.