Replica set configuration basics in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initiate a replica set with the name 'rs0'.

MongoDB

rs.initiate({ _id: [1], members: [{ _id: 0, host: "localhost:27017" }] })

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

A"rs0"

B0

C"replicaSet"

Drs0

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around the replica set name.

Using a number instead of a string for the replica set name.

The replica set name must be a string, so it should be enclosed in quotes.

2fill in blank

medium

Complete the code to add a second member to the replica set with host 'localhost:27018'.

MongoDB

rs.initiate({ _id: "rs0", members: [{ _id: 0, host: "localhost:27017" }, { _id: [1], host: "localhost:27018" }] })

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

A1

B"1"

C2

D"2"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of an integer for _id.

Using a duplicate _id value.

Member _id values must be unique integers starting from 0.

3fill in blank

hard

Fix the error in the code to correctly configure the priority of the first member to 2.

MongoDB

rs.initiate({ _id: "rs0", members: [{ _id: 0, host: "localhost:27017", priority: [1] }] })

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

A"2"

Bpriority: 2

Ctrue

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Putting quotes around the priority number.

Including the key 'priority:' inside the blank.

The priority value must be a number without quotes.

4fill in blank

hard

Fill both blanks to configure a hidden member with _id 2 and host 'localhost:27019'.

MongoDB

rs.initiate({ _id: "rs0", members: [{ _id: 0, host: "localhost:27017" }, { _id: 1, host: "localhost:27018" }, { _id: [1], host: [2], hidden: true }] })

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

A2

B"localhost:27019"

C"localhost:27020"

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong _id number.

Using the wrong host string.

The hidden member should have _id 2 and the correct host string.

5fill in blank

hard

Fill all three blanks to configure an arbiter with _id 3, host 'localhost:27020', and votes set to 1.

MongoDB

rs.initiate({ _id: "rs0", members: [{ _id: 0, host: "localhost:27017" }, { _id: 1, host: "localhost:27018" }, { _id: 2, host: "localhost:27019", hidden: true }, { _id: [1], host: [2], arbiterOnly: true, votes: [3] }] })

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

A2

B"localhost:27020"

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using duplicate _id values.

Setting votes to 0 or forgetting to set votes.

The arbiter must have a unique _id 3, the correct host string, and votes set to 1.

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