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.

Practice

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

easy

A. To backup data manually every hour

B. To keep copies of data on multiple servers for safety and availability

C. To store data in different formats on the same server

D. To speed up queries by using multiple CPUs on one server

Solution

1. Step 1: Understand replica set function

   A replica set keeps copies of data on multiple servers to protect against failure.

2. Step 2: Compare options

   Only To keep copies of data on multiple servers for safety and availability describes this purpose correctly; others describe unrelated tasks.

3. Final Answer:

   To keep copies of data on multiple servers for safety and availability -> Option B

4. Quick Check:

   Replica set = data copies on servers [OK]

Hint: Replica sets = multiple copies for safety [OK]

Common Mistakes:

• Confusing replica sets with sharding
• Thinking replica sets speed up CPU usage
• Believing backups are manual in replica sets

2. Which of the following is the correct way MongoDB decides the primary in a replica set?

easy

A. MongoDB automatically elects a primary among members

B. The server with the most data becomes primary

C. The oldest server is always primary

D. The user manually sets the primary server

Solution

1. Step 1: Recall primary election process

   MongoDB automatically elects one server as primary to manage writes.

2. Step 2: Eliminate incorrect options

   Primary is not chosen by data size, age, or manual setting.

3. Final Answer:

   MongoDB automatically elects a primary among members -> Option A

4. Quick Check:

   Primary election = automatic [OK]

Hint: Primary is chosen automatically, not manually [OK]

Common Mistakes:

• Assuming user sets primary manually
• Thinking data size decides primary
• Believing oldest server is primary

3. Consider a replica set with one primary and two secondaries. If the primary goes down, what happens next?

medium

A. All secondaries become primary at the same time

B. The replica set stops working until the original primary returns

C. One of the secondaries is automatically elected as the new primary

D. The data is lost because there is no primary

Solution

1. Step 1: Understand failover in replica sets

   If the primary fails, MongoDB elects a new primary from secondaries automatically.

2. Step 2: Check other options

   Replica sets do not stop working, do not have multiple primaries, and do not lose data immediately.

3. Final Answer:

   One of the secondaries is automatically elected as the new primary -> Option C

4. Quick Check:

   Failover = automatic new primary [OK]

Hint: Primary failure triggers automatic new primary election [OK]

Common Mistakes:

• Thinking replica set stops on primary failure
• Believing multiple primaries exist simultaneously
• Assuming data loss happens immediately

4. You set up a replica set but notice writes are failing. Which of these could be the cause?

medium

A. All members are healthy and synced

B. The replica set has only one member

C. Secondary nodes are accepting writes

D. No primary is elected in the replica set

Solution

1. Step 1: Identify write requirements in replica sets

   Writes must go to the primary; if no primary exists, writes fail.

2. Step 2: Analyze options

   Healthy members or single member replica sets usually allow writes; secondaries do not accept writes.

3. Final Answer:

   No primary is elected in the replica set -> Option D

4. Quick Check:

   Writes need primary elected [OK]

Hint: Writes fail if no primary exists [OK]

Common Mistakes:

• Thinking secondaries accept writes
• Assuming single member replica set blocks writes
• Ignoring primary election status

5. You want to ensure your MongoDB replica set remains available even if two servers fail. What is the minimum number of members you should have in your replica set?

hard

A. 5 members

B. 4 members

C. 3 members

D. 6 members

Solution

1. Step 1: Understand majority voting in replica sets

   Replica sets use majority votes to elect primary; to tolerate 2 failures, majority must be > half of total members.

2. Step 2: Calculate minimum members

   With 5 members, majority is 3; losing 2 still leaves 3 members to form majority and keep primary.

3. Final Answer:

   5 members -> Option A

4. Quick Check:

   To survive 2 failures, need 5 members [OK]

Hint: Majority = more than half; 5 members survive 2 failures [OK]

Common Mistakes:

• Choosing 3 or 4 members which can't tolerate 2 failures
• Not considering majority voting
• Assuming even number members are better