Election process concept in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all candidates in the election collection.

MongoDB

db.election.[1]()

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

Ainsert

Bupdate

Cfind

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert() instead of find()

Using update() when you want to read data

The find() method retrieves documents from a collection.

2fill in blank

medium

Complete the code to count how many votes a candidate named 'Alice' has.

MongoDB

db.votes.countDocuments({ candidate: [1] })

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

A'John'

B'Alice'

C'Eve'

D'Bob'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the name

Using the wrong candidate name

We count documents where the candidate field equals 'Alice'.

3fill in blank

hard

Fix the error in the query to find votes where the count is greater than 100.

MongoDB

db.votes.find({ count: { [1]: 100 } })

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

A$gt

B$eq

C$lt

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt (less than) instead of $gt

Using $eq which means equal

The operator $gt means 'greater than'.

4fill in blank

hard

Fill both blanks to update the vote count for candidate 'Bob' to 250.

MongoDB

db.votes.updateOne({ candidate: [1] }, { [2]: { count: 250 } })

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

A'Bob'

B$set

C$inc

D'Alice'

Attempts:

3 left

💡 Hint

Common Mistakes

Using $inc instead of $set

Using the wrong candidate name

We match candidate 'Bob' and use $set to change the count to 250.

5fill in blank

hard

Fill all three blanks to create a dictionary with candidate names as keys and their vote counts as values, but only for counts greater than 100.

MongoDB

result = { [1]: [2] for doc in db.votes.find({ count: { [3]: 100 } }) }

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

Adoc.candidate

Bdoc.count

C$gt

Ddoc.name

Attempts:

3 left

💡 Hint

Common Mistakes

Using doc.name instead of doc.candidate

Using $lt instead of $gt

Swapping keys and values

We use doc.candidate as keys, doc.count as values, and filter with $gt for counts greater than 100.

Practice

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

easy

A. To select a primary server that manages writes

B. To delete old data from the database

C. To backup the database automatically

D. To create new collections in the database

Solution

1. Step 1: Understand the role of election in MongoDB

   In a replica set, election chooses a primary server to handle write operations.

2. Step 2: Identify the correct purpose

   The primary server manages writes and coordinates data replication.

3. Final Answer:

   To select a primary server that manages writes -> Option A

4. Quick Check:

   Election = select primary server [OK]

Hint: Election picks the primary server for writes [OK]

Common Mistakes:

• Thinking election deletes data
• Confusing election with backup process
• Believing election creates collections

2. Which MongoDB command shows the current election status in a replica set?

easy

A. db.showElection()

B. db.electionStatus()

C. rs.status()

D. rs.election()

Solution

1. Step 1: Recall the command to check replica set status

   The command rs.status() shows detailed replica set info including election status.

2. Step 2: Verify other options

   Other commands like db.showElection() or rs.election() do not exist in MongoDB.

3. Final Answer:

   rs.status() -> Option C

4. Quick Check:

   Check election with rs.status() [OK]

Hint: Use rs.status() to see election info [OK]

Common Mistakes:

• Using non-existent commands
• Confusing db commands with rs commands
• Trying to use election-specific commands that don't exist

3. Given the following output from rs.status(), which member is currently the primary?

{
  "members": [
    {"_id": 0, "stateStr": "SECONDARY"},
    {"_id": 1, "stateStr": "PRIMARY"},
    {"_id": 2, "stateStr": "SECONDARY"}
  ]
}

medium

A. Member with _id 0

B. No primary elected yet

C. Member with _id 2

D. Member with _id 1

Solution

1. Step 1: Read the stateStr values for each member

   Member with _id 1 has stateStr "PRIMARY", others are "SECONDARY".

2. Step 2: Identify the primary member

   The member with stateStr "PRIMARY" is the current primary server.

3. Final Answer:

   Member with _id 1 -> Option D

4. Quick Check:

   Primary = stateStr "PRIMARY" member [OK]

Hint: Primary member has stateStr 'PRIMARY' in rs.status() [OK]

Common Mistakes:

• Choosing a secondary member
• Assuming no primary if unsure
• Ignoring the stateStr field

4. You run rs.status() but get an error: not authorized on admin to execute command. What is the likely cause?

medium

A. You lack proper user permissions to run rs.status()

B. You are not connected to the primary server

C. The replica set is not initialized

D. The command rs.status() is misspelled

Solution

1. Step 1: Understand the error message

   The error says 'not authorized', meaning permission is denied.

2. Step 2: Match error to cause

   Lack of user permissions to run the command causes this error, not connection or spelling issues.

3. Final Answer:

   You lack proper user permissions to run rs.status() -> Option A

4. Quick Check:

   Authorization error = permission issue [OK]

Hint: Authorization errors mean missing user permissions [OK]

Common Mistakes:

• Assuming connection problem
• Thinking replica set not initialized
• Blaming command spelling

5. In a MongoDB replica set with 5 members, what minimum number of votes is needed to elect a primary during an election?

hard

A. 2 votes

B. 3 votes

C. 4 votes

D. 5 votes

Solution

1. Step 1: Recall election voting rules in MongoDB

   Primary election requires a majority of votes from members.

2. Step 2: Calculate majority for 5 members

   Majority is more than half, so 3 out of 5 votes are needed.

3. Final Answer:

   3 votes -> Option B

4. Quick Check:

   Majority of 5 = 3 votes [OK]

Hint: Majority votes needed: half members + 1 [OK]

Common Mistakes:

• Choosing half votes instead of majority
• Thinking all members must vote
• Confusing votes with number of members