Write concern basics in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the write concern to acknowledge writes on the primary only.

MongoDB

db.collection.insertOne({name: "Alice"}, {writeConcern: {w: [1])

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

A0

Bmajority

C1

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'majority' when only primary acknowledgment is needed.

Using 0 which means no acknowledgment.

Setting w: 1 means the write is acknowledged by the primary only.

2fill in blank

medium

Complete the code to set the write concern to require acknowledgment from the majority of replica set members.

MongoDB

db.collection.insertOne({name: "Bob"}, {writeConcern: {w: [1])

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

Aall

B1

C0

Dmajority

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 'majority' for stronger write guarantees.

Using 0 which means no acknowledgment.

Setting w: 'majority' ensures the write is acknowledged by most nodes in the replica set.

3fill in blank

hard

Fix the error in the write concern option to wait for acknowledgment from two nodes.

MongoDB

db.collection.insertOne({name: "Carol"}, {writeConcern: {w: [1])

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

Amajority

B2

C"two"

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string like 'two' instead of the number 2.

Using 'majority' when a specific number is required.

The value 2 means the write must be acknowledged by two nodes. It must be a number, not a string.

4fill in blank

hard

Fill both blanks to set write concern to wait for majority acknowledgment and a timeout of 5000 milliseconds.

MongoDB

db.collection.insertOne({name: "Dave"}, {writeConcern: {w: [1], wtimeout: [2])

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

Amajority

B5000

C1000

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a timeout value too low or too high.

Using numeric w instead of 'majority' for majority acknowledgment.

Setting w: 'majority' waits for most nodes, and wtimeout: 5000 sets a 5-second timeout.

5fill in blank

hard

Fill all three blanks to set write concern to wait for 3 nodes, enable journaling, and set a timeout of 2000 milliseconds.

MongoDB

db.collection.insertOne({name: "Eve"}, {writeConcern: {w: [1], j: [2], wtimeout: [3])

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

Atrue

B2000

C3

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using string 'true' instead of boolean true.

Setting wtimeout too high or too low.

Setting w: 3 waits for three nodes, j: true enables journaling, and wtimeout: 2000 sets a 2-second timeout.

Practice

1.

What does write concern in MongoDB control?

easy

A. The size of the database files

B. The speed of reading data from the database

C. The number of users connected to the database

D. How sure MongoDB is that your data is saved

Solution

1. Step 1: Understand the role of write concern

   Write concern defines the level of acknowledgment requested from MongoDB when writing data.

2. Step 2: Identify what write concern controls

   It controls how sure the database is that the data has been saved successfully.

3. Final Answer:

   How sure MongoDB is that your data is saved -> Option D

4. Quick Check:

   Write concern = Data save confirmation [OK]

Hint: Write concern = data save confirmation level [OK]

Common Mistakes:

• Confusing write concern with read speed
• Thinking it controls database size
• Assuming it manages user connections

2.

Which of the following is the correct way to set a write concern of w: 1 in a MongoDB insert operation?

db.collection.insertOne({name: 'Alice'}, {writeConcern: ???})

easy

A. {w: 1}

B. {w: 'majority'}

C. {w: 0}

D. {w: true}

Solution

1. Step 1: Recall write concern syntax

   Write concern is set as an object with key w and a value indicating the level.

2. Step 2: Match the correct value for w: 1

   The correct syntax is {w: 1} to wait for acknowledgment from one server.

3. Final Answer:

   {w: 1} -> Option A

4. Quick Check:

   Write concern syntax = {w: 1} [OK]

Hint: Use {w: 1} to wait for one server acknowledgment [OK]

Common Mistakes:

• Using string 'true' instead of number 1
• Confusing 'majority' with numeric 1
• Setting w to 0 which means no acknowledgment

3.

What will happen if you run this MongoDB command?

db.orders.insertOne({item: 'book'}, {writeConcern: {w: 0}})

medium

A. The insert does not wait for any confirmation

B. The insert waits for confirmation from the server

C. The insert waits for confirmation from majority of servers

D. The insert throws a syntax error

Solution

1. Step 1: Understand writeConcern w: 0 meaning

   Setting w: 0 means no acknowledgment is required from the server.

2. Step 2: Predict behavior of insertOne with w: 0

   The insert operation will send data but not wait for any confirmation, so it returns immediately.

3. Final Answer:

   The insert does not wait for any confirmation -> Option A

4. Quick Check:

   w: 0 means no wait for confirmation [OK]

Hint: w: 0 means fire-and-forget, no wait [OK]

Common Mistakes:

• Thinking w: 0 waits for server confirmation
• Assuming syntax error due to w: 0
• Confusing w: 0 with majority write concern

4.

Identify the error in this MongoDB write concern usage:

db.users.insertOne({name: 'Bob'}, {writeConcern: {w: 'two'}})

medium

A. The write concern should be set as a string without quotes

B. The value 'two' is invalid for write concern w

C. The insertOne method does not accept write concern

D. The document format is incorrect

Solution

1. Step 1: Check valid values for write concern w

   Write concern w accepts numbers or 'majority', not arbitrary strings like 'two'.

2. Step 2: Identify the error in the given code

   Using 'two' is invalid and will cause an error.

3. Final Answer:

   The value 'two' is invalid for write concern w -> Option B

4. Quick Check:

   Write concern w must be number or 'majority' [OK]

Hint: Write concern w must be number or 'majority' only [OK]

Common Mistakes:

• Using invalid string values for w
• Thinking writeConcern is not allowed in insertOne
• Confusing quotes usage in write concern

5.

You want to ensure your MongoDB write operation waits for confirmation from the majority of replica set members but also times out if it takes more than 5 seconds. Which write concern option should you use?

hard

A. {w: 0, wtimeout: 5000}

B. {w: 1, wtimeout: 5000}

C. {w: 'majority', wtimeout: 5000}

D. {w: 'majority', wtimeout: 0}

Solution

1. Step 1: Understand write concern for majority

   To wait for majority confirmation, w must be set to 'majority'.

2. Step 2: Add timeout for waiting

   Use wtimeout to specify max wait time in milliseconds; 5000 means 5 seconds.

3. Step 3: Combine options correctly

   The correct option is {w: 'majority', wtimeout: 5000} to wait for majority with 5 seconds timeout.

4. Final Answer:

   {w: 'majority', wtimeout: 5000} -> Option C

5. Quick Check:

   Majority + 5s timeout = {w: 'majority', wtimeout: 5000} [OK]

Hint: Use w: 'majority' with wtimeout in ms for timeout [OK]

Common Mistakes:

• Using w: 1 instead of 'majority' for majority confirmation
• Setting wtimeout to 0 which means no timeout
• Using w: 0 which disables waiting