Write concern levels (w: 1, majority) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

MongoDB

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

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

A0

Ball

Cmajority

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 means no acknowledgment.

Using 'majority' waits for more nodes.

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

2fill in blank

medium

Complete the code to set write concern to wait for majority of nodes to acknowledge.

MongoDB

db.collection.updateOne({_id: 1}, {$set: {score: 100}}, {writeConcern: {w: [1])

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

A1

B0

Cmajority

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 waits only for primary.

Using 0 means no acknowledgment.

Using w: 'majority' waits for most nodes to confirm the write.

3fill in blank

hard

Fix the error in the write concern option to correctly wait for majority acknowledgment.

MongoDB

db.collection.deleteOne({_id: 5}, {writeConcern: {w: [1])

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

Amajority

B1

CMajority

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using capitalized 'Majority' causes error.

Using numeric values won't wait for majority.

The write concern value 'majority' must be lowercase string exactly.

4fill in blank

hard

Fill both blanks to set write concern to acknowledge by primary and timeout after 5000ms.

MongoDB

db.collection.insertMany(docs, {writeConcern: {w: [1], wtimeout: [2])

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

A1

Bmajority

C5000

D1000

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'majority' with a low timeout may cause errors.

Using 0 for w means no acknowledgment.

Setting w: 1 waits for primary acknowledgment and wtimeout: 5000 sets 5 seconds timeout.

5fill in blank

hard

Fill all three blanks to set write concern to majority, timeout 2000ms, and journal enabled.

MongoDB

db.collection.updateMany(filter, update, {writeConcern: {w: [1], wtimeout: [2], j: [3])

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

A1

B2000

CTrue

Dmajority

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'true' as string instead of boolean.

Using 0 for w disables acknowledgment.

Using w: 'majority' waits for majority, wtimeout: 2000 sets 2 seconds timeout, and j: true enables journaling.

Practice

1. What does the write concern w: 1 mean in MongoDB?

easy

A. The write is acknowledged by all servers in the cluster.

B. The write is acknowledged by only one server before returning success.

C. The write is acknowledged by the majority of servers before returning success.

D. The write is not acknowledged by any server.

Solution

1. Step 1: Understand write concern w: 1

   This means the write operation waits for acknowledgment from only one server.

2. Step 2: Compare with other write concerns

   w: majority waits for most servers, so w: 1 is less strict and faster.

3. Final Answer:

   The write is acknowledged by only one server before returning success. -> Option B

4. Quick Check:

   Write concern w: 1 = one server acknowledgment [OK]

Hint: Remember: w:1 means wait for one server only [OK]

Common Mistakes:

• Confusing w:1 with w:majority
• Thinking w:1 waits for all servers
• Assuming w:1 means no acknowledgment

2. Which of the following is the correct syntax to set write concern to majority in a MongoDB write operation?

easy

A. { writeConcern: { w: majority } }

B. { writeConcern: { w: "1" } }

C. { writeConcern: { w: majority'' } }

D. { writeConcern: { w: 'majority' } }

Solution

1. Step 1: Identify correct syntax for string values

   Write concern values like 'majority' must be strings enclosed in quotes.

2. Step 2: Check each option

   { writeConcern: { w: 'majority' } } uses quotes correctly. { writeConcern: { w: majority } } misses quotes. { writeConcern: { w: majority'' } } has syntax error with extra quotes. { writeConcern: { w: "1" } } sets w to string '1', not 'majority'.

3. Final Answer:

   { writeConcern: { w: 'majority' } } -> Option D

4. Quick Check:

   Use quotes for string write concern values [OK]

Hint: Write concern strings need quotes like 'majority' [OK]

Common Mistakes:

• Omitting quotes around 'majority'
• Using invalid quotes or syntax
• Confusing w:1 string with w:'majority'

3. Consider this MongoDB write operation with write concern:

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

What does this guarantee about the write?

medium

A. The write is confirmed by the majority of replica set members before success.

B. The write is not confirmed by any server before success.

C. The write is confirmed by at least one server before success.

D. The write is confirmed by all servers including secondaries and arbiters.

Solution

1. Step 1: Understand w: 'majority' write concern

   This means the write waits for acknowledgment from most members of the replica set.

2. Step 2: Compare with other options

   The write is confirmed by at least one server before success. is w:1. The write is not confirmed by any server before success. means no acknowledgment. The write is confirmed by all servers including secondaries and arbiters. is incorrect because arbiters do not store data and are not counted.

3. Final Answer:

   The write is confirmed by the majority of replica set members before success. -> Option A

4. Quick Check:

   w: 'majority' means majority confirmation [OK]

Hint: Majority means most data-bearing members confirm write [OK]

Common Mistakes:

• Thinking arbiters count towards majority
• Confusing majority with all servers
• Assuming w:majority is same as w:1

4. You wrote this code:

db.collection.insertOne({item: 'book'}, {writeConcern: {w: majority}})

But it throws an error. What is the problem?

medium

A. The write concern value majority must be a string in quotes.

B. The writeConcern option is misspelled.

C. The insertOne method does not accept writeConcern option.

D. The document to insert is missing required fields.

Solution

1. Step 1: Check syntax of write concern value

   The value majority must be a string, so it needs quotes: 'majority'.

2. Step 2: Verify other parts

   writeConcern is spelled correctly, insertOne accepts writeConcern, and document is valid.

3. Final Answer:

   The write concern value majority must be a string in quotes. -> Option A

4. Quick Check:

   Write concern strings need quotes [OK]

Hint: Always quote string write concern values like 'majority' [OK]

Common Mistakes:

• Forgetting quotes around 'majority'
• Assuming writeConcern is misspelled
• Thinking insertOne disallows writeConcern

5. You want to ensure your MongoDB writes are safe and confirmed by most servers, but you also want to avoid waiting for all servers. Which write concern should you choose and why?

hard

A. Use w: 1 because it waits for only one server, making writes fastest.

B. Use w: 0 because it does not wait, making writes fastest but unsafe.

C. Use w: majority because it waits for most servers, balancing safety and speed.

D. Use w: all because it waits for every server, ensuring maximum safety.

Solution

1. Step 1: Understand the trade-offs

   w: 1 is fast but less safe. w: majority waits for most servers, giving good safety without waiting for all.

2. Step 2: Evaluate options

   w: all waits for every server, which can be slow. w: 0 is unsafe as it doesn't wait for acknowledgment.

3. Final Answer:

   Use w: majority because it waits for most servers, balancing safety and speed. -> Option C

4. Quick Check:

   Majority write concern balances safety and speed [OK]

Hint: Majority write concern balances safety and speed well [OK]

Common Mistakes:

• Choosing w:1 for safety
• Assuming w:all is always best
• Ignoring performance impact of write concern