Write concern levels (w: 1, majority) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It means the write operation is acknowledged by the primary node only. The operation is confirmed as saved on the primary before returning success.

intermediate

Explain the write concern level w: majority in MongoDB.

The write operation is acknowledged only after a majority of replica set members confirm the write. This ensures better data durability and consistency.

intermediate

Why would you choose w: majority over w: 1?

Choosing w: majority ensures data is saved on most nodes, reducing risk of data loss if the primary fails. w: 1 is faster but less safe.

beginner

What happens if you set write concern to w: 0?

The write operation is unacknowledged. The client does not wait for confirmation, so it is faster but you don't know if the write succeeded.

intermediate

How does write concern affect application performance and reliability?

Higher write concern levels (like w: majority) increase reliability but add latency. Lower levels (like w: 1) are faster but risk data loss.

What does w: 1 guarantee in MongoDB write concern?

A Write is unacknowledged

B Write is confirmed by all replica set members

C Write is confirmed by a majority of nodes

D Write is confirmed by the primary node only

w: 1 means the primary node confirms the write before success is returned.

Which write concern level waits for most replica set members to confirm the write?

A w: 0

B w: majority

C w: 1

D w: all

w: majority waits for a majority of nodes to confirm the write.

What is a risk of using w: 0 write concern?

A Slower writes

B Write waits for majority confirmation

C No confirmation of write success

D Data is saved on all nodes

w: 0 means no acknowledgement, so you don't know if the write succeeded.

Which write concern level provides the highest data durability?

A w: majority

B w: 1

C w: 0

D w: 0 with journaling

w: majority ensures data is saved on most nodes, improving durability.

If you want the fastest write speed and can tolerate some data loss, which write concern would you choose?

A w: 0

B w: 1

C w: majority

D w: all

w: 0 is fastest but does not confirm writes.

Describe the differences between w: 1 and w: majority write concern levels in MongoDB.

Think about how many nodes confirm the write and the tradeoff between speed and safety.

Explain how write concern levels affect data safety and application performance.

Consider the balance between speed and reliability.

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