Write concern basics in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is write concern in MongoDB?

Write concern is a setting that controls the level of acknowledgment requested from MongoDB when performing write operations. It tells MongoDB how sure you want to be that your data was saved.

beginner

What does w: 1 mean in write concern?

It means the write operation must be acknowledged by the primary server only before reporting success. This is the default level.

intermediate

What happens if you set w: 'majority' in write concern?

The write operation waits until a majority of replica set members have acknowledged the write. This increases data safety by ensuring replication.

intermediate

What is the role of wtimeout in write concern?

It sets a time limit (in milliseconds) for how long MongoDB waits for the write concern to be satisfied. If the time expires, an error is returned.

advanced

What does w: 0 mean and when might it be used?

It means no acknowledgment is requested from the server. The client does not wait for confirmation. It can be used for very fast writes where data loss is acceptable.

What does write concern control in MongoDB?

A The speed of queries

B The size of the database

C The number of indexes

D How many servers acknowledge a write operation

Write concern controls the level of acknowledgment from servers for write operations.

Which write concern value waits for acknowledgment from the primary only?

A w: 1

B w: all

C w: majority

D w: 0

w: 1 means acknowledgment from the primary server only.

What does setting wtimeout do?

A Defines the size of the write batch

B Sets the number of writes per second

C Limits how long to wait for write acknowledgment

D Specifies the number of indexes to update

wtimeout sets a time limit for waiting for write concern acknowledgment.

If you want the fastest write without waiting for confirmation, which write concern do you use?

A w: majority

B w: 0

C w: 1

D w: 2

w: 0 means no acknowledgment is requested, so writes are fastest but less safe.

What does w: 'majority' ensure?

A Write is acknowledged by a majority of replica set members

B Write is acknowledged by the primary only

C Write is acknowledged by all members

D Write is not acknowledged

w: 'majority' waits for acknowledgment from most replica set members.

Explain what write concern is and why it matters in MongoDB.

Think about how sure you want to be that your data is saved.

Describe the differences between write concern values w: 0, w: 1, and w: majority.

Consider how many servers confirm the write.

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