Write concern basics in MongoDB - Mini Project: Build & Apply

Write Concern Basics in MongoDB

📖 Scenario: You are managing a small online store database using MongoDB. You want to ensure that when you add new products, the database confirms the write operation properly to avoid data loss.

🎯 Goal: Learn how to set up and use writeConcern in MongoDB to control the acknowledgment of write operations.

📋 What You'll Learn

Create a MongoDB collection named products with sample product documents.

Define a writeConcern configuration variable with w: 1.

Insert a new product document using the defined writeConcern.

Complete the insert operation with the writeConcern option applied.

💡 Why This Matters

🌍 Real World

Write concern is important in real-world applications to ensure data is safely written to the database and to avoid data loss in case of failures.

💼 Career

Understanding write concern is essential for database administrators and backend developers to maintain data integrity and reliability in production systems.

1

Create the products collection with sample data

Create a variable called products that holds an array with these exact product documents: { name: "Pen", price: 1.5 }, { name: "Notebook", price: 3 }, and { name: "Eraser", price: 0.5 }.

MongoDB

// Create the products array with the specified documents
// Your code here

Hint

Use a JavaScript array with objects for each product.

2

Define the writeConcern configuration

Create a variable called writeConcern and set it to an object with w: 1 to require acknowledgment from the primary server.

MongoDB

const products = [
  { name: "Pen", price: 1.5 },
  { name: "Notebook", price: 3 },
  { name: "Eraser", price: 0.5 }
];

// Define writeConcern with w: 1
// Your code here

Hint

Use an object with the key w set to 1.

3

Insert a new product using the writeConcern

Write a line to insert a new product document { name: "Marker", price: 2 } into the products collection using the writeConcern variable for the write concern option.

MongoDB

const products = [
  { name: "Pen", price: 1.5 },
  { name: "Notebook", price: 3 },
  { name: "Eraser", price: 0.5 }
];

const writeConcern = { w: 1 };

// Insert the new product with writeConcern
// Your code here

Hint

Use db.products.insertOne() with the second argument as { writeConcern }.

4

Complete the insert operation with the writeConcern

Add a line to assign the result of the insert operation to a variable called insertResult using the writeConcern option, completing the database write with acknowledgment.

MongoDB

const products = [
  { name: "Pen", price: 1.5 },
  { name: "Notebook", price: 3 },
  { name: "Eraser", price: 0.5 }
];

const writeConcern = { w: 1 };

const insertResult = db.products.insertOne({ name: "Marker", price: 2 }, { writeConcern });

// Finalize the write concern setup
// Your code here

Hint

Ensure the insert operation uses the writeConcern option and assigns the result to insertResult.

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