Write concern levels (w: 1, majority) in MongoDB - Mini Project: Build & Apply

Understanding MongoDB Write Concern Levels (w: 1, majority)

📖 Scenario: You are managing a MongoDB database for an online store. 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 different write concern levels in MongoDB to control the acknowledgment of write operations, specifically using w: 1 and w: majority.

📋 What You'll Learn

Create a MongoDB collection named products with sample documents.

Set a write concern variable with w: 1.

Insert a new product document using the w: 1 write concern.

Change the write concern to w: majority and insert another product document.

💡 Why This Matters

🌍 Real World

Write concerns help ensure data is safely written to the database, which is critical for applications like online stores where data loss can cause serious problems.

💼 Career

Understanding write concerns is important for database administrators and backend developers to balance data safety and performance in MongoDB.

1

Create the products collection with initial documents

Create a MongoDB collection called products and insert these exact documents: { _id: 1, name: "Laptop", price: 1200 } and { _id: 2, name: "Smartphone", price: 800 }.

MongoDB

// Insert initial documents into the products collection
// Your code here

Hint

Use insertMany to add multiple documents to the products collection.

2

Set a write concern variable with w: 1

Create a variable called writeConcernOne and set it to an object with w: 1.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Laptop", price: 1200 },
  { _id: 2, name: "Smartphone", price: 800 }
])

// Create writeConcernOne variable with w: 1
// Your code here

Hint

Use const to declare the variable and set w: 1 inside an object.

3

Insert a new product using the w: 1 write concern

Use db.products.insertOne() to insert the document { _id: 3, name: "Tablet", price: 600 } with the write concern set to the variable writeConcernOne.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Laptop", price: 1200 },
  { _id: 2, name: "Smartphone", price: 800 }
])

const writeConcernOne = { w: 1 }

// Insert a new product with writeConcernOne
// Your code here

Hint

Pass the write concern as the second argument in insertOne.

4

Set w: majority write concern and insert another product

Create a variable called writeConcernMajority with w: "majority". Then insert the document { _id: 4, name: "Monitor", price: 300 } into products using writeConcernMajority.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Laptop", price: 1200 },
  { _id: 2, name: "Smartphone", price: 800 }
])

const writeConcernOne = { w: 1 }

db.products.insertOne({ _id: 3, name: "Tablet", price: 600 }, { writeConcern: writeConcernOne })

// Create writeConcernMajority and insert a new product with it
// Your code here

Hint

Use w: "majority" in the object and pass it as the write concern option in insertOne.

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