Write concern and data durability in MongoDB - Mini Project: Build & Apply

Understanding Write Concern and Data Durability in MongoDB

📖 Scenario: You are managing a small online store database using MongoDB. You want to make sure that when you add new orders, the data is safely saved and acknowledged properly to avoid losing any important information.

🎯 Goal: Build a MongoDB script that inserts order documents with different write concern levels to understand how data durability works.

📋 What You'll Learn

Create a MongoDB collection named orders with sample order documents.

Set a write concern variable to control acknowledgment level.

Insert a new order document using the specified write concern.

Add a final command to check the write concern settings applied.

💡 Why This Matters

🌍 Real World

In real-world applications, ensuring data is safely written to the database is critical to avoid data loss, especially in systems like online stores where orders must be reliably saved.

💼 Career

Understanding write concern and data durability is essential for database administrators and backend developers to configure MongoDB for reliable and consistent data storage.

1

Create the orders collection with sample data

Create a variable called orders that represents a MongoDB collection. Insert these exact documents into orders: { orderId: 1, item: "Book", quantity: 3 } and { orderId: 2, item: "Pen", quantity: 10 }.

MongoDB

// Create the orders collection and insert sample documents
// Your code here

Hint

Use db.collection('orders') to get the collection. Use insertMany to add multiple documents.

2

Set the write concern variable

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

MongoDB

const orders = db.collection('orders');
orders.insertMany([
  { orderId: 1, item: "Book", quantity: 3 },
  { orderId: 2, item: "Pen", quantity: 10 }
]);

// Set write concern to w: 1
// Your code here

Hint

Write concern is an object with a w property. Use { w: 1 } for primary acknowledgment.

3

Insert a new order with the write concern

Use the insertOne method on orders to add this document: { orderId: 3, item: "Notebook", quantity: 5 }. Pass the writeConcern variable as the write concern option.

MongoDB

const orders = db.collection('orders');
orders.insertMany([
  { orderId: 1, item: "Book", quantity: 3 },
  { orderId: 2, item: "Pen", quantity: 10 }
]);

const writeConcern = { w: 1 };

// Insert new order with write concern
// Your code here

Hint

Pass the write concern as an option object in the second argument of insertOne.

4

Check the write concern settings

Create a variable called currentWriteConcern and assign it the value of orders.writeConcern to check the current write concern settings of the collection.

MongoDB

const orders = db.collection('orders');
orders.insertMany([
  { orderId: 1, item: "Book", quantity: 3 },
  { orderId: 2, item: "Pen", quantity: 10 }
]);

const writeConcern = { w: 1 };

orders.insertOne(
  { orderId: 3, item: "Notebook", quantity: 5 },
  { writeConcern: writeConcern }
);

// Check current write concern
// Your code here

Hint

Access the writeConcern property of the collection to see its settings.

Practice

1. What does the writeConcern option in MongoDB control?

easy

A. How MongoDB confirms that data is saved to the database

B. The format of data stored in MongoDB

C. The speed of data retrieval from MongoDB

D. The size limit of documents in MongoDB

Solution

1. Step 1: Understand the role of write concern

   Write concern tells MongoDB how to confirm that a write operation is saved and durable.

2. Step 2: Differentiate from other options

   It does not control data format, retrieval speed, or document size.

3. Final Answer:

   How MongoDB confirms that data is saved to the database -> Option A

4. Quick Check:

   Write concern = data save confirmation [OK]

Hint: Write concern = confirmation of data saved [OK]

Common Mistakes:

• Confusing write concern with data format
• Thinking it controls read speed
• Assuming it limits document size

2. Which of the following is the correct way to specify a write concern that waits for acknowledgment from the primary in MongoDB?

easy

A. { writeConcern: { w: -1 } }

B. { writeConcern: { w: 0 } }

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

D. { writeConcern: { w: 1 } }

Solution

1. Step 1: Recall write concern values

   w:1 means wait for acknowledgment from the primary node only.

2. Step 2: Check other options

   w:0 means no acknowledgment; w:'majority' waits for majority nodes; w:-1 is invalid.

3. Final Answer:

   { writeConcern: { w: 1 } } -> Option D

4. Quick Check:

   w:1 = primary ack wait [OK]

Hint: w:1 waits for primary acknowledgment [OK]

Common Mistakes:

• Using w:0 which means no acknowledgment
• Confusing 'majority' with primary only
• Using invalid negative values

3. Consider this MongoDB insert command with write concern:

db.collection.insertOne({name: 'Alice'}, {writeConcern: {w: 'majority', wtimeout: 5000}})

What happens if the majority of nodes do not acknowledge the write within 5 seconds?

medium

A. The write operation succeeds immediately without waiting

B. The write operation retries indefinitely until acknowledged

C. The write operation fails with a timeout error

D. The write operation ignores the write concern and writes anyway

Solution

1. Step 1: Understand w:'majority' and wtimeout

   The write waits for majority nodes to confirm within 5000 ms (5 seconds).

2. Step 2: Outcome if timeout exceeded

   If majority nodes don't respond in time, MongoDB returns a timeout error and the write is considered failed.

3. Final Answer:

   The write operation fails with a timeout error -> Option C

4. Quick Check:

   w:'majority' + wtimeout = fail if no timely ack [OK]

Hint: wtimeout limits wait; timeout causes failure [OK]

Common Mistakes:

• Assuming write succeeds immediately
• Thinking MongoDB retries forever
• Ignoring write concern on timeout

4. You wrote this code to insert a document with write concern:

db.collection.insertOne({item: 'book'}, {writeConcern: {w: 2, wtimeout: '3000'}})

But it throws an error. What is the problem?

medium

A. wtimeout must be a number, not a string

B. w:2 is invalid; only 0,1, or 'majority' allowed

C. insertOne does not accept writeConcern option

D. The document field 'item' is not allowed

Solution

1. Step 1: Check writeConcern syntax

   wtimeout expects a number (milliseconds), not a string.

2. Step 2: Validate other parts

   w:2 is valid if the replica set has at least 2 nodes; insertOne accepts writeConcern; 'item' field is valid.

3. Final Answer:

   wtimeout must be a number, not a string -> Option A

4. Quick Check:

   wtimeout type must be number [OK]

Hint: wtimeout needs number, not quotes [OK]

Common Mistakes:

• Using string for wtimeout
• Thinking w:2 is invalid always
• Assuming insertOne disallows writeConcern

5. You want to ensure that a write is saved to disk on the primary and replicated to at least two secondary nodes before confirming success. Which write concern setting achieves this?

hard

A. { writeConcern: { w: 1, j: true } }

B. { writeConcern: { w: 3, j: true } }

C. { writeConcern: { w: 'majority', j: false } }

D. { writeConcern: { w: 2, j: false } }

Solution

1. Step 1: Understand w and j options

   w:3 means wait for acknowledgment from primary plus two secondaries (total 3 nodes). j:true means wait for write to be committed to journal (disk) on primary.

2. Step 2: Check other options

   B waits only for primary with journaling; C waits for majority but no journaling; D waits for 2 nodes but no journaling.

3. Final Answer:

   { writeConcern: { w: 3, j: true } } -> Option B

4. Quick Check:

   w:3 + j:true = 3 nodes + disk write [OK]

Hint: w:number = nodes ack; j:true = disk commit [OK]

Common Mistakes:

• Ignoring journaling for durability
• Using w:'majority' without knowing node count
• Assuming w:2 means two secondaries only