Read concern levels (local, majority, snapshot) in MongoDB - Mini Project: Build & Apply

Understanding MongoDB Read Concern Levels

📖 Scenario: You are managing a MongoDB database for a small online store. You want to understand how different read concern levels affect the data you read from the database.

🎯 Goal: Learn to set and use different MongoDB read concern levels: local, majority, and snapshot in queries.

📋 What You'll Learn

Create a MongoDB collection with sample documents

Set a read concern level variable

Query the collection using the specified read concern

Complete the query with the correct read concern option

💡 Why This Matters

🌍 Real World

Understanding read concern levels helps ensure your application reads data with the right consistency and freshness, important for financial or inventory systems.

💼 Career

Database administrators and backend developers often configure read concerns to balance performance and data accuracy in distributed databases.

1

Create a collection with sample data

Create a MongoDB collection called products and insert these exact documents: { _id: 1, name: 'Pen', stock: 100 }, { _id: 2, name: 'Notebook', stock: 200 }, and { _id: 3, name: 'Eraser', stock: 150 }.

MongoDB

// Your code here to create 'products' collection and insert documents

Hint

Use insertMany to add multiple documents to the products collection.

2

Set the read concern level

Create a variable called readConcernLevel and set it to the string 'majority'.

MongoDB

db.products.insertMany([
  { _id: 1, name: 'Pen', stock: 100 },
  { _id: 2, name: 'Notebook', stock: 200 },
  { _id: 3, name: 'Eraser', stock: 150 }
])

// Create variable readConcernLevel and set to 'majority'
// Your code here

Hint

Use const to create the variable readConcernLevel and assign the string 'majority'.

3

Query the collection with the read concern

Write a query to find all documents in the products collection using the read concern level stored in readConcernLevel. Use db.products.find() with the readConcern option set to { level: readConcernLevel }.

MongoDB

db.products.insertMany([
  { _id: 1, name: 'Pen', stock: 100 },
  { _id: 2, name: 'Notebook', stock: 200 },
  { _id: 3, name: 'Eraser', stock: 150 }
])

const readConcernLevel = 'majority';

// Query products with readConcern option using readConcernLevel
// Your code here

Hint

Use db.products.find({}, { readConcern: { level: readConcernLevel } }) to query with the read concern.

4

Complete the query to use snapshot read concern

Change the readConcernLevel variable to the string 'snapshot' and update the query to use this new read concern level.

MongoDB

db.products.insertMany([
  { _id: 1, name: 'Pen', stock: 100 },
  { _id: 2, name: 'Notebook', stock: 200 },
  { _id: 3, name: 'Eraser', stock: 150 }
])

// Change readConcernLevel to 'snapshot'
// Update the query to use the new readConcernLevel
// Your code here

Hint

Update the variable readConcernLevel to 'snapshot' and use it in the query options.

Practice

1. Which MongoDB read concern level provides the fastest read but may return data that is not yet acknowledged by majority of nodes?

easy

A. local

B. majority

C. snapshot

D. linearizable

Solution

1. Step 1: Understand read concern levels

   local returns the most recent data on the node but does not wait for replication.

2. Step 2: Compare with other levels

   majority waits for data to be acknowledged by majority nodes, snapshot is for transactions, and linearizable is stricter and slower.

3. Final Answer:

   local -> Option A

4. Quick Check:

   Fastest read = local [OK]

Hint: Fastest read is always local [OK]

Common Mistakes:

• Confusing local with majority
• Thinking snapshot is fastest
• Assuming linearizable is default

2. Which of the following is the correct syntax to set read concern to majority in a MongoDB query?

easy

A. db.collection.find.readConcern('majority')

B. db.collection.find().readConcern = 'majority'

C. db.collection.find().readConcern('majority')

D. db.collection.find().withReadConcern('majority')

Solution

1. Step 1: Recall MongoDB read concern syntax

   The correct method to set read concern on a query is readConcern('level').

2. Step 2: Check options for syntax correctness

   Some options look similar, but one omits parentheses after find. The correct chaining is db.collection.find().readConcern('majority').

3. Final Answer:

   db.collection.find().readConcern('majority') -> Option C

4. Quick Check:

   Correct method call = db.collection.find().readConcern('majority') [OK]

Hint: Use .readConcern('level') after find() [OK]

Common Mistakes:

• Using assignment instead of method call
• Using non-existent method names
• Placing parentheses incorrectly

3. Given a MongoDB transaction using snapshot read concern, what will be the behavior if a document is updated after the transaction starts but before it reads the document?

medium

A. The transaction reads the updated document reflecting the change.

B. The transaction reads the document state as it was at the start of the transaction.

C. The transaction throws an error due to data inconsistency.

D. The transaction reads the document from the primary node only.

Solution

1. Step 1: Understand snapshot read concern in transactions

   snapshot ensures a consistent view of data as of the start of the transaction.

2. Step 2: Analyze document update timing

   Since the document was updated after the transaction started, the transaction will not see this change.

3. Final Answer:

   The transaction reads the document state as it was at the start of the transaction. -> Option B

4. Quick Check:

   snapshot = consistent start view [OK]

Hint: Snapshot reads see data as of transaction start [OK]

Common Mistakes:

• Assuming transaction sees latest updates
• Thinking snapshot causes errors on updates
• Confusing snapshot with majority read concern

4. You wrote this code snippet to set read concern to majority but it throws an error:

db.collection.find().readConcern = 'majority'

What is the error and how to fix it?

medium

A. No error, code is correct.

B. Error: readConcern must be set on the client, not query.

C. Error: 'majority' is not a valid read concern level.

D. Error: readConcern is a method, use readConcern('majority') instead.

Solution

1. Step 1: Identify the error cause

   Assigning a string to readConcern property is invalid because it is a method, not a property.

2. Step 2: Correct the syntax

   Use method call syntax: readConcern('majority') to set the read concern level.

3. Final Answer:

   Error: readConcern is a method, use readConcern('majority') instead. -> Option D

4. Quick Check:

   Use method call, not assignment [OK]

Hint: Use parentheses for readConcern, not assignment [OK]

Common Mistakes:

• Assigning string instead of calling method
• Setting read concern on wrong object
• Using invalid read concern levels

5. You want to ensure your MongoDB transaction reads a consistent snapshot of data and also that your reads outside transactions see only data confirmed by majority of nodes. Which read concern levels should you use respectively?

hard

A. snapshot for transactions, majority for outside reads

B. local for transactions, snapshot for outside reads

C. majority for transactions, local for outside reads

D. snapshot for transactions, local for outside reads

Solution

1. Step 1: Identify read concern for transactions

   snapshot read concern ensures consistent view inside transactions.

2. Step 2: Identify read concern for outside transactions

   majority ensures reads see data confirmed by majority of nodes, providing safety.

3. Final Answer:

   snapshot for transactions, majority for outside reads -> Option A

4. Quick Check:

   Transaction = snapshot, Outside = majority [OK]

Hint: Use snapshot in transactions, majority outside [OK]

Common Mistakes:

• Using local inside transactions
• Confusing majority with snapshot
• Using local for safe reads