Why consistency levels matter in MongoDB - See It in Action

Why Consistency Levels Matter in MongoDB

📖 Scenario: You are managing a small online store's database using MongoDB. You want to understand how different consistency levels affect the data your customers see when they browse products and place orders.

🎯 Goal: Build a simple MongoDB setup that demonstrates how to read data with different consistency levels and why it matters for accurate and reliable information.

📋 What You'll Learn

Create a MongoDB collection called products with specific product documents

Set a read preference variable to control consistency level

Write a query to find a product by product_id using the read preference

Add a final configuration to ensure the read preference is applied correctly

💡 Why This Matters

🌍 Real World

Online stores and many applications need to read data consistently to avoid showing outdated or incorrect information to users.

💼 Career

Understanding consistency levels in databases like MongoDB is important for roles such as database administrators, backend developers, and data engineers to ensure reliable data access.

1

DATA SETUP: Create the products collection with sample data

Create a MongoDB collection called products and insert these exact documents: { product_id: 1, name: "Laptop", stock: 50 }, { product_id: 2, name: "Smartphone", stock: 100 }, and { product_id: 3, name: "Headphones", stock: 75 }.

MongoDB

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

Need a hint?

Use insertMany on db.products with an array of the exact product objects.

2

CONFIGURATION: Set the read preference to primary

Create a variable called readPref and set it to the string "primary" to specify the consistency level for reading data.

MongoDB

db.products.insertMany([
  { product_id: 1, name: "Laptop", stock: 50 },
  { product_id: 2, name: "Smartphone", stock: 100 },
  { product_id: 3, name: "Headphones", stock: 75 }
])

// Set read preference to primary
// Your code here

Need a hint?

Use const readPref = "primary" to set the read preference.

3

CORE LOGIC: Query the products collection using the read preference

Write a query to find the product with product_id equal to 2 from the products collection. Use the readPref variable to set the read preference option in the query.

MongoDB

db.products.insertMany([
  { product_id: 1, name: "Laptop", stock: 50 },
  { product_id: 2, name: "Smartphone", stock: 100 },
  { product_id: 3, name: "Headphones", stock: 75 }
])

const readPref = "primary"

// Find product with product_id 2 using readPref
// Your code here

Need a hint?

Use db.products.findOne with a filter for product_id: 2 and pass { readPreference: readPref } as options.

4

COMPLETION: Confirm the read preference is applied in the query options

Add a final line to assign the read preference option explicitly in the query options object to ensure consistency level is respected.

MongoDB

db.products.insertMany([
  { product_id: 1, name: "Laptop", stock: 50 },
  { product_id: 2, name: "Smartphone", stock: 100 },
  { product_id: 3, name: "Headphones", stock: 75 }
])

const readPref = "primary"

const product = db.products.findOne({ product_id: 2 }, { readPreference: readPref })

// Ensure readPreference is set correctly in options
// Your code here

Need a hint?

Define const options = { readPreference: readPref } to explicitly set the read preference options.