Read preference for replica sets in MongoDB - Mini Project: Build & Apply

Read Preference for Replica Sets in MongoDB

📖 Scenario: You are managing a MongoDB database with a replica set. You want to control how your application reads data from the primary and secondary members of the replica set to balance load and ensure data freshness.

🎯 Goal: Build a MongoDB query setup that uses different read preferences to read data from the replica set members.

📋 What You'll Learn

Create a MongoDB collection named products with sample documents.

Set a read preference variable to secondaryPreferred.

Write a query that uses the read preference variable to read documents from the products collection.

Add a final configuration to confirm the read preference is applied.

💡 Why This Matters

🌍 Real World

In real applications, controlling read preference helps balance load between primary and secondary database servers, improving performance and availability.

💼 Career

Database administrators and backend developers often configure read preferences to optimize data reads in distributed MongoDB setups.

1

Create the products collection with sample data

Create a MongoDB collection called products and insert these exact documents: { _id: 1, name: "Pen", price: 1.5 }, { _id: 2, name: "Notebook", price: 3 }, and { _id: 3, name: "Eraser", price: 0.5 }.

MongoDB

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

Need a hint?

Use db.products.insertMany() with an array of documents.

2

Set the read preference variable

Create a variable called readPref and set it to the string "secondaryPreferred" to specify the read preference.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Pen", price: 1.5 },
  { _id: 2, name: "Notebook", price: 3 },
  { _id: 3, name: "Eraser", price: 0.5 }
])

// Create the readPref variable and set it to "secondaryPreferred"
// Your code here

Need a hint?

Use const readPref = "secondaryPreferred"; to set the variable.

3

Query the products collection using the read preference

Write a query that finds all documents in the products collection using the readPref variable as the read preference option.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Pen", price: 1.5 },
  { _id: 2, name: "Notebook", price: 3 },
  { _id: 3, name: "Eraser", price: 0.5 }
])

const readPref = "secondaryPreferred";

// Use the readPref variable to read all documents from products
// Your code here

Need a hint?

Use db.products.find().readPref(readPref) to apply the read preference.

4

Confirm the read preference is applied

Add a line to assign the read preference readPref to the cursor variable explicitly to confirm the read preference is set.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Pen", price: 1.5 },
  { _id: 2, name: "Notebook", price: 3 },
  { _id: 3, name: "Eraser", price: 0.5 }
])

const readPref = "secondaryPreferred";

const cursor = db.products.find().readPref(readPref);

// Confirm the read preference is set on cursor
// Your code here

Need a hint?

Call cursor.readPref() to confirm the read preference is applied.