Read from secondaries trade-offs in MongoDB - Mini Project: Build & Apply

Understanding Read from Secondaries Trade-offs in MongoDB

📖 Scenario: You are managing a MongoDB database for an online bookstore. To improve read performance and reduce load on the primary server, you want to read data from secondary replicas. However, you need to understand the trade-offs involved in reading from secondaries, such as data consistency and replication lag.

🎯 Goal: Build a MongoDB query setup that reads book data from secondary replicas using readPreference. Learn how to configure the read preference and understand the impact on data freshness and consistency.

📋 What You'll Learn

Create a MongoDB collection named books with sample book documents.

Set a read preference variable to read from secondary replicas.

Write a query to find all books using the secondary read preference.

Add a final configuration to handle potential replication lag.

💡 Why This Matters

🌍 Real World

Reading from secondary replicas helps distribute read load and improve performance in real-world applications like online stores or social media platforms.

💼 Career

Understanding read preferences and replication lag is important for database administrators and backend developers to ensure data consistency and performance.

1

Create the books collection with sample data

Create a MongoDB collection called books and insert these exact documents: { title: "The Great Gatsby", author: "F. Scott Fitzgerald", year: 1925 }, { title: "1984", author: "George Orwell", year: 1949 }, and { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960 }.

MongoDB

// Insert the sample book documents into the books collection
// Your code here

Need a hint?

Use db.books.insertMany() with an array of objects containing the exact book details.

2

Set the read preference to secondary

Create a variable called readPref and set it to { readPreference: "secondary" } to configure queries to read from secondary replicas.

MongoDB

db.books.insertMany([
  { title: "The Great Gatsby", author: "F. Scott Fitzgerald", year: 1925 },
  { title: "1984", author: "George Orwell", year: 1949 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960 }
])

// Create a variable readPref with readPreference set to 'secondary'
// Your code here

Need a hint?

Use const readPref = { readPreference: "secondary" } to set the read preference.

3

Query the books collection using the secondary read preference

Write a query using db.books.find() with the readPref variable to read all documents from the secondary replicas.

MongoDB

db.books.insertMany([
  { title: "The Great Gatsby", author: "F. Scott Fitzgerald", year: 1925 },
  { title: "1984", author: "George Orwell", year: 1949 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960 }
])

const readPref = { readPreference: "secondary" }

// Use db.books.find() with readPref to read from secondary replicas
// Your code here

Need a hint?

Use db.books.find({}, null, readPref) to query with the read preference.

4

Add a maxStalenessSeconds option to handle replication lag

Extend the readPref variable to include maxStalenessSeconds: 90 to limit how stale the data can be when reading from secondaries.

MongoDB

db.books.insertMany([
  { title: "The Great Gatsby", author: "F. Scott Fitzgerald", year: 1925 },
  { title: "1984", author: "George Orwell", year: 1949 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960 }
])

// Update readPref to include maxStalenessSeconds: 90
const readPref = { readPreference: "secondary" }

const booksFromSecondary = db.books.find({}, null, readPref)

// Your code here

Need a hint?

Add maxStalenessSeconds: 90 inside the readPref object.