When transactions are necessary vs unnecessary in MongoDB - Hands-On Comparison

Understanding When Transactions Are Necessary vs Unnecessary in MongoDB

📖 Scenario: You are managing a small online bookstore database using MongoDB. You want to understand when you need to use transactions to keep your data safe and when you can safely update data without transactions.

🎯 Goal: Build a simple MongoDB setup to practice inserting and updating documents with and without transactions, so you can see when transactions are necessary and when they are not.

📋 What You'll Learn

Create a collection called books with sample book documents

Create a variable session to start a transaction

Write a transaction that updates two documents atomically

Write a simple update outside a transaction to show when it is unnecessary

💡 Why This Matters

🌍 Real World

In real-world applications, transactions help keep data consistent when multiple related changes must happen together, like transferring stock between products.

💼 Career

Understanding when to use transactions is important for database administrators and backend developers to ensure data integrity and avoid unnecessary performance costs.

1

Create the books collection with sample documents

Create a books collection with these exact documents: { _id: 1, title: "Book A", stock: 5 } and { _id: 2, title: "Book B", stock: 3 }.

MongoDB

// Create the books collection with two documents
// Your code here

Need a hint?

Use insertMany to add multiple documents to the books collection.

2

Start a session to use transactions

Create a variable called session and start a session using db.getMongo().startSession().

MongoDB

db.books.insertMany([
  { _id: 1, title: "Book A", stock: 5 },
  { _id: 2, title: "Book B", stock: 3 }
])

// Start a session for transactions
// Your code here

Need a hint?

Use db.getMongo().startSession() to create a session for transactions.

3

Use a transaction to update stock atomically

Use session.startTransaction() to start a transaction. Then update the stock of Book A by subtracting 1 and update Book B by adding 1 inside the transaction. Commit the transaction with session.commitTransaction().

MongoDB

db.books.insertMany([
  { _id: 1, title: "Book A", stock: 5 },
  { _id: 2, title: "Book B", stock: 3 }
])

const session = db.getMongo().startSession()

// Start transaction and update stocks atomically
// Your code here

Need a hint?

Use session.startTransaction() to begin, pass { session } to update commands, and call session.commitTransaction() to finish.

4

Update a single document without a transaction

Update the stock of Book A by adding 2 without using a transaction. This shows when transactions are unnecessary for single updates.

MongoDB

db.books.insertMany([
  { _id: 1, title: "Book A", stock: 5 },
  { _id: 2, title: "Book B", stock: 3 }
])

const session = db.getMongo().startSession()

session.startTransaction()
db.books.updateOne({ _id: 1 }, { $inc: { stock: -1 } }, { session })
db.books.updateOne({ _id: 2 }, { $inc: { stock: 1 } }, { session })
session.commitTransaction()

// Update Book A stock by 2 without transaction
// Your code here

Need a hint?

Use db.books.updateOne without a session to update a single document.