Multi-document transactions in MongoDB - Mini Project: Build & Apply

Using Multi-Document Transactions in MongoDB

📖 Scenario: You are managing a simple banking system where users can transfer money between accounts. Each account is stored as a document in a MongoDB collection. To keep the data consistent, you need to update both the sender's and receiver's account balances in a single transaction.

🎯 Goal: Build a multi-document transaction in MongoDB that safely transfers money from one account to another, ensuring both accounts are updated together or not at all.

📋 What You'll Learn

Create two account documents with specific balances

Set up a MongoDB session for transactions

Write a transaction that deducts money from one account and adds it to another

Commit the transaction to save changes atomically

💡 Why This Matters

🌍 Real World

Banking and financial applications require multi-document transactions to keep account balances consistent during transfers.

💼 Career

Understanding MongoDB transactions is important for backend developers working with NoSQL databases to ensure data integrity.

1

Create the accounts collection with two account documents

Create a MongoDB collection called accounts and insert two documents with these exact fields and values: { _id: 1, name: "Alice", balance: 500 } and { _id: 2, name: "Bob", balance: 300 }.

MongoDB

// Insert two account documents into the accounts collection
// Your code here

Need a hint?

Use insertMany on the accounts collection to add both documents at once.

2

Start a client session for the transaction

Create a variable called session by starting a client session using client.startSession().

MongoDB

db.accounts.insertMany([
  { _id: 1, name: "Alice", balance: 500 },
  { _id: 2, name: "Bob", balance: 300 }
])

// Start a client session and assign it to session
// Your code here

Need a hint?

Use client.startSession() to create a session for transactions.

3

Write the transaction to transfer money

Use session.withTransaction() to run a transaction function that deducts 100 from Alice's balance and adds 100 to Bob's balance using updateOne with { $inc: { balance: -100 } } and { $inc: { balance: 100 } } respectively. Use session as an option in both updates.

MongoDB

db.accounts.insertMany([
  { _id: 1, name: "Alice", balance: 500 },
  { _id: 2, name: "Bob", balance: 300 }
])

const session = client.startSession()

// Use session.withTransaction() to transfer 100 from Alice to Bob
// Your code here

Need a hint?

Inside withTransaction, use updateOne twice with the session option to update both accounts.

4

End the session after the transaction

Call session.endSession() to properly close the client session after the transaction.

MongoDB

db.accounts.insertMany([
  { _id: 1, name: "Alice", balance: 500 },
  { _id: 2, name: "Bob", balance: 300 }
])

const session = client.startSession()

await session.withTransaction(async () => {
  await db.accounts.updateOne({ _id: 1 }, { $inc: { balance: -100 } }, { session })
  await db.accounts.updateOne({ _id: 2 }, { $inc: { balance: 100 } }, { session })
})

// End the session
// Your code here

Need a hint?

Always call endSession() to close the session after finishing the transaction.