Transaction read-then-write pattern in Firebase - Mini Project: Build & Apply

Transaction read-then-write pattern

📖 Scenario: You are building a simple inventory system using Firebase Realtime Database. You want to safely update the stock count of a product only if the current stock is enough to fulfill an order.

🎯 Goal: Build a Firebase transaction that reads the current stock of a product, checks if there is enough stock, and if yes, reduces the stock by the order quantity.

📋 What You'll Learn

Create a reference to the product stock in Firebase Realtime Database

Set an order quantity variable

Use a Firebase transaction to read the current stock

Inside the transaction, check if stock is enough and update it

Return the updated stock or abort the transaction if not enough stock

💡 Why This Matters

🌍 Real World

This pattern is used in inventory management systems to prevent overselling products by ensuring stock counts are updated safely even with multiple users.

💼 Career

Understanding Firebase transactions is important for developers working on real-time applications that require safe concurrent data updates.

1

Create a Firebase reference to product stock

Create a constant called productStockRef that points to the Firebase Realtime Database path 'products/product123/stock' using firebase.database().ref().

Firebase

// Your code here

Need a hint?

Use firebase.database().ref('path') to get a reference to the database location.

2

Set the order quantity variable

Create a constant called orderQuantity and set it to the number 3.

Firebase

const productStockRef = firebase.database().ref('products/product123/stock');
// Your code here

Need a hint?

Use const orderQuantity = 3; to set the order quantity.

3

Write the transaction to read and update stock

Use productStockRef.transaction with a callback function that takes currentStock as input. Inside the function, if currentStock is null, return null. If currentStock is less than orderQuantity, return undefined to abort. Otherwise, return currentStock - orderQuantity.

Firebase

const productStockRef = firebase.database().ref('products/product123/stock');
const orderQuantity = 3;
// Your code here

Need a hint?

The transaction callback receives the current value. Return the new value to update or undefined to abort.

4

Add completion handler to transaction

Add a second argument to productStockRef.transaction which is a callback function with parameters error, committed, and snapshot. Inside, if error exists, handle it. If committed is true, the transaction succeeded. Use snapshot.val() to get the updated stock.

Firebase

const productStockRef = firebase.database().ref('products/product123/stock');
const orderQuantity = 3;

productStockRef.transaction(currentStock => {
  if (currentStock === null) {
    return null;
  }
  if (currentStock  {
  // Your code here
});

Need a hint?

The second callback handles the transaction result. Check error and committed flags.