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

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

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

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
});

Hint

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

Practice

1. What is the main purpose of using a transaction in Firebase when updating data?

easy

A. To speed up data retrieval from the database

B. To store data permanently without changes

C. To safely read and write data together without conflicts

D. To delete data automatically after a set time

Solution

1. Step 1: Understand transaction role

   Transactions allow reading and writing data atomically, meaning both happen together safely.

2. Step 2: Identify conflict prevention

   This prevents conflicts when multiple users try to update the same data at once.

3. Final Answer:

   To safely read and write data together without conflicts -> Option C

4. Quick Check:

   Transaction = safe read-write [OK]

Hint: Transactions combine read and write safely [OK]

Common Mistakes:

• Thinking transactions only speed up reads
• Assuming transactions delete data
• Believing transactions store data permanently without changes

2. Which of the following is the correct syntax to start a transaction in Firebase Firestore using JavaScript?

easy

A. db.startTransaction(() => { /* code */ })

B. db.transactionStart(() => { /* code */ })

C. db.beginTransaction(async () => { /* code */ })

D. db.runTransaction(async (transaction) => { /* code */ })

Solution

1. Step 1: Recall Firestore transaction syntax

   The correct method to run a transaction is db.runTransaction with an async callback.

2. Step 2: Check other options

   Other options like transactionStart, beginTransaction, or startTransaction are not valid Firestore methods.

3. Final Answer:

   db.runTransaction(async (transaction) => { /* code */ }) -> Option D

4. Quick Check:

   runTransaction is correct method [OK]

Hint: Use db.runTransaction with async callback [OK]

Common Mistakes:

• Using non-existent methods like transactionStart
• Forgetting async keyword in callback
• Confusing transaction syntax with other database APIs

3. Given this transaction code snippet, what will be the final value of the 'count' field if the document initially has count = 5?

db.runTransaction(async (transaction) => {
  const docRef = db.collection('counters').doc('counter1');
  const doc = await transaction.get(docRef);
  if (!doc.exists) {
    transaction.set(docRef, { count: 1 });
  } else {
    const newCount = doc.data().count + 1;
    transaction.update(docRef, { count: newCount });
  }
});

medium

A. 6

B. 1

C. 5

D. 0

Solution

1. Step 1: Check if document exists

   The document exists with count = 5, so the else block runs.

2. Step 2: Calculate new count

   New count is 5 + 1 = 6, and transaction updates the document with this value.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Existing count + 1 = 6 [OK]

Hint: If doc exists, increment count by 1 [OK]

Common Mistakes:

• Assuming count resets to 1 even if doc exists
• Forgetting to add 1 to existing count
• Confusing set and update behavior

4. Identify the error in this transaction code snippet:

db.runTransaction(async (transaction) => {
  const docRef = db.collection('items').doc('item1');
  const doc = await transaction.get(docRef);
  const currentStock = doc.data().stock;
  if (currentStock > 0) {
    transaction.update(docRef, { stock: currentStock - 1 });
  }
});

medium

A. Missing check if document exists before accessing data

B. Using transaction.update instead of transaction.set

C. Not awaiting transaction.update call

D. Incorrect collection name syntax

Solution

1. Step 1: Check document existence

   The code accesses doc.data().stock without verifying if doc.exists, which can cause errors if doc is missing.

2. Step 2: Confirm other parts

   transaction.update is correct for partial update, awaiting update is not needed as transaction methods handle it, and collection name syntax is correct.

3. Final Answer:

   Missing check if document exists before accessing data -> Option A

4. Quick Check:

   Always check doc.exists before data [OK]

Hint: Always check doc.exists before reading data [OK]

Common Mistakes:

• Skipping doc.exists check causing runtime errors
• Confusing update with set methods
• Expecting to await transaction.update explicitly

5. You want to implement a transaction that increments a user's score only if the current score is less than 100. Which of the following code snippets correctly applies the transaction read-then-write pattern to achieve this?

hard

A. db.runTransaction(async (transaction) => {
const userRef = db.collection('users').doc('user1');
const doc = await transaction.get(userRef);
if (doc.data().score < 100) {
transaction.set(userRef, { score: doc.data().score + 1 });
}
});

B. db.runTransaction(async (transaction) => {
const userRef = db.collection('users').doc('user1');
const doc = await transaction.get(userRef);
if (doc.exists && doc.data().score < 100) {
transaction.update(userRef, { score: doc.data().score + 1 });
}
});

C. db.runTransaction(async (transaction) => {
const userRef = db.collection('users').doc('user1');
const doc = await transaction.get(userRef);
transaction.update(userRef, { score: 100 });
});

D. db.runTransaction(async (transaction) => {
const userRef = db.collection('users').doc('user1');
transaction.update(userRef, { score: 1 });
});

Solution

1. Step 1: Check document existence and condition

   db.runTransaction(async (transaction) => { const userRef = db.collection('users').doc('user1'); const doc = await transaction.get(userRef); if (doc.exists && doc.data().score < 100) { transaction.update(userRef, { score: doc.data().score + 1 }); } }); correctly checks if the document exists and if the score is less than 100 before updating.

2. Step 2: Verify update method and logic

   It uses transaction.update to increment the score by 1 only when condition is met, preserving existing data.

3. Final Answer:

   db.runTransaction(async (transaction) => { const userRef = db.collection('users').doc('user1'); const doc = await transaction.get(userRef); if (doc.exists && doc.data().score < 100) { transaction.update(userRef, { score: doc.data().score + 1 }); } }); -> Option B

4. Quick Check:

   Check exists + condition before update [OK]

Hint: Check doc exists and condition before update [OK]

Common Mistakes:

• Not checking if doc exists before reading data
• Using set instead of update causing overwrite
• Updating without condition or setting fixed value