Batch writes in Firebase - Mini Project: Build & Apply

Batch writes

📖 Scenario: You are building a simple app that needs to add multiple user profiles to a Firestore database at once. To keep the data consistent and save time, you will use batch writes.

🎯 Goal: Create a Firestore batch write that adds three user documents with exact data in a single commit.

📋 What You'll Learn

Create a Firestore batch instance

Add three user documents with specified IDs and fields to the batch

Commit the batch to write all documents at once

💡 Why This Matters

🌍 Real World

Batch writes are used to perform multiple database changes at once, ensuring all succeed or fail together. This is useful for syncing related data or improving performance.

💼 Career

Understanding batch writes is important for backend and full-stack developers working with Firestore to maintain data consistency and optimize database operations.

1

Initialize Firestore and batch

Create a variable called db that initializes Firestore using getFirestore(). Then create a variable called batch that initializes a batch write using writeBatch(db).

Firebase

# Your code here

Hint

Use getFirestore() to get the Firestore instance and writeBatch(db) to start a batch.

2

Prepare user document references

Create three variables called userRef1, userRef2, and userRef3. Each should use doc(db, 'users', 'user1'), doc(db, 'users', 'user2'), and doc(db, 'users', 'user3') respectively to reference documents in the 'users' collection.

Firebase

import { getFirestore, writeBatch, doc } from 'firebase/firestore';

const db = getFirestore();
const batch = writeBatch(db);

# Your code here

Hint

Use doc(db, 'users', 'userX') to get document references for each user.

3

Add user data to batch

Use batch.set() to add three user documents to the batch. For userRef1, set {name: 'Alice', age: 30}. For userRef2, set {name: 'Bob', age: 25}. For userRef3, set {name: 'Charlie', age: 35}.

Firebase

import { getFirestore, writeBatch, doc } from 'firebase/firestore';

const db = getFirestore();
const batch = writeBatch(db);

const userRef1 = doc(db, 'users', 'user1');
const userRef2 = doc(db, 'users', 'user2');
const userRef3 = doc(db, 'users', 'user3');

# Your code here

Hint

Use batch.set(documentRef, data) to add each user document to the batch.

4

Commit the batch write

Add a line to commit the batch write using await batch.commit(). Make sure the function is async or use a top-level await if supported.

Firebase

import { getFirestore, writeBatch, doc } from 'firebase/firestore';

const db = getFirestore();
const batch = writeBatch(db);

const userRef1 = doc(db, 'users', 'user1');
const userRef2 = doc(db, 'users', 'user2');
const userRef3 = doc(db, 'users', 'user3');

batch.set(userRef1, { name: 'Alice', age: 30 });
batch.set(userRef2, { name: 'Bob', age: 25 });
batch.set(userRef3, { name: 'Charlie', age: 35 });

# Your code here

Hint

Use await batch.commit() to send all writes to Firestore at once.

Practice

1. What is the main purpose of using batch writes in Firebase?

easy

A. To group multiple data changes into a single request

B. To read data faster from the database

C. To create multiple databases at once

D. To backup data automatically

Solution

1. Step 1: Understand batch writes concept

   Batch writes allow grouping many data changes into one request to keep data consistent.

2. Step 2: Identify the main purpose

   The main purpose is to combine multiple changes to save time and ensure consistency.

3. Final Answer:

   To group multiple data changes into a single request -> Option A

4. Quick Check:

   Batch writes = group changes [OK]

Hint: Batch writes combine changes to save time and keep data consistent [OK]

Common Mistakes:

• Thinking batch writes speed up data reading
• Confusing batch writes with database creation
• Assuming batch writes automatically backup data

2. Which of the following is the correct way to start a batch write in Firebase?

easy

A. const batch = firestore.batch();

B. const batch = firestore.startBatch();

C. const batch = firestore.createBatchWrite();

D. const batch = firestore.newBatch();

Solution

1. Step 1: Recall Firebase batch write syntax

   The correct method to start a batch is firestore.batch().

2. Step 2: Compare options

   Only const batch = firestore.batch(); uses the exact Firebase API method firestore.batch().

3. Final Answer:

   const batch = firestore.batch(); -> Option A

4. Quick Check:

   Start batch with firestore.batch() [OK]

Hint: Use firestore.batch() to start batch writes [OK]

Common Mistakes:

• Using non-existent methods like startBatch or newBatch
• Confusing batch creation with other Firestore methods
• Misspelling the batch method name

3. What will happen if you run this Firebase batch write code?

const batch = firestore.batch();
const docRef1 = firestore.collection('users').doc('user1');
batch.set(docRef1, {name: 'Alice'});
const docRef2 = firestore.collection('users').doc('user2');
batch.update(docRef2, {age: 30});
await batch.commit();

medium

A. Both documents 'user1' and 'user2' will be updated or created as specified

B. Only 'user1' document will be created; 'user2' update will fail silently

C. The batch commit will fail if 'user2' document does not exist

D. The batch will commit but no changes will be made

Solution

1. Step 1: Understand batch set and update behavior

   batch.set creates or overwrites a document; batch.update requires the document to exist.

2. Step 2: Analyze the code effect

   Since 'user2' might not exist, batch.update will cause the entire batch commit to fail.

3. Final Answer:

   The batch commit will fail if 'user2' document does not exist -> Option C

4. Quick Check:

   batch.update needs existing doc = fail if missing [OK]

Hint: batch.update fails if document missing, causing batch failure [OK]

Common Mistakes:

• Assuming batch.update creates documents
• Thinking batch.commit partially applies changes
• Ignoring that batch fails on any single error

4. Identify the error in this Firebase batch write code:

const batch = firestore.batch();
const docRef = firestore.collection('items').doc('item1');
batch.update(docRef, {price: 100});
batch.set(docRef, {name: 'Book'});
await batch.commit();

medium

A. Calling batch.update before batch.set on the same document causes an error

B. batch.set cannot be used after batch.update in the same batch

C. batch.update requires a callback function

D. No error; batch.commit will succeed and apply both changes

Solution

1. Step 1: Review batch operation order rules

   Firebase batch allows multiple operations on the same doc; last write wins.

2. Step 2: Analyze the code

   batch.update then batch.set on same doc means set overwrites update fields.

3. Final Answer:

   No error; batch.commit will succeed and apply both changes -> Option D

4. Quick Check:

   Multiple ops allowed; last write wins [OK]

Hint: Batch allows multiple ops on same doc; last write applies [OK]

Common Mistakes:

• Thinking batch.update must come after batch.set
• Assuming batch.set overwrites cause errors
• Believing batch.update needs a callback

5. You want to update multiple user profiles atomically using batch writes. Which approach ensures all updates succeed or none apply?

hard

A. Use batch.set for each user document and commit once

B. Use batch.update for each user document and commit once

C. Use separate batch commits for each user document update

D. Use batch.delete for each user document before updating

Solution

1. Step 1: Understand atomic batch writes

   Batch writes commit all changes together; if one fails, none apply.

2. Step 2: Choose correct method for updating existing profiles

   batch.update modifies existing docs without overwriting all fields, suitable for updates.

3. Step 3: Analyze options

   Use batch.update for each user document and commit once uses batch.update for all users and commits once, ensuring atomicity.

4. Final Answer:

   Use batch.update for each user document and commit once -> Option B

5. Quick Check:

   batch.update + single commit = atomic update [OK]

Hint: Use batch.update and one commit for atomic multi-doc updates [OK]

Common Mistakes:

• Using multiple commits losing atomicity
• Using batch.set which overwrites entire documents
• Deleting docs before updating unnecessarily