Batch limits and best practices in Firebase - Mini Project: Build & Apply

Batch Limits and Best Practices with Firebase

📖 Scenario: You are managing a Firebase Firestore database for a small online store. You want to update multiple product prices at once using batch writes. However, Firestore limits batch writes to 500 operations per batch. You need to handle this limit properly to avoid errors.

🎯 Goal: Build a Firebase batch write process that updates product prices in batches of 500 or less, following best practices to handle batch limits.

📋 What You'll Learn

Create a list of 600 product IDs with their new prices.

Set a batch size limit variable to 500.

Write a loop that processes the product updates in batches of 500 or less.

Commit each batch properly to Firestore.

💡 Why This Matters

🌍 Real World

Batch writes are essential when updating many documents in Firestore efficiently and within limits.

💼 Career

Understanding batch limits and best practices is important for backend developers and cloud engineers working with Firebase.

1

Create the product updates list

Create a list called productUpdates containing 600 objects. Each object should have id as a string from "product1" to "product600" and newPrice as a number starting from 10 and increasing by 1 for each product.

Firebase

# Create the productUpdates list with 600 products
# Your code here

Hint

Use a list comprehension with range(1, 601) to create 600 products.

2

Set the batch size limit

Create a variable called batchSize and set it to 500 to represent the Firestore batch write limit.

Firebase

productUpdates = [{"id": f"product{i}", "newPrice": 10 + i - 1} for i in range(1, 601)]
# Set the batchSize variable to 500
# Your code here

Hint

Just assign 500 to batchSize.

3

Write the batch processing loop

Write a for loop using range with variables start and end to process productUpdates in slices of size batchSize. Inside the loop, create a new batch with db.batch() and add update operations for each product in the current slice using batch.update(). Use db.collection('products').doc(product['id']) as the document reference and update the price field with product['newPrice']. Do not commit the batch yet.

Firebase

productUpdates = [{"id": f"product{i}", "newPrice": 10 + i - 1} for i in range(1, 601)]
batchSize = 500
# Write a for loop to process productUpdates in batches
# Your code here

Hint

Use range(0, len(productUpdates), batchSize) to loop in steps of 500.

4

Commit each batch

Inside the for loop after adding all updates, commit the batch by calling batch.commit().

Firebase

productUpdates = [{"id": f"product{i}", "newPrice": 10 + i - 1} for i in range(1, 601)]
batchSize = 500
for start in range(0, len(productUpdates), batchSize):
    end = start + batchSize
    batch = db.batch()
    for product in productUpdates[start:end]:
        docRef = db.collection('products').doc(product['id'])
        batch.update(docRef, {"price": product['newPrice']})
    # Commit the batch here
    # Your code here

Hint

Call batch.commit() after adding all updates in the batch.

Practice

1. What is the maximum number of operations allowed in a single Firestore batch write?

easy

A. Unlimited operations

B. 1000 operations

C. 200 operations

D. 500 operations

Solution

1. Step 1: Recall Firestore batch write limits

   Firestore limits batch writes to a maximum of 500 operations to ensure performance and consistency.

2. Step 2: Confirm the exact number

   The official Firestore documentation states the limit is 500 operations per batch.

3. Final Answer:

   500 operations -> Option D

4. Quick Check:

   Batch limit = 500 operations [OK]

Hint: Remember Firestore batch max is 500 ops per batch [OK]

Common Mistakes:

• Thinking batch limit is 1000 or unlimited
• Confusing batch limit with document size limit
• Assuming batch size depends on network speed

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

easy

A. const batch = firestore.startBatch();

B. const batch = firestore.batch();

C. const batch = new firestore.BatchWrite();

D. const batch = firestore.createBatchWrite();

Solution

1. Step 1: Identify Firestore batch syntax

   The Firestore JavaScript SDK uses firestore.batch() to create a new batch instance.

2. Step 2: Verify method names

   Methods like startBatch(), BatchWrite(), or createBatchWrite() do not exist in Firestore SDK.

3. Final Answer:

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

4. Quick Check:

   Correct batch start method = batch() [OK]

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

Common Mistakes:

• Using non-existent methods like startBatch()
• Trying to instantiate batch with new keyword
• Confusing batch with transaction methods

3. Consider this code snippet in Firestore JavaScript SDK:

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

What will be printed after successful execution?

medium

A. Batch committed

B. Error: update failed

C. No output

D. Batch commit pending

Solution

1. Step 1: Understand batch commit behavior

   Calling await batch.commit() waits for all operations to complete successfully.

2. Step 2: Confirm console output after commit

   After commit resolves, console.log prints 'Batch committed'. No errors occur if data is valid.

3. Final Answer:

   Batch committed -> Option A

4. Quick Check:

   Successful commit logs 'Batch committed' [OK]

Hint: Await batch.commit() before logging success message [OK]

Common Mistakes:

• Expecting output before commit finishes
• Assuming update fails without checking document existence
• Ignoring async/await behavior

4. You try to batch write 600 operations in Firestore but get an error. What is the best way to fix this?

medium

A. Split the 600 operations into two batches of 300 each

B. Increase Firestore batch limit in settings

C. Write all 600 operations in one batch anyway

D. Use a transaction instead of batch

Solution

1. Step 1: Recognize Firestore batch limit

   Firestore limits batch writes to 500 operations max, so 600 exceeds this limit causing errors.

2. Step 2: Apply best practice to split batches

   Splitting 600 operations into smaller batches (e.g., two batches of 300) avoids limit errors and keeps writes manageable.

3. Final Answer:

   Split the 600 operations into two batches of 300 each -> Option A

4. Quick Check:

   Split large batches to ≤ 500 ops [OK]

Hint: Split batches over 500 ops into smaller chunks [OK]

Common Mistakes:

• Trying to increase batch limit (not possible)
• Ignoring batch size causing runtime errors
• Using transactions which have different limits

5. You have 1200 user records to update in Firestore. Which approach follows best practices for batch writes?

hard

A. Update users one by one without batching

B. Send all 1200 updates in a single batch to save time

C. Divide updates into 3 batches of 400 operations each and commit sequentially

D. Use a single transaction for all 1200 updates

Solution

1. Step 1: Understand Firestore batch and transaction limits

   Batch writes are limited to 500 operations; transactions have similar or stricter limits.

2. Step 2: Apply best practice for large updates

   Dividing 1200 updates into smaller batches (e.g., 3 batches of 400) avoids limit errors and ensures reliable commits.

3. Step 3: Avoid single large batch or single transaction

   Sending all 1200 in one batch or transaction exceeds limits and risks failure; updating one by one is inefficient.

4. Final Answer:

   Divide updates into 3 batches of 400 operations each and commit sequentially -> Option C

5. Quick Check:

   Split large updates into batches ≤ 500 ops [OK]

Hint: Split large updates into batches under 500 ops [OK]

Common Mistakes:

• Trying to batch all 1200 updates at once
• Using transactions for very large writes
• Updating records one by one causing slow performance