Batch writes in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Batch Write Atomicity Behavior

You perform a batch write in Firebase Firestore with 5 document updates. One update tries to write to a document that does not exist. What happens to the batch operation?

Firebase

const batch = db.batch();
const docRef1 = db.collection('users').doc('user1');
const docRef2 = db.collection('users').doc('user2');

batch.update(docRef1, {age: 30});
batch.update(docRef2, {age: 25});
// Assume docRef2 does not exist

await batch.commit();

A The entire batch fails and no document is updated.

B Only the update to the non-existent document fails; others succeed.

C The batch partially commits and skips the non-existent document update.

D The batch creates the non-existent document and updates it.

💡 Hint

Think about whether batch writes are atomic or not.

Batch writes in Firestore are atomic. If any write in the batch fails, the entire batch fails and no changes are applied.

❓ Configuration

intermediate

Maximum Operations in a Batch Write

What is the maximum number of write operations you can include in a single Firestore batch write?

A 1000

B 100

C 500

D 2000

💡 Hint

Check Firestore batch write limits.

Firestore limits batch writes to a maximum of 500 operations per batch.

❓ Architecture

advanced

Designing Efficient Batch Writes for High Throughput

You need to update 10,000 documents in Firestore as quickly as possible using batch writes. Which approach is best?

A Create one batch with all 10,000 writes and commit once.

B Create 20 batches with 500 writes each and commit them in parallel.

C Create 20 batches with 500 writes each and commit them sequentially.

D Update documents one by one without batching.

💡 Hint

Consider Firestore batch size limits and concurrency.

Firestore limits batch writes to 500 operations. Committing batches in parallel improves throughput compared to sequential commits.

❓ security

advanced

Security Rules Impact on Batch Writes

You have a batch write with 3 document updates. One document update violates Firestore security rules. What happens?

A The batch write ignores security rules and commits all updates.

B Only the document violating security rules is skipped; others succeed.

C The batch write partially commits and logs a warning.

D The entire batch write fails and no documents are updated.

💡 Hint

Think about how Firestore enforces security rules on batch writes.

Firestore enforces security rules atomically on batch writes. If any write violates rules, the entire batch fails.

✅ Best Practice

expert

Handling Large Batch Writes with Firestore Transaction Limits

You want to update 2000 documents atomically but Firestore batch writes limit is 500 operations. Which is the best approach?

A Use a cloud function to queue and process batches asynchronously with retries.

B Use Firestore transactions to update all 2000 documents in one transaction.

C Split updates into batches of 500 and chain them sequentially with error handling.

D Use multiple batch writes of 500 operations each and rely on eventual consistency.

💡 Hint

Consider Firestore limits and how to ensure reliability and atomicity at scale.

Firestore transactions and batch writes have limits. Using a cloud function to queue and process batches asynchronously with retries ensures reliability and handles large updates gracefully.

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