Batch writes in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a batch write in Firebase?

A batch write lets you group multiple write operations (like create, update, delete) together so they all happen at once. This means either all succeed or none do.

beginner

Why use batch writes instead of individual writes?

Batch writes save time and reduce errors by sending many changes in one go. They also keep data consistent because all changes apply together or not at all.

intermediate

What is the maximum number of operations allowed in a single Firebase batch write?

A single batch write can include up to 500 operations.

intermediate

How does Firebase handle errors in batch writes?

If any operation in the batch fails, none of the changes are applied. This keeps your data safe and consistent.

intermediate

Can batch writes read data before writing in Firebase?

No, batch writes only perform write operations. To read data before writing, use transactions instead.

What happens if one operation in a Firebase batch write fails?

A All operations are rolled back; no changes are made.

B Only the failed operation is skipped; others succeed.

C The batch write retries automatically until success.

D The batch write partially applies changes.

Firebase batch writes are atomic. If one operation fails, none of the changes are applied.

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

A 1000

B 200

C 100

D 500

Firebase batch writes support up to 500 operations per batch.

Which Firebase feature should you use if you need to read data before writing?

A Batch writes

B Cloud Functions

C Transactions

D Realtime Database

Transactions allow reading data and then writing based on that data.

Why are batch writes useful in Firebase?

A They allow multiple writes to happen at once atomically.

B They automatically back up your data.

C They speed up data reads.

D They encrypt data before writing.

Batch writes group multiple write operations so they succeed or fail together.

Can batch writes be used to update multiple documents in Firebase?

A No, batch writes only delete documents.

B Yes, batch writes can update multiple documents at once.

C No, batch writes only create new documents.

D Yes, but only if documents are in the same collection.

Batch writes can create, update, or delete multiple documents in one atomic operation.

Explain what a batch write is in Firebase and why it is useful.

Think about doing many changes at once safely.

Describe the difference between batch writes and transactions in Firebase.

One can read before writing, the other cannot.

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