Transaction basics in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What is the output of this Firebase transaction code?

Consider this Firebase transaction on a document with initial field count: 5. What will be the final value of count after this transaction completes successfully?

Firebase

const docRef = firestore.collection('counters').doc('counter1');

await firestore.runTransaction(async (transaction) => {
  const doc = await transaction.get(docRef);
  const newCount = (doc.data()?.count ?? 0) + 3;
  transaction.update(docRef, { count: newCount });
});

A 0

B 3

C 5

D 8

💡 Hint

Think about how the transaction reads the current value and adds 3.

The transaction reads the current count value (5), adds 3, and updates it to 8.

🧠 Conceptual

intermediate

Which statement about Firebase transactions is true?

Choose the correct statement about Firebase transactions.

A Transactions can only update one document at a time.

B Transactions automatically retry if data changes during the transaction.

C Transactions do not guarantee atomicity across multiple documents.

D Transactions can only read data but cannot write updates.

💡 Hint

Think about how Firebase handles concurrent updates inside transactions.

Firebase transactions automatically retry if the data they read changes before commit, ensuring atomicity.

❓ Configuration

advanced

What error occurs with this Firebase transaction code?

What error will this Firebase transaction code raise when executed?

Firebase

const docRef = firestore.collection('users').doc('user1');

await firestore.runTransaction(async (transaction) => {
  transaction.update(docRef, { score: 10 });
  const doc = await transaction.get(docRef);
  console.log(doc.data());
});

A TransactionError: Cannot update before reading document

B SyntaxError: Missing await before transaction.update

C No error, transaction runs successfully

D TypeError: transaction.update is not a function

💡 Hint

Check the order of operations allowed in Firebase transactions.

Firebase transactions allow updates before reads; this code runs without error.

❓ security

advanced

Which Firebase security rule prevents unauthorized transaction writes?

Given a Firestore collection orders, which security rule correctly prevents users from updating orders they do not own, even inside transactions?

A allow update: if request.auth.uid == resource.data.ownerId;

B allow update: if request.auth.uid != resource.data.ownerId;

C allow update: if request.auth.token.admin == true;

D allow update: if request.time < timestamp.date(2025, 1, 1);

💡 Hint

Think about how to restrict updates to only the owner of the document.

Rule A allows updates only if the authenticated user ID matches the document's owner ID, securing transaction writes.

❓ Architecture

expert

What is the best explanation for why Firebase transactions retry automatically?

Firebase transactions retry automatically when data changes during the transaction. Why is this behavior important in distributed systems?

A To ensure eventual consistency by resolving conflicts caused by concurrent updates.

B To reduce network traffic by batching multiple writes into one request.

C To prevent any writes from happening if the user is offline.

D To guarantee that transactions always complete within a fixed time limit.

💡 Hint

Consider how multiple users might update the same data at the same time.

Automatic retries help maintain data consistency by handling conflicts from concurrent updates in distributed environments.

Practice

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

easy

A. To speed up data retrieval

B. To ensure data changes are safe and consistent

C. To store large files

D. To send notifications to users

Solution

1. Step 1: Understand transaction purpose

   Transactions are designed to keep data changes safe and consistent when multiple users might update the same data.

2. Step 2: Eliminate unrelated options

   Speeding up retrieval, storing files, or sending notifications are not the main goals of transactions.

3. Final Answer:

   To ensure data changes are safe and consistent -> Option B

4. Quick Check:

   Transaction purpose = safe and consistent data [OK]

Hint: Transactions protect data integrity during concurrent updates [OK]

Common Mistakes:

• Thinking transactions speed up data retrieval
• Confusing transactions with file storage
• Assuming transactions send notifications

2. Which of the following is the correct way to start a transaction in Firebase Realtime Database?

easy

A. database.ref('path').transaction(updateFunction)

B. database.ref('path').setTransaction(updateFunction)

C. database.transaction('path', updateFunction)

D. database.ref('path').runTransaction(updateFunction)

Solution

1. Step 1: Recall Firebase transaction syntax

   The correct method to start a transaction is calling transaction() on a database reference.

2. Step 2: Check each option

   database.ref('path').transaction(updateFunction) uses ref('path').transaction(updateFunction), which is correct. Options A, B, and D use incorrect method names or call patterns.

3. Final Answer:

   database.ref('path').transaction(updateFunction) -> Option A

4. Quick Check:

   Correct method is .transaction() on ref [OK]

Hint: Use .transaction() on a database reference to start [OK]

Common Mistakes:

• Using .setTransaction() which does not exist
• Calling transaction directly on database object
• Using .runTransaction() which is invalid

3. Consider this transaction code snippet:

database.ref('counter').transaction(currentValue => {
  if (currentValue === null) {
    return 1;
  } else {
    return currentValue + 1;
  }
});

What will be the value of 'counter' after this transaction if it was initially 5?

medium

A. 6

B. 1

C. null

D. 5

Solution

1. Step 1: Analyze the transaction function logic

   The function checks if the current value is null; if so, it returns 1. Otherwise, it returns currentValue + 1.

2. Step 2: Apply initial value 5

   Since currentValue is 5 (not null), the function returns 5 + 1 = 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   5 + 1 = 6 [OK]

Hint: If value exists, transaction adds 1; else sets to 1 [OK]

Common Mistakes:

• Assuming value stays the same
• Confusing null check logic
• Returning 1 regardless of current value

4. What is wrong with this Firebase transaction code?

database.ref('score').transaction(() => {
  database.ref('score').set(10);
  return 20;
});

medium

A. You cannot return a value in a transaction function

B. Calling set() inside transaction causes an error

C. Reads must happen before writes inside the transaction function

D. Transaction function must be async

Solution

1. Step 1: Understand transaction function rules

   All reads must happen before any writes inside the transaction function to ensure consistency.

2. Step 2: Identify the problem in code

   The code calls set(10) inside the transaction function before returning a new value, which violates the rule.

3. Final Answer:

   Reads must happen before writes inside the transaction function -> Option C

4. Quick Check:

   Transaction rule: read before write [OK]

Hint: Do all reads before any writes inside transaction function [OK]

Common Mistakes:

• Calling set() inside transaction function
• Returning a value is allowed and required
• Thinking transaction function must be async

5. You want to increment a user's points only if their current points are less than 100. Which transaction function correctly implements this logic?

database.ref('users/user1/points').transaction(currentPoints => {
  // Your code here
});

hard

A. if (currentPoints === null) { return 0; } else { return 100; }

B. if (currentPoints >= 100) { return currentPoints + 1; } else { return currentPoints; }

C. return currentPoints + 1;

D. if (currentPoints === null || currentPoints < 100) { return (currentPoints || 0) + 1; } else { return currentPoints; }

Solution

1. Step 1: Understand the condition to increment points

   Points should increment only if currentPoints is less than 100 or if it is null (no points yet).

2. Step 2: Check each option logic

   if (currentPoints === null || currentPoints < 100) { return (currentPoints || 0) + 1; } else { return currentPoints; } increments points by 1 if currentPoints is null or less than 100, else returns currentPoints unchanged. This matches the requirement. if (currentPoints >= 100) { return currentPoints + 1; } else { return currentPoints; } increments when points are 100 or more, which is wrong. return currentPoints + 1; always increments ignoring the limit. if (currentPoints === null) { return 0; } else { return 100; } resets points incorrectly.

3. Final Answer:

   if (currentPoints === null || currentPoints < 100) { return (currentPoints || 0) + 1; } else { return currentPoints; } -> Option D

4. Quick Check:

   Increment only if points < 100 or null [OK]

Hint: Check null or less than 100 before incrementing [OK]

Common Mistakes:

• Incrementing points when already 100 or more
• Ignoring null initial value
• Resetting points instead of incrementing