Topic-based messaging in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

How does topic-based messaging deliver notifications?

In Firebase Cloud Messaging, when you send a message to a topic, how does the service deliver the message?

A The message is delivered to all devices subscribed to the topic, regardless of their connection state.

B The message is stored on the server and delivered only when the user opens the app.

C The message is sent to a random device subscribed to the topic.

D The message is sent only to devices currently connected to the server.

💡 Hint

Think about how topics help reach multiple devices at once.

Topic-based messaging delivers messages to all devices subscribed to that topic, whether they are currently connected or not. Firebase handles storing and forwarding messages as needed.

❓ Architecture

intermediate

Choosing topics for user groups

You want to send notifications to users grouped by their interests. Which topic naming strategy is best to organize these groups?

A Create a topic per user and send messages to each topic individually.

B Use a single topic for all users and filter messages on the client side.

C Create one topic per interest, like 'sports', 'music', and subscribe users accordingly.

D Use random topic names and assign users randomly.

💡 Hint

Think about how topics help target groups efficiently.

Creating one topic per interest allows sending messages only to users interested in that topic, making messaging efficient and organized.

❓ security

advanced

Securing topic subscriptions

Which method best prevents unauthorized users from subscribing to sensitive topics?

A Use public topic names and rely on obscurity to protect sensitive topics.

B Allow all users to subscribe to any topic without restrictions.

C Use Firebase Authentication and verify user identity before subscribing to topics on the client.

D Subscribe users to topics only on the server after verifying permissions.

💡 Hint

Consider where subscription control should happen for security.

Subscribing users to topics on the server after verifying permissions ensures only authorized users get subscribed, preventing unauthorized access.

❓ Configuration

advanced

Correct topic subscription code snippet

Which code snippet correctly subscribes a device to a topic using Firebase Cloud Messaging in JavaScript?

Firebase

import { getMessaging, getToken, onMessage } from 'firebase/messaging';

const messaging = getMessaging();

// Which option correctly subscribes to 'news' topic?

A messaging.subscribe('news');

B getToken(messaging).then(token => fetch('/subscribe', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ token, topic: 'news' }) }));

C messaging.subscribeToTopic('news');

D messaging.subscribeTopic('news');

💡 Hint

Firebase client SDK does not directly subscribe to topics; it requires server interaction.

Firebase client SDK does not provide a direct method to subscribe to topics. Instead, the client sends the device token to the server, which subscribes the token to the topic using Firebase Admin SDK.

✅ Best Practice

expert

Handling topic messaging at scale

Your app has millions of users subscribed to multiple topics. What is the best practice to avoid hitting Firebase Cloud Messaging limits when sending topic messages?

A Batch topic messages and send them with delays to respect Firebase rate limits.

B Send one message per topic for all topics at once, ignoring rate limits.

C Send messages to individual device tokens instead of topics to avoid limits.

D Use random topic names to distribute load evenly.

💡 Hint

Think about how to manage large scale messaging without errors.

Firebase enforces rate limits on messaging. Batching messages and adding delays helps avoid hitting these limits and ensures reliable delivery.