Topic-based messaging in Firebase - Mini Project: Build & Apply

Firebase Topic-Based Messaging Setup

📖 Scenario: You are building a mobile app that sends notifications to users based on their interests. You want to use Firebase Cloud Messaging (FCM) to send messages to groups of users subscribed to specific topics.

🎯 Goal: Create a Firebase Cloud Messaging setup that subscribes users to a topic, configures a message payload, sends a message to the topic, and completes the messaging configuration.

📋 What You'll Learn

Create a topic subscription variable

Add a message configuration object

Send a message to the topic using Firebase Admin SDK

Complete the messaging setup with proper topic and message structure

💡 Why This Matters

🌍 Real World

Topic-based messaging allows apps to send notifications to groups of users interested in specific subjects, like news or sports updates.

💼 Career

Understanding Firebase topic messaging is useful for cloud engineers and mobile developers who build scalable notification systems.

1

Create a topic subscription variable

Create a variable called topic and set it to the string "news" to represent the topic users will subscribe to.

Firebase

// Create a variable called topic and set it to "news"
// Your code here

Hint

Use const topic = "news"; to define the topic name.

2

Add a message configuration object

Create a constant called message that is an object with a notification property. The notification should have title set to "Breaking News" and body set to "New updates available.". Also add a topic property set to the variable topic.

Firebase

const topic = "news";
// Create a message object with notification title and body, and topic property
// Your code here

Hint

Define message as an object with notification and topic properties.

3

Send a message to the topic using Firebase Admin SDK

Use the Firebase Admin SDK's messaging().send() method to send the message object. Assign the result to a constant called response. Use await to wait for the send operation to complete.

Firebase

const topic = "news";
const message = {
  notification: {
    title: "Breaking News",
    body: "New updates available."
  },
  topic: topic
};

// Send the message using Firebase Admin SDK and assign the result to response
// Your code here

Hint

Use await admin.messaging().send(message); and assign it to response.

4

Complete the messaging setup with proper topic and message structure

Add a subscription call using admin.messaging().subscribeToTopic() to subscribe an array with a single token "user_token_123" to the topic. Assign the result to a constant called subscribeResponse using await.

Firebase

const topic = "news";
const message = {
  notification: {
    title: "Breaking News",
    body: "New updates available."
  },
  topic: topic
};

const response = await admin.messaging().send(message);

// Subscribe the token "user_token_123" to the topic and assign the result to subscribeResponse
// Your code here

Hint

Use await admin.messaging().subscribeToTopic(["user_token_123"], topic); and assign it to subscribeResponse.

Practice

1. What is the main purpose of topic-based messaging in Firebase Cloud Messaging?

easy

A. To send messages to multiple devices subscribed to a topic

B. To send messages only to a single device

C. To store messages in a database

D. To authenticate users before sending messages

Solution

1. Step 1: Understand topic-based messaging concept

   Topic-based messaging allows grouping devices under a topic to send messages to all subscribers at once.

2. Step 2: Identify the main purpose

   The main purpose is to send messages to multiple devices subscribed to the same topic efficiently.

3. Final Answer:

   To send messages to multiple devices subscribed to a topic -> Option A

4. Quick Check:

   Topic-based messaging = send to many devices [OK]

Hint: Topic messaging targets groups, not single devices [OK]

Common Mistakes:

• Confusing topic messaging with single device messaging
• Thinking it stores messages in database
• Assuming it handles user authentication

2. Which Firebase function is used to subscribe a device to a topic?

easy

A. unsubscribeFromTopic()

B. sendToTopic()

C. getToken()

D. subscribeToTopic()

Solution

1. Step 1: Identify subscription function

   To add a device to a topic, Firebase provides the subscribeToTopic() function.

2. Step 2: Differentiate from other functions

   sendToTopic() sends messages, getToken() gets device token, unsubscribeFromTopic() removes subscription.

3. Final Answer:

   subscribeToTopic() -> Option D

4. Quick Check:

   Subscribe device = subscribeToTopic() [OK]

Hint: Subscribe uses subscribeToTopic(), not sendToTopic() [OK]

Common Mistakes:

• Confusing sendToTopic() with subscription
• Using getToken() to subscribe
• Mixing unsubscribe with subscribe

3. Given this code snippet, what will happen?

const message = {
  topic: 'news',
  notification: { title: 'Hello', body: 'World' }
};
admin.messaging().sendToTopic(message.topic, message.notification);

medium

A. A notification with title 'Hello' and body 'World' is sent to all devices subscribed to 'news'

B. An error occurs because sendToTopic requires a message object, not separate topic and notification

C. The message is sent only to one device

D. The message is stored but not sent

Solution

1. Step 1: Analyze sendToTopic usage

   The sendToTopic method expects a topic string followed by a payload object. Here, message.topic provides the string 'news' and message.notification provides the payload {title: 'Hello', body: 'World'}.

2. Step 2: Determine the outcome

   This correctly sends the notification to all devices subscribed to the 'news' topic.

3. Final Answer:

   A notification with title 'Hello' and body 'World' is sent to all devices subscribed to 'news' -> Option A

4. Quick Check:

   sendToTopic(topic, payload) sends to subscribers [OK]

Hint: sendToTopic(topic_string, payload_object) sends to topic subscribers [OK]

Common Mistakes:

• Confusing sendToTopic(topic, payload) with send(full_message_object)
• Thinking it sends only to one device
• Believing the message is stored but not sent

4. You wrote this code to send a message to a topic:

const message = {
  topic: 'sports',
  notification: { title: 'Game', body: 'Tonight at 8' }
};
admin.messaging().sendToTopic(message);

But it throws an error. What is the likely cause?

medium

A. sendToTopic expects the message object, but you passed only the topic string

B. The topic field should be inside the notification object

C. sendToTopic expects a topic string, not a message object

D. sendToTopic expects the message object, but you passed the whole object correctly

Solution

1. Step 1: Check sendToTopic parameter type

   In Firebase Admin SDK, sendToTopic expects two parameters: topic string and payload object, not a single message object.

2. Step 2: Identify mismatch

   Passing the whole message object as one argument causes an error because the function signature is different.

3. Final Answer:

   sendToTopic expects a topic string, not a message object -> Option C

4. Quick Check:

   sendToTopic(topic, payload) not sendToTopic(message) [OK]

Hint: sendToTopic needs topic string first, then payload [OK]

Common Mistakes:

• Passing message object instead of topic string
• Putting topic inside notification
• Assuming one argument is accepted

5. You want to send a message only to devices subscribed to both 'news' and 'sports' topics. How can you achieve this using Firebase topic messaging?

hard

A. Send two separate messages, one to 'news' and one to 'sports' topics

B. Use condition parameter with "'news' in topics && 'sports' in topics" in sendToTopic

C. Subscribe devices to a combined topic 'news_sports' and send to that topic

D. Send a message to 'news' topic and filter devices manually

Solution

1. Step 1: Understand topic combination options

   Firebase supports sending messages to multiple topics using conditions with logical operators.

2. Step 2: Use condition parameter for AND logic

   Using condition: "'news' in topics && 'sports' in topics" sends message only to devices subscribed to both topics.

3. Step 3: Evaluate other options

   Sending two messages sends to each topic separately, subscribing to combined topic requires extra setup, filtering manually is inefficient.

4. Final Answer:

   Use condition parameter with "'news' in topics && 'sports' in topics" in sendToTopic -> Option B

5. Quick Check:

   Use condition for multiple topic targeting [OK]

Hint: Use condition with && for multiple topic targeting [OK]

Common Mistakes:

• Sending separate messages instead of using condition
• Creating combined topic unnecessarily
• Filtering devices manually after sending