Topic-based messaging in Firebase - Step-by-Step Execution

Process Flow - Topic-based messaging

Client subscribes to topic

↓

Topic subscription stored

↓

Server sends message to topic

↓

Message delivered to all subscribed clients

↓

Clients receive and process message

Clients subscribe to a topic. Server sends messages to that topic. All subscribed clients receive the message.

Execution Sample

Firebase

admin.messaging().subscribeToTopic(token, 'news')
admin.messaging().sendToTopic('news', messagePayload)

Subscribe a client to 'news' topic and send a message to all clients subscribed to 'news'.

Process Table

Step	Action	Input	Result	Notes
1	Subscribe client to topic	token='abc123', topic='news'	Subscription stored	Client token linked to 'news' topic
2	Send message to topic	topic='news', message='Hello subscribers!'	Message queued for topic	Server prepares message for all subscribers
3	Deliver message	topic='news'	Message sent to token='abc123'	All clients subscribed receive message
4	Client receives message	token='abc123'	Message displayed	Client app shows message content
5	End	-	-	No more actions

💡 All subscribed clients have received the message; process ends.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
subscriptionList	{}	{'news': ['abc123']}	{'news': ['abc123']}	{'news': ['abc123']}	{'news': ['abc123']}
messageQueue	[]	[]	[{'topic':'news','message':'Hello subscribers!'}]	[]	[]

Key Moments - 2 Insights

Why does the message get delivered only after subscribing?

Because the server sends messages to topics, only clients subscribed to that topic receive messages, as shown in execution_table step 3.

What happens if no clients are subscribed to a topic?

The message is queued but no delivery occurs, so no clients receive it. This is implied by the empty subscriptionList in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution table, what is the subscriptionList after step 1?

A {'news': []}

B {'news': ['abc123']}

C {}

D ['abc123']

💡 Hint

Check the variable_tracker after Step 1 for subscriptionList value.

At which step does the message get sent to the client?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look at execution_table row where message is sent to token='abc123'.

If a client unsubscribes before step 3, what changes in the subscriptionList?

A The messageQueue is cleared

B The subscriptionList remains the same

C The client token is removed from the topic list

D The topic is deleted

💡 Hint

Consider how subscriptionList tracks clients per topic in variable_tracker.

Concept Snapshot

Topic-based messaging lets clients subscribe to named topics.
Server sends messages to topics, not individual clients.
Only subscribed clients receive messages.
Useful for broadcasting updates efficiently.
Subscribe with subscribeToTopic(token, topic).
Send with sendToTopic(topic, message).

Full Transcript

Topic-based messaging in Firebase works by clients subscribing to topics. When a client subscribes, their token is stored linked to that topic. The server sends messages to a topic, which are then delivered to all clients subscribed to that topic. This allows efficient broadcast messaging. The execution steps show subscription storage, message queuing, delivery to clients, and client reception. Variables track subscriptions and message queues. Key points include that messages only reach subscribed clients and unsubscribed clients do not receive messages. This method is useful for sending updates to groups of users without addressing each individually.