Topics and subscriptions in GCP - Step-by-Step Execution

Process Flow - Topics and subscriptions

Create Topic

↓

Create Subscription

↓

Publish Message

↓

Message Stored in Topic

↓

Subscription Receives Message

↓

Subscriber Processes Message

↓

Acknowledge Message

↓

Message Removed from Subscription

This flow shows how a message moves from creation in a topic to delivery and acknowledgment by a subscription.

Execution Sample

GCP

1. Create topic 'news-updates'
2. Create subscription 'user1-sub'
3. Publish message 'Hello World'
4. Subscription receives message
5. Subscriber acknowledges message

This example shows creating a topic and subscription, publishing a message, and the subscriber receiving and acknowledging it.

Process Table

Step	Action	State Change	Message State	Subscription State
1	Create topic 'news-updates'	Topic 'news-updates' created	No messages	No subscriptions
2	Create subscription 'user1-sub'	Subscription 'user1-sub' created	No messages	Subscription 'user1-sub' active, no messages
3	Publish message 'Hello World'	Message added to topic	Message 'Hello World' in topic	Subscription 'user1-sub' active, message 'Hello World' pending delivery
4	Subscription receives message	Message delivered to subscription	Message 'Hello World' in topic	Subscription 'user1-sub' has message 'Hello World' pending ack
5	Subscriber acknowledges message	Message acknowledged and removed from subscription	Message 'Hello World' in topic	Subscription 'user1-sub' no pending messages

💡 Message acknowledged, no pending messages in subscription, flow ends

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
Topic	None	Created, no messages	Created with 1 message	Created with 1 message	Created with 1 message
Subscription	None	Created, no messages	Created with 1 message pending delivery	Has 1 message pending ack	No messages pending
Message	None	None	'Hello World' in topic	'Hello World' delivered to subscription	Acknowledged and removed

Key Moments - 3 Insights

Why does the message stay in the topic even after the subscription receives it?

Because Pub/Sub topics retain messages for a configurable retention duration (default 7 days), independent of acknowledgments by subscriptions. The subscription gets its own copy to process.

What happens if the subscriber does not acknowledge the message?

The message stays in the subscription as pending and can be redelivered until acknowledged, ensuring no message loss.

Can multiple subscriptions receive the same message from one topic?

Yes, each subscription gets its own copy of the message independently.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the subscription first receive the message?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Check the 'Subscription State' column for when the message appears in the subscription

According to the variable tracker, what is the state of the message after step 5?

A Message acknowledged and removed

B Message still pending in subscription

C Message deleted from topic

D Message duplicated in subscription

💡 Hint

Look at the 'Message' row under 'After Step 5' in variable_tracker

If the subscriber never acknowledges the message, what would happen according to the flow?

A Message is removed from topic immediately

B Subscription loses the message

C Message remains pending in subscription and can be redelivered

D Topic deletes the subscription

💡 Hint

Refer to the key moment about message acknowledgment and subscription state

Concept Snapshot

Topics hold messages published by senders.
Subscriptions receive copies of messages from topics.
Subscribers pull messages from subscriptions.
Messages stay until acknowledged by subscribers.
Multiple subscriptions get independent copies.
Acknowledgment removes message from subscription.

Full Transcript

This visual execution shows how Google Cloud Pub/Sub topics and subscriptions work together. First, a topic is created to hold messages. Then, a subscription is created to receive messages from that topic. When a message is published, it is stored in the topic. The subscription gets its own copy of the message to deliver to subscribers. The subscriber processes the message and sends an acknowledgment. Only after acknowledgment is the message removed from the subscription. The topic retains the message for a retention period (default 7 days), regardless of acknowledgments. This ensures reliable delivery and no message loss. Multiple subscriptions can independently receive the same message from one topic.