Message retention and acknowledgment in GCP - Step-by-Step Execution

Process Flow - Message retention and acknowledgment

Message Published

↓

Message Stored in Topic

↓

Subscriber Receives Message

↓

Subscriber Processes Message

↓

Subscriber Sends Acknowledgment

↓

Message Removed from Subscription

↓

If No Ack Before Deadline

↓

Message Redelivered

Messages are published and stored. Subscribers receive and process them, then send acknowledgment. If ack is missing before deadline, message is redelivered.

Execution Sample

GCP

Publish message -> Subscriber receives -> Process message -> Send ack -> Message removed

Shows the flow of message delivery, processing, acknowledgment, and removal from subscription.

Process Table

Step	Action	Message State	Ack Deadline	Result
1	Message published to topic	Stored in topic	N/A	Waiting for subscriber
2	Subscriber receives message	Delivered to subscriber	10 seconds	Subscriber starts processing
3	Subscriber processes message	In processing	7 seconds left	Processing ongoing
4	Subscriber sends acknowledgment	Acknowledged	N/A	Message removed from subscription
5	No new message	No message pending	N/A	Waiting for next message
6	If ack not sent before deadline	Not acknowledged	0 seconds	Message redelivered to subscriber

💡 Message removed after acknowledgment or redelivered if ack deadline missed

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 6
Message State	Stored in topic	Delivered to subscriber	In processing	Acknowledged	Redelivered
Ack Deadline (seconds)	N/A	10	7	N/A	0

Key Moments - 3 Insights

Why does the message get redelivered even if it was already sent once?

If the subscriber does not send acknowledgment before the ack deadline (see step 6 in execution_table), the message is considered unprocessed and is redelivered to ensure it is handled.

What happens to the message after the subscriber sends acknowledgment?

According to step 4 in execution_table, once acknowledgment is sent, the message is removed from the subscription and will not be delivered again.

Does the message stay in the topic after delivery to subscriber?

Yes, the message stays in the topic. It is delivered to the subscription and tracked there until acknowledged or redelivered (see steps 1 and 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the message state after the subscriber sends acknowledgment?

A In processing

B Delivered to subscriber

C Acknowledged

D Stored in topic

💡 Hint

Check step 4 in the execution_table under 'Message State'

At which step does the message get redelivered if no acknowledgment is received?

A Step 3

B Step 6

C Step 4

D Step 2

💡 Hint

Look at the 'Result' column in step 6 of the execution_table

If the ack deadline is 10 seconds, what happens if the subscriber processes the message for 12 seconds without sending ack?

A Message is redelivered after 10 seconds

B Message stays in processing indefinitely

C Message is removed immediately

D Message is deleted from topic

💡 Hint

Refer to the ack deadline and redelivery logic in execution_table steps 2, 3, and 6

Concept Snapshot

Message retention means messages stay until acknowledged.
Subscribers must send acknowledgment before deadline.
If no ack, message is redelivered.
Ack removes message from subscription.
This ensures reliable message processing.

Full Transcript

In Google Cloud Pub/Sub, messages are published to a topic and stored until subscribers receive them. When a subscriber gets a message, it has a limited time called the acknowledgment deadline to process and acknowledge it. If the subscriber sends acknowledgment in time, the message is removed from the subscription and won't be sent again. If the subscriber fails to acknowledge before the deadline, the message is redelivered to ensure it is processed. This flow guarantees messages are not lost and are processed reliably.