Dead letter topics in GCP - Step-by-Step Execution

Process Flow - Dead letter topics

Message Published

↓

Message Delivered to Subscriber

↓

Subscriber Processes Message

↓

Processing Success?

No→Retry Attempts

↓

Max Retries Reached?

No→Retry Processing

↓

Message Acknowledged

↓

END

↓

Dead Letter Topic Stores Message

Messages flow from publisher to subscriber. If processing fails repeatedly, messages move to a dead letter topic for later inspection.

Execution Sample

GCP

Publish message -> Subscriber receives -> Process message
If fails, retry up to max attempts
If still fails, send to dead letter topic

Shows how messages are retried on failure and sent to dead letter topic after max retries.

Process Table

Step	Action	Message State	Retry Count	Result
1	Message published	New	0	Delivered to subscriber
2	Subscriber processes message	Processing	0	Fails
3	Retry attempt 1	Retrying	1	Fails
4	Retry attempt 2	Retrying	2	Fails
5	Retry attempt 3	Retrying	3	Fails
6	Max retries reached	Failed	3	Sent to dead letter topic
7	Message stored in dead letter topic	Dead letter	3	Stored for inspection

💡 Processing failed after max retries; message moved to dead letter topic

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
Message State	New	Processing	Retrying	Retrying	Retrying	Failed	Dead letter
Retry Count	0	0	1	2	3	3	3

Key Moments - 3 Insights

Why does the message go to the dead letter topic after retries?

Because the message failed processing after the maximum retry count was reached, as shown in steps 5 and 6 of the execution table.

Does the retry count reset after each failed attempt?

No, the retry count increments with each failure and does not reset until the message is acknowledged or moved to dead letter topic, as seen in the variable tracker.

What happens if the message processing succeeds before max retries?

The message is acknowledged and removed from the queue, ending the flow early, which is not shown here because all attempts failed.

Visual Quiz - 3 Questions

Look at the execution table, what is the retry count at step 4?

A 2

B 3

C 1

D 0

💡 Hint

Check the 'Retry Count' column for step 4 in the execution table.

At which step does the message get sent to the dead letter topic?

A Step 5

B Step 6

C Step 3

D Step 7

💡 Hint

Look for the step where 'Max retries reached' and 'Sent to dead letter topic' appear in the execution table.

If the message succeeded at retry attempt 2, what would happen next?

A Message sent to dead letter topic

B Retry count resets and retries continue

C Message acknowledged and processing ends

D Retry count increments and message retries again

💡 Hint

Refer to the key moment about success before max retries and the flow in the concept flow diagram.

Concept Snapshot

Dead letter topics handle messages that fail processing after retries.
Messages are retried up to a max count.
If still failing, they move to dead letter topic.
This helps isolate problematic messages for later review.
Retry count increments on each failure.
Successful processing acknowledges and removes message.

Full Transcript

In Google Cloud Pub/Sub, messages are sent from publishers to subscribers. When a subscriber receives a message, it tries to process it. If processing fails, the system retries up to a maximum number of times. Each retry increments a retry count. If the message still fails after the maximum retries, it is sent to a dead letter topic. This special topic stores failed messages separately for later inspection or manual handling. This flow ensures that problematic messages do not block the system and can be reviewed later. The retry count helps track how many attempts were made before moving the message to the dead letter topic.