Message retention and acknowledgment in GCP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is message retention in Google Cloud Pub/Sub?

Message retention is the time period during which Pub/Sub keeps a message available for delivery, even if it has been acknowledged. This allows subscribers to receive messages that were published before they subscribed or to reprocess messages if needed.

beginner

What does acknowledging a message mean in Pub/Sub?

Acknowledging a message means telling Pub/Sub that the message has been successfully received and processed. Once acknowledged, Pub/Sub stops delivering that message to the subscriber.

intermediate

How long is the default message retention period in Pub/Sub?

By default, Pub/Sub retains unacknowledged messages for 7 days. This means messages can be redelivered within this time if not acknowledged.

intermediate

What happens if a subscriber does not acknowledge a message within the ack deadline?

If a message is not acknowledged within the ack deadline (default 10 seconds), Pub/Sub considers it unacknowledged and will redeliver it to the same or another subscriber.

advanced

How can you extend the time a subscriber has to acknowledge a message?

Subscribers can modify the ack deadline by sending a ModifyAckDeadline request to extend the time before the message is redelivered, allowing more time to process the message.

What does acknowledging a message in Pub/Sub do?

A Extends the message retention period

B Stops redelivery of that message

C Deletes the message from the topic

D Publishes a new message

Acknowledging a message tells Pub/Sub that the message was processed successfully and should not be redelivered.

What is the default ack deadline for a message in Pub/Sub?

A 60 seconds

B 5 seconds

C 30 seconds

D 10 seconds

The default ack deadline is 10 seconds, after which unacknowledged messages are redelivered.

How long does Pub/Sub retain unacknowledged messages by default?

A 1 day

B 3 days

C 7 days

D 30 days

Pub/Sub retains unacknowledged messages for 7 days by default.

What happens if a message is not acknowledged before the ack deadline?

A It is redelivered

B It is sent to a dead-letter queue automatically

C It is archived

D It is deleted permanently

If not acknowledged in time, the message is redelivered to subscribers.

Which method allows a subscriber to extend the ack deadline?

A modifyAckDeadline

B extendMessageRetention

C acknowledgeMessage

D renewSubscription

The modifyAckDeadline method lets subscribers extend the time to acknowledge a message.

Explain how message retention and acknowledgment work together in Google Cloud Pub/Sub.

Think about how Pub/Sub ensures messages are not lost and are processed at least once.

Describe what happens when a subscriber does not acknowledge a message within the ack deadline.

Focus on the flow of message delivery and retry.

Practice

1. What happens to a message in Google Cloud Pub/Sub if it is not acknowledged within the retention period?

easy

A. The message is automatically acknowledged.

B. The message is deleted and no longer available for delivery.

C. The message is moved to a dead-letter queue.

D. The message is duplicated and sent again.

Solution

1. Step 1: Understand message retention

   Message retention defines how long unacknowledged messages are kept before deletion.

2. Step 2: Effect of no acknowledgment within retention

   If a message is not acknowledged before retention expires, it is deleted and lost.

3. Final Answer:

   The message is deleted and no longer available for delivery. -> Option B

4. Quick Check:

   Unacknowledged message + retention expired = deleted [OK]

Hint: Unacknowledged messages expire and get deleted after retention [OK]

Common Mistakes:

• Thinking messages auto-acknowledge after retention
• Assuming messages move to dead-letter queue automatically
• Believing messages duplicate on retention expiry

2. Which of the following is the correct way to acknowledge a message in Google Cloud Pub/Sub client libraries?

easy

A. Call message.ack() after processing the message.

B. Call message.delete() to remove the message.

C. Call message.complete() to finish processing.

D. Call message.finish() to acknowledge.

Solution

1. Step 1: Identify acknowledgment method

   In Pub/Sub client libraries, acknowledging a message is done by calling ack() on the message object.

2. Step 2: Verify other options

   Methods like delete(), complete(), or finish() are not valid for acknowledgment.

3. Final Answer:

   Call message.ack() after processing the message. -> Option A

4. Quick Check:

   Use message.ack() to acknowledge [OK]

Hint: Use message.ack() to confirm message processed [OK]

Common Mistakes:

• Using incorrect method names like delete or finish
• Not acknowledging messages at all
• Confusing acknowledgment with message deletion

3. Consider this code snippet using Google Cloud Pub/Sub subscriber client:

def callback(message):
    print(f"Received: {message.data.decode('utf-8')}")
    # No acknowledgment called here

subscriber.subscribe(subscription_path, callback=callback)

What will happen to the messages if the callback never calls message.ack()?

medium

A. Messages will be acknowledged automatically after printing.

B. Messages will be lost immediately after printing.

C. Messages will be redelivered until acknowledged or retention expires.

D. Messages will be sent to a dead-letter topic instantly.

Solution

1. Step 1: Understand callback behavior without ack

   If message.ack() is not called, Pub/Sub considers the message unacknowledged.

2. Step 2: Effect on message delivery

   Unacknowledged messages are redelivered repeatedly until acknowledged or retention period expires.

3. Final Answer:

   Messages will be redelivered until acknowledged or retention expires. -> Option C

4. Quick Check:

   No ack means redelivery until retention ends [OK]

Hint: No ack means message redelivery until retention ends [OK]

Common Mistakes:

• Assuming messages auto-acknowledge after callback
• Thinking messages are lost immediately without ack
• Believing messages go to dead-letter topic without config

4. You have set a message retention duration of 7 days on your Pub/Sub subscription. However, messages are disappearing after 1 day. What is the most likely cause?

medium

A. Messages are being acknowledged by the subscriber before 1 day.

B. The retention duration setting is ignored by Pub/Sub.

C. The subscription is a push type and does not support retention.

D. Messages are automatically deleted after 24 hours regardless of settings.

Solution

1. Step 1: Check retention vs acknowledgment

   Retention controls how long unacknowledged messages stay. If messages are acknowledged, they are removed immediately.

2. Step 2: Analyze disappearance cause

   If messages disappear early, likely they were acknowledged by the subscriber before 1 day.

3. Final Answer:

   Messages are being acknowledged by the subscriber before 1 day. -> Option A

4. Quick Check:

   Acknowledged messages removed before retention ends [OK]

Hint: Acknowledged messages delete immediately, ignoring retention [OK]

Common Mistakes:

• Assuming retention is ignored by Pub/Sub
• Believing push subscriptions don't support retention
• Thinking messages auto-delete after 24 hours always

5. You want to ensure that messages in your Pub/Sub subscription are retained for 3 days even if they are not acknowledged, but also want to avoid message duplication after acknowledgment. Which configuration should you apply?

hard

A. Set message retention duration to 0 and rely on subscriber retries.

B. Set message retention duration to 3 days and disable acknowledgments.

C. Set message retention duration to 7 days and never acknowledge messages.

D. Set message retention duration to 3 days and ensure subscriber acknowledges messages promptly.

Solution

1. Step 1: Understand retention and acknowledgment roles

   Retention keeps unacknowledged messages for specified time. Acknowledgment removes messages to avoid duplicates.

2. Step 2: Choose configuration to meet goals

   Setting retention to 3 days keeps messages if unacknowledged. Prompt acknowledgment removes processed messages, preventing duplicates.

3. Final Answer:

   Set message retention duration to 3 days and ensure subscriber acknowledges messages promptly. -> Option D

4. Quick Check:

   Retention + prompt ack = no duplicates, messages kept [OK]

Hint: Retention keeps unacked; ack removes to prevent duplicates [OK]

Common Mistakes:

• Disabling acknowledgments causing duplicates
• Setting retention to 0 losing unacked messages quickly
• Never acknowledging causing message buildup and duplicates