Message retention and acknowledgment in GCP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Message acknowledgment timing in Pub/Sub

In Google Cloud Pub/Sub, when a subscriber receives a message, what happens if the subscriber does not acknowledge the message within the acknowledgment deadline?

A The message is redelivered to the subscriber or another subscriber after the acknowledgment deadline expires.

B The message is immediately deleted from the subscription and not redelivered.

C The message is moved to a dead-letter topic automatically without redelivery.

D The message acknowledgment deadline is extended indefinitely until the subscriber acknowledges.

💡 Hint

Think about what happens when a message is not confirmed as processed in time.

If a subscriber does not acknowledge a message within the acknowledgment deadline, Pub/Sub assumes the message was not processed and makes it available for redelivery to the same or another subscriber.

❓ Architecture

intermediate

Configuring message retention duration

You want to ensure that unacknowledged messages in a Pub/Sub subscription are retained for 7 days before being deleted. Which configuration should you set?

A Set the topic's <code>messageRetentionDuration</code> to 7 days.

B Set the subscription's <code>ackDeadlineSeconds</code> to 7 days.

C Set the topic's <code>ackDeadlineSeconds</code> to 7 days.

D Set the subscription's <code>messageRetentionDuration</code> to 7 days.

💡 Hint

Retention duration controls how long messages stay in the subscription after delivery.

The messageRetentionDuration on the subscription controls how long unacknowledged messages are retained before deletion. Setting it to 7 days keeps messages for that duration.

❓ security

advanced

Preventing message loss with acknowledgment deadlines

You have a subscriber that processes messages slowly, sometimes taking longer than the default acknowledgment deadline. What is the best practice to prevent message loss or duplicate processing?

A Use the subscriber client to extend the acknowledgment deadline dynamically while processing.

B Set the subscription's message retention duration to zero.

C Disable message acknowledgment to avoid redelivery.

D Increase the subscription's acknowledgment deadline to a very high value (e.g., 1 hour).

💡 Hint

Think about how to keep the message invisible to others while processing.

Extending the acknowledgment deadline dynamically allows the subscriber to keep the message invisible to others while processing, preventing premature redelivery and duplicate processing.

🧠 Conceptual

advanced

Dead-letter topic behavior in Pub/Sub

What happens to a message in a Pub/Sub subscription configured with a dead-letter topic if the message is not acknowledged after the maximum delivery attempts?

A The message is redelivered indefinitely until acknowledged.

B The message is deleted permanently without being sent anywhere else.

C The message is sent to the dead-letter topic for further inspection or processing.

D The message is automatically acknowledged to prevent further delivery.

💡 Hint

Dead-letter topics are used to handle problematic messages.

When a message exceeds the maximum delivery attempts without acknowledgment, Pub/Sub sends it to the configured dead-letter topic for separate handling.

✅ Best Practice

expert

Designing a reliable message processing system with Pub/Sub

You are designing a system that must process messages exactly once using Google Cloud Pub/Sub. Which combination of features and practices will best help achieve this?

A Acknowledge messages immediately upon receipt without processing to speed up throughput.

B Use at-least-once delivery with message deduplication logic in the subscriber and enable dead-letter topics for failed messages.

C Use exactly-once delivery mode in Pub/Sub and disable dead-letter topics to prevent message loss.

D Set acknowledgment deadline to zero and disable message retention to avoid duplicates.

💡 Hint

Consider how to handle duplicates and failures safely.

Pub/Sub provides at-least-once delivery, so to achieve exactly-once processing, the subscriber must handle duplicates. Using dead-letter topics helps manage failed messages safely.

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