Message retention and acknowledgment in GCP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the acknowledgment deadline for a Pub/Sub subscription.

GCP

subscription = pubsub_client.subscription('projects/my-project/subscriptions/my-subscription')
subscription.modify_ack_deadline(seconds=[1])

Drag options to blanks, or click blank then click option'

A10

B0

C60

D300

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the deadline to 0 causes immediate redelivery.

Using a very high value delays message redelivery unnecessarily.

The acknowledgment deadline is set in seconds. 60 seconds is a common default to allow enough time for message processing before redelivery.

2fill in blank

medium

Complete the code to acknowledge a received message in Pub/Sub.

GCP

def callback(message):
    print(f'Received message: {message.data}')
    message.[1]()

Drag options to blanks, or click blank then click option'

Areject

Back

Cdelete

Dcancel

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reject' instead of 'ack' does not acknowledge the message.

Calling 'delete' is not a valid method on the message object.

Calling ack() on the message tells Pub/Sub that the message was processed successfully and should not be redelivered.

3fill in blank

hard

Fix the error in the code to extend the acknowledgment deadline for a message.

GCP

def callback(message):
    # Extend ack deadline by 30 seconds
    message.modify_ack_deadline(seconds=[1])
    message.ack()

Drag options to blanks, or click blank then click option'

A30

B300

C60

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes the message to be redelivered immediately.

Using too large a value may delay redelivery unnecessarily.

The modify_ack_deadline method extends the deadline by the given seconds. 30 seconds is a typical extension time.

4fill in blank

hard

Fill both blanks to create a subscription with a message retention duration and enable message ordering.

GCP

subscription = pubsub_client.create_subscription(
    name='projects/my-project/subscriptions/my-subscription',
    topic='projects/my-project/topics/my-topic',
    message_retention_duration=[1],
    enable_message_ordering=[2]
)

Drag options to blanks, or click blank then click option'

A'604800s'

B'3600s'

CTrue

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using integer instead of string for retention duration.

Setting enable_message_ordering to False disables ordering.

The message retention duration is set in seconds as a string with 's' suffix. '604800s' equals 7 days. Enabling message ordering requires setting enable_message_ordering to True.

5fill in blank

hard

Fill all three blanks to create a pull subscription with a custom acknowledgment deadline and enable exactly-once delivery.

GCP

subscription = pubsub_client.create_subscription(
    name='projects/my-project/subscriptions/my-subscription',
    topic='projects/my-project/topics/my-topic',
    ack_deadline_seconds=[1],
    enable_exactly_once_delivery=[2],
    retain_acked_messages=[3]
)

Drag options to blanks, or click blank then click option'

A20

BTrue

CFalse

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Setting ack_deadline_seconds too high or zero.

Disabling exactly-once delivery by setting it to False.

The acknowledgment deadline is set to 10 seconds. Exactly-once delivery and retaining acknowledged messages are enabled by setting their flags to True.

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