The correct choice is to use push subscriptions with a dead-letter topic. This configuration ensures at-least-once delivery because Cloud Pub/Sub will automatically retry pushing a message if the subscriber endpoint returns an error, and only after exhausting the maximum delivery attempts—defaulting to five—does the system forward the undelivered message to a dead-letter topic for later analysis or reprocessing. This guarantees that no message is silently lost; it is either successfully processed or preserved for manual intervention, directly satisfying the at-least-once requirement. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how Pub/Sub’s retry mechanism and dead-lettering work together to handle processing failures without dropping data. A common trap is assuming a pull subscription with manual acknowledgment alone provides the same guarantee, but without a dead-letter topic, messages that repeatedly fail can be lost if the subscriber stops acknowledging them. Memory tip: think “push, retry, then dead-letter” as the three-step safety net for at-least-once delivery.
PCD Practice Question: Designing highly scalable, available, and reliable cloud-native applications
This PCD practice question tests your understanding of designing highly scalable, available, and reliable cloud-native applications. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A developer is building a Cloud Pub/Sub-based event-driven system. They need to ensure that messages are processed at least once, and they want to handle processing failures. What should they do?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "least"
Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use push subscriptions with a dead-letter topic
Option D is correct because using push subscriptions with a dead-letter topic ensures at-least-once delivery and provides a mechanism to handle processing failures. When a push subscription fails to deliver a message (e.g., due to a downstream error), Pub/Sub automatically retries delivery. After exhausting the maximum delivery attempts (default 5), the message is forwarded to a dead-letter topic, where it can be analyzed or reprocessed without losing the message. This guarantees that every message is either processed successfully or stored for manual intervention, satisfying the at-least-once requirement.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use pull subscriptions with auto-acknowledgment
Why it's wrong here
Auto-acknowledgment may lose messages if processing fails.
✗
Configure max delivery attempts on the subscription
Why it's wrong here
This alone doesn't handle messages after max attempts; a dead-letter topic is needed.
✗
Use Cloud Tasks instead of Pub/Sub
Why it's wrong here
Cloud Tasks is for distributed task queues, not event streaming.
✓
Use push subscriptions with a dead-letter topic
Why this is correct
Push subscriptions with a dead-letter topic provide retries and failure handling.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that simply increasing delivery attempts (Option B) is sufficient for failure handling, but the trap is that without a dead-letter topic, messages are permanently lost after the final attempt, violating the at-least-once requirement.
Detailed technical explanation
How to think about this question
Under the hood, Pub/Sub push subscriptions use HTTPS POST requests to a configured endpoint; if the endpoint returns a non-2xx status or does not respond within the acknowledgment deadline (default 10 seconds), Pub/Sub retries the delivery with exponential backoff. The dead-letter topic is configured on the subscription with a maximum delivery attempt count (default 5) and a dead-letter topic reference; once the count is exceeded, the message is published to the dead-letter topic with attributes indicating the original subscription and failure reason. This pattern is critical in real-world scenarios like payment processing, where a transient failure (e.g., database timeout) should not cause message loss, and persistent failures (e.g., invalid payload) must be quarantined for debugging.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Designing highly scalable, available, and reliable cloud-native applications — This question tests Designing highly scalable, available, and reliable cloud-native applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use push subscriptions with a dead-letter topic — Option D is correct because using push subscriptions with a dead-letter topic ensures at-least-once delivery and provides a mechanism to handle processing failures. When a push subscription fails to deliver a message (e.g., due to a downstream error), Pub/Sub automatically retries delivery. After exhausting the maximum delivery attempts (default 5), the message is forwarded to a dead-letter topic, where it can be analyzed or reprocessed without losing the message. This guarantees that every message is either processed successfully or stored for manual intervention, satisfying the at-least-once requirement.
What should I do if I get this PCD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "least". You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A developer is building an event-driven system using Cloud Pub/Sub. They need to ensure reliable message delivery and processing. Which three practices should they follow?
medium
A.Set a minimum number of delivery attempts.
✓ B.Use pull subscriptions with synchronous acknowledgment.
C.Use message ordering.
✓ D.Configure a dead-letter topic.
✓ E.Use exponential backoff for pull subscriptions.
Why B: Pull subscriptions with synchronous acknowledgment (option B) ensure that a message is not acknowledged until the subscriber has successfully processed it. This prevents premature acknowledgment and message loss, because Cloud Pub/Sub will redeliver the message if the acknowledgment deadline expires without a synchronous ack. This is a core pattern for reliable processing.
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This PCD practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCD exam.
Question Discussion
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