The correct answer is to make the function idempotent using a deduplication field. This works because Pub/Sub guarantees at-least-once delivery, meaning the same message can be delivered multiple times, so your Cloud Function must be designed to handle duplicates safely. By checking a unique deduplication key—such as the Pub/Sub message ID or a custom business identifier—before processing, the function can skip or safely reapply the operation, preventing duplicate side effects like double database writes or duplicate email sends. On the Google Professional Cloud Developer exam, this scenario tests your understanding of event-driven architecture pitfalls and the principle of idempotency in serverless functions; a common trap is assuming Pub/Sub provides exactly-once delivery or that retries alone solve the problem. Remember the memory tip: “Dedupe before you dupe”—always check a unique key first to keep your functions idempotent.
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. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 company uses Cloud Functions to process events from Pub/Sub. They notice that occasionally the same message is processed more than once. What can they do to ensure idempotent processing?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Make the function idempotent using a deduplication field
Option A is correct because making the function idempotent using a deduplication field ensures that even if the same Pub/Sub message is delivered more than once (Pub/Sub offers at-least-once delivery), the function processes it only once. By checking a unique message ID or a custom deduplication key before processing, the function can skip or safely reapply the operation, preventing duplicate side effects.
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.
✓
Make the function idempotent using a deduplication field
Why this is correct
Ensures that processing a duplicate message has no effect.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure retry policies to only retry once
Why it's wrong here
Reduces duplicates but does not eliminate them; messages may still be delivered multiple times.
✗
Use Cloud Tasks instead
Why it's wrong here
Cloud Tasks provides at-least-once delivery; still need idempotency.
✗
Use Cloud Scheduler
Why it's wrong here
Cloud Scheduler is for scheduled jobs, not relevant.
✗
Increase the acknowledgement deadline for the subscription
Why it's wrong here
Reduces redelivery but does not prevent duplicates.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think increasing the acknowledgement deadline or reducing retries will prevent duplicate processing, but they fail to understand that Pub/Sub's at-least-once delivery guarantee means duplicates can still occur due to network issues or subscriber crashes, making idempotency the only reliable solution.
Detailed technical explanation
How to think about this question
Under the hood, Pub/Sub uses a pull-based model with a configurable acknowledgement deadline (default 10 seconds). If a subscriber fails to ack within that window, the message is redelivered. Idempotency is typically achieved by storing processed message IDs (e.g., in Cloud Firestore or a Redis cache) with a TTL equal to the subscription's message retention duration (up to 7 days). This ensures that even if the same message is delivered to multiple function instances, only the first one performs the side effect. In real-world scenarios, a payment processing function using a deduplication field (like an order ID) prevents double charges when Pub/Sub redelivers the same event.
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: Make the function idempotent using a deduplication field — Option A is correct because making the function idempotent using a deduplication field ensures that even if the same Pub/Sub message is delivered more than once (Pub/Sub offers at-least-once delivery), the function processes it only once. By checking a unique message ID or a custom deduplication key before processing, the function can skip or safely reapply the operation, preventing duplicate side effects.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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