The correct answer is to use Cloud Firestore to record the state of each processed event, which directly addresses the need for Cloud Functions idempotency with duplicate events. By generating a unique idempotency key for every incoming event—such as a Cloud Storage object ID or a Pub/Sub message ID—and storing that key in Firestore before processing, the function can check the database upon each invocation. If the key already exists, the function safely skips reprocessing, preventing data corruption or duplicate side effects. This pattern is critical for the Google Professional Cloud Developer exam because it tests your understanding of at-least-once delivery semantics, where Cloud Pub/Sub or Cloud Storage notifications may resend events. A common trap is relying solely on function retries without external state tracking, which fails when duplicates arrive from the source itself. Remember the memory tip: “Key to idempotency is a key in the DB”—always persist your idempotency keys in Firestore to guarantee exactly-once processing.
PCD Integrating Google Cloud services Practice Question
This PCD practice question tests your understanding of integrating google cloud services. 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.
You are designing a serverless application using Cloud Functions that processes events from Cloud Storage and Cloud Pub/Sub. The function must be idempotent and handle duplicate events. Which three best practices should you implement? (Choose THREE.)
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Generate a unique idempotency key for each event and store processed keys in a database.
Option A is correct because generating a unique idempotency key for each event and storing processed keys in a database (such as Cloud Firestore) ensures that if the same event is delivered multiple times (e.g., due to at-least-once delivery semantics in Cloud Pub/Sub or Cloud Storage notifications), the function can check the key before processing and skip duplicates. This pattern is essential for idempotent serverless functions, as Cloud Functions may be retried on failure or receive duplicate events from the source.
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.
✓
Generate a unique idempotency key for each event and store processed keys in a database.
Why this is correct
Idempotency keys prevent duplicate processing.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Invoke the function synchronously to avoid duplicates.
Why it's wrong here
Synchronous invocation is not supported for background functions.
✓
Implement a deduplication logic that checks the event's publish time against a threshold.
Why this is correct
Timestamps can help filter duplicates.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✓
Use Cloud Firestore to record the state of each processed event.
Why this is correct
Transactional state helps idempotency.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Set the function timeout to maximum (540 seconds) to ensure processing completes.
Why it's wrong here
Timeout does not prevent duplicates.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse timeout settings or synchronous invocation with duplicate prevention, but neither addresses the root cause of duplicate events from at-least-once delivery systems.
Detailed technical explanation
How to think about this question
Under the hood, Cloud Pub/Sub uses at-least-once delivery, meaning the same message can be redelivered if the subscriber does not acknowledge it in time. Cloud Storage notifications also have similar behavior. Storing processed event IDs in Cloud Firestore (or another transactional store) allows atomic checks and writes, ensuring exactly-once processing semantics. In a real-world scenario, a payment processing function must use idempotency keys to avoid charging a customer twice if a Pub/Sub message is retried.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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.
Integrating Google Cloud services — This question tests Integrating Google Cloud services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Generate a unique idempotency key for each event and store processed keys in a database. — Option A is correct because generating a unique idempotency key for each event and storing processed keys in a database (such as Cloud Firestore) ensures that if the same event is delivered multiple times (e.g., due to at-least-once delivery semantics in Cloud Pub/Sub or Cloud Storage notifications), the function can check the key before processing and skip duplicates. This pattern is essential for idempotent serverless functions, as Cloud Functions may be retried on failure or receive duplicate events from the source.
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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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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