- A
Use a Cloud Function with a retry policy to ensure delivery, and deduplicate using a Cloud Bigtable row key.
Why wrong: Bigtable row keys can help but the primary pattern is idempotency.
- B
Use Cloud Dataflow with exactly-once processing mode and write to Firestore using a custom sink.
Why wrong: Dataflow exactly-once still requires idempotent writes to the sink.
- C
Use a Cloud Run service to pull messages and write to Firestore; rely on Firestore's built-in deduplication using document IDs.
Why wrong: Firestore does not automatically deduplicate based on document ID.
- D
Make the message processor idempotent by using a unique event ID as the Firestore document ID, and perform upsert operations.
Idempotent processing with upsert ensures exactly-once effect.
Quick Answer
The answer is to make the message processor idempotent by using a unique event ID as the Firestore document ID and performing upsert operations. This works because idempotent writes ensure that processing the same Pub/Sub message multiple times—a consequence of its at-least-once delivery guarantee—results in only one effective state change in Firestore. By deriving the document ID from a unique, deterministic event attribute, any duplicate write simply overwrites the same document with the same data, making the operation harmless and achieving exactly-once processing semantics without relying on external deduplication. On the Google Professional Cloud Developer exam, this question tests your understanding that Pub/Sub guarantees at-least-once delivery, so you must design for idempotency at the sink; a common trap is assuming Firestore or Dataflow can magically deduplicate for you. Remember the mnemonic: "ID for ID" — use the unique event ID as the document ID to make your writes idempotent.
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 system that ingests high-velocity event streams from IoT devices using Pub/Sub. Each event must be processed exactly once to update a Firestore database. However, due to the distributed nature, at-least-once delivery is guaranteed by Pub/Sub. Which design pattern should you use to achieve exactly-once processing?
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Make the message processor idempotent by using a unique event ID as the Firestore document ID, and perform upsert operations.
Option D is correct because making the processor idempotent using a unique event ID as the Firestore document ID ensures that duplicate writes are harmless. Option A is wrong because Firestore does not provide built-in deduplication. Option B is wrong because Dataflow's exactly-once mode still requires idempotent sinks. Option C is wrong because Bigtable row keys can help but idempotency is the key.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 a Cloud Function with a retry policy to ensure delivery, and deduplicate using a Cloud Bigtable row key.
Why it's wrong here
Bigtable row keys can help but the primary pattern is idempotency.
- ✗
Use Cloud Dataflow with exactly-once processing mode and write to Firestore using a custom sink.
Why it's wrong here
Dataflow exactly-once still requires idempotent writes to the sink.
- ✗
Use a Cloud Run service to pull messages and write to Firestore; rely on Firestore's built-in deduplication using document IDs.
Why it's wrong here
Firestore does not automatically deduplicate based on document ID.
- ✓
Make the message processor idempotent by using a unique event ID as the Firestore document ID, and perform upsert operations.
Why this is correct
Idempotent processing with upsert ensures exactly-once effect.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PCD NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this PCD question test?
Integrating Google Cloud services — This question tests Integrating Google Cloud services — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Make the message processor idempotent by using a unique event ID as the Firestore document ID, and perform upsert operations. — Option D is correct because making the processor idempotent using a unique event ID as the Firestore document ID ensures that duplicate writes are harmless. Option A is wrong because Firestore does not provide built-in deduplication. Option B is wrong because Dataflow's exactly-once mode still requires idempotent sinks. Option C is wrong because Bigtable row keys can help but idempotency is the key.
What should I do if I get this PCD question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PCD NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
About these practice questions
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Last reviewed: Jun 24, 2026
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.
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