The answer is Cloud Functions with a Pub/Sub trigger. This is the correct choice because it is purpose-built for serverless pubsub to bigquery cloud functions workloads that must scale to zero when idle, automatically spinning up from zero to thousands of concurrent invocations based on message volume and natively integrating with Pub/Sub via background functions that process each message and write results to BigQuery. On the Google Professional Cloud Developer exam, this scenario tests your understanding of serverless compute options for event-driven architectures, often appearing as a distractor where Cloud Run or App Engine might seem viable but lack the same granular, per-message scaling and zero-idle capability. A common trap is selecting Cloud Run, which can scale to zero but requires a container and HTTP endpoint, whereas Cloud Functions directly binds to the Pub/Sub trigger without extra overhead. Memory tip: think “Function triggers, Run serves”—Cloud Functions is the only option that natively subscribes to a Pub/Sub topic and fires per message, making it the leanest choice for bursty, event-driven pipelines.
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 developer is designing a serverless event-driven application that processes messages from Pub/Sub and writes results to BigQuery. The workload is unpredictable but must scale to zero when idle. Which compute option should they choose?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Cloud Functions with Pub/Sub trigger
Cloud Functions with a Pub/Sub trigger is the correct choice because it is purpose-built for event-driven, serverless workloads that scale to zero when idle. It automatically scales from zero to thousands of concurrent invocations based on the volume of Pub/Sub messages, and it natively integrates with Pub/Sub via a background function that is invoked for each message, making it ideal for unpredictable, bursty workloads that must process messages and write results to BigQuery.
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.
✗
Cloud Run with Pub/Sub push subscription
Why it's wrong here
Cloud Run can scale to zero but requires an intermediary like Eventarc.
✓
Cloud Functions with Pub/Sub trigger
Why this is correct
Cloud Functions is serverless, scales to zero, and has native Pub/Sub integration.
Related concept
Read the scenario before looking for a memorised answer.
✗
Compute Engine with managed instance groups
Why it's wrong here
Compute Engine is not serverless and requires manual scaling.
✗
Google Kubernetes Engine with Horizontal Pod Autoscaler
Why it's wrong here
GKE does not scale to zero and has overhead for serverless workloads.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that Cloud Run is always the best serverless option, but the trap here is that Cloud Functions is the native, simpler choice for pure event-driven Pub/Sub processing, while Cloud Run is better suited for HTTP request-driven workloads or when you need longer request timeouts or custom runtimes.
Detailed technical explanation
How to think about this question
Under the hood, Cloud Functions with a Pub/Sub trigger uses a push-based subscription where Pub/Sub delivers messages to a Cloud Functions endpoint via HTTP POST requests, and the function runtime automatically manages concurrency and scaling based on the number of unacknowledged messages. A subtle behavior is that Cloud Functions has a maximum timeout of 9 minutes (540 seconds) per invocation, so if message processing requires longer durations, you must design for idempotency and retries, or consider Cloud Run for longer-running tasks. In a real-world scenario, this pattern is ideal for streaming analytics pipelines where messages arrive in bursts and must be processed quickly without maintaining any always-on infrastructure.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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: Cloud Functions with Pub/Sub trigger — Cloud Functions with a Pub/Sub trigger is the correct choice because it is purpose-built for event-driven, serverless workloads that scale to zero when idle. It automatically scales from zero to thousands of concurrent invocations based on the volume of Pub/Sub messages, and it natively integrates with Pub/Sub via a background function that is invoked for each message, making it ideal for unpredictable, bursty workloads that must process messages and write results to BigQuery.
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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