The answer is to set min instances to 0 and max instances to 1000. This configuration is correct because Cloud Run’s autoscaling engine can scale a service down to zero instances when idle, saving costs, and then scale out horizontally to a maximum of 1000 instances to absorb unpredictable traffic spikes. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how min and max instances control both cost efficiency and burst capacity, often appearing in questions about serverless scaling trade-offs. A common trap is assuming min instances must be set above zero for reliability, but for truly unpredictable workloads, starting from zero is ideal since Cloud Run’s cold start latency is minimal. Remember the memory tip: “Zero to hero” — min 0 saves money, max 1000 handles the spike.
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. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 startup is building a REST API on Cloud Run. They expect unpredictable traffic spikes and want to ensure the service can scale from 0 to many instances automatically. What scaling configuration should they use?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Set min instances to 0 and max instances to 1000.
Option B is correct because Cloud Run's autoscaling allows min instances to be set to 0, enabling the service to scale down to zero when idle (cost-efficient), and max instances to 1000 to handle unpredictable traffic spikes by scaling out horizontally. This configuration ensures the service can start from zero and automatically add instances up to the maximum limit as demand increases, which is ideal for unpredictable workloads.
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.
✗
Set max instances to 1 to control costs.
Why it's wrong here
With max 1, the service cannot scale to handle traffic spikes, likely causing errors.
✓
Set min instances to 0 and max instances to 1000.
Why this is correct
This configuration allows the service to scale from zero to a high number as needed, handling spikes while minimizing cost during idle periods.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use manual scaling with a fixed number of instances.
Why it's wrong here
Manual scaling does not automatically adjust to load; it requires operator intervention.
✗
Set min instances to 5 and max to 100.
Why it's wrong here
This keeps at least 5 instances always running, increasing costs, and does not scale to zero.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'min instances' with 'max instances' or assume that setting min instances to 0 will cause the service to be unavailable during cold starts, but Cloud Run handles cold starts transparently, and the question specifically asks for scaling from 0 to many instances, which requires min=0 and a high max limit.
Detailed technical explanation
How to think about this question
Cloud Run uses the Knative serving autoscaler, which relies on the concurrency target (default 80 concurrent requests per instance) and the 'container concurrency' setting to decide when to add or remove instances. When min instances is 0, the autoscaler can reduce the number of instances to zero after a period of inactivity (controlled by the 'idle timeout' setting, default 5 minutes), and then scale up from zero using a 'cold start' mechanism that initializes a new container instance on the first request. This behavior is critical for cost optimization in serverless architectures, but cold starts can introduce latency, so for latency-sensitive applications, a higher min instances value might be preferred.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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: Set min instances to 0 and max instances to 1000. — Option B is correct because Cloud Run's autoscaling allows min instances to be set to 0, enabling the service to scale down to zero when idle (cost-efficient), and max instances to 1000 to handle unpredictable traffic spikes by scaling out horizontally. This configuration ensures the service can start from zero and automatically add instances up to the maximum limit as demand increases, which is ideal for unpredictable workloads.
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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