The answer is a regional managed instance group (MIG) with instances deployed across three zones. This architecture achieves 99.99% availability because it can tolerate both a full zonal outage and a simultaneous single-instance failure in one of the remaining zones, ensuring that at least two zones remain operational to serve traffic. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how regional MIGs distribute fault domains; a common trap is assuming two zones are sufficient, but two zones only survive one failure—not the combination of a zone loss plus an instance crash. Remember the memory tip: “Three zones for 99.99%—two zones for 99.9%.”
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.
An organization runs a critical application on Compute Engine with a regional managed instance group. They want to achieve 99.99% availability. Which architecture 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
✓
Regional MIG with instances in three zones
To achieve 99.99% availability, the architecture must tolerate both a zonal failure and a single instance failure. A regional managed instance group (MIG) with instances in three zones ensures that even if one zone becomes unavailable, the remaining two zones can still serve traffic, meeting the 99.99% uptime target. Three zones provide the necessary redundancy because a two-zone regional MIG can only survive a single zone failure but not a simultaneous instance failure in the remaining zone, whereas three zones allow for a rolling update or failure of one zone while still maintaining quorum.
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.
✗
Regional MIG with instances in two zones
Why it's wrong here
Two zones provide good availability but three zones are more robust for 99.99%.
✗
Single zone MIG with multiple instances
Why it's wrong here
Single zone MIG cannot survive a zone outage.
✓
Regional MIG with instances in three zones
Why this is correct
Three zones provide higher availability within a region.
Related concept
Read the scenario before looking for a memorised answer.
✗
Multi-region deployment with global load balancer
Why it's wrong here
Multi-region is for disaster recovery, exceeding the 99.99% requirement.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that two zones are sufficient for 99.99% availability, but the trap here is that two zones only provide 99.9% availability because they cannot tolerate a simultaneous instance failure in the remaining zone during a zonal outage or maintenance event.
Detailed technical explanation
How to think about this question
Google Compute Engine's regional MIG uses a regional instance template and distributes instances across zones within a region. The 99.99% availability target corresponds to approximately 52.56 minutes of downtime per year, which requires surviving both a zonal failure and a single instance failure. Under the hood, the regional MIG uses a health check-based autohealing and can be configured with a target distribution shape (e.g., BALANCED or ANY_SINGLE_ZONE) to ensure capacity is maintained. In practice, a three-zone deployment allows for a zone to be taken down for maintenance or failure while the other two zones continue to serve, and the MIG can automatically redistribute instances if one zone becomes unhealthy.
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: Regional MIG with instances in three zones — To achieve 99.99% availability, the architecture must tolerate both a zonal failure and a single instance failure. A regional managed instance group (MIG) with instances in three zones ensures that even if one zone becomes unavailable, the remaining two zones can still serve traffic, meeting the 99.99% uptime target. Three zones provide the necessary redundancy because a two-zone regional MIG can only survive a single zone failure but not a simultaneous instance failure in the remaining zone, whereas three zones allow for a rolling update or failure of one zone while still maintaining quorum.
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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