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. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 company runs a stateful application on Compute Engine with regional persistent disks. They want to achieve high availability with automatic failover in case of a zone failure. Which architecture meets these requirements?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use regional persistent disks with a managed instance group spanning two zones
Option D is correct because regional persistent disks synchronously replicate data across two zones, and when combined with a managed instance group (MIG) spanning those same two zones, the application can automatically fail over to the healthy zone if one zone fails. The MIG's autohealing and health-check mechanisms detect the failure and recreate instances in the surviving zone, while the regional PD remains accessible from either zone, ensuring high availability without manual intervention.
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.
✗
Use Cloud Storage FUSE to mount a multi-regional bucket as a filesystem
Why it's wrong here
Cloud Storage is not a filesystem for stateful applications with strong consistency needs.
✗
Use standard persistent disks with scheduled snapshots to a multi-region bucket
Why it's wrong here
Snapshots are point-in-time and not real-time replication.
✗
Use zonal persistent disks with a managed instance group in a single zone
Why it's wrong here
If the zone fails, all instances are lost.
✓
Use regional persistent disks with a managed instance group spanning two zones
Why this is correct
Regional disks provide synchronous replication; instance group autohealing restarts VMs on failure.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse zonal persistent disks with regional persistent disks, or assume that snapshots (Option B) provide automatic failover, when in reality snapshots are for backup/DR and require manual recovery steps, not instant zone-failure recovery.
Detailed technical explanation
How to think about this question
Regional persistent disks use synchronous replication at the block level between two zones within the same region, ensuring that writes are acknowledged only after being committed to both zones. When a MIG spans two zones, it uses a health-check probe (e.g., HTTP or TCP) to monitor instance health; if a zone fails, the MIG automatically terminates instances in the failed zone and creates new instances in the surviving zone, which can attach the existing regional PD (which remains available) without data loss. This architecture provides a Recovery Point Objective (RPO) of zero and a Recovery Time Objective (RTO) of a few minutes, limited by instance startup and health-check intervals.
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.
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: Use regional persistent disks with a managed instance group spanning two zones — Option D is correct because regional persistent disks synchronously replicate data across two zones, and when combined with a managed instance group (MIG) spanning those same two zones, the application can automatically fail over to the healthy zone if one zone fails. The MIG's autohealing and health-check mechanisms detect the failure and recreate instances in the surviving zone, while the regional PD remains accessible from either zone, ensuring high availability without manual intervention.
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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