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 instances with persistent disks. The application must be highly available and be able to recover from a zonal failure with minimal data loss. The current architecture uses a single instance in one zone. Which design should the team implement?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a regional persistent disk attached to a managed instance group across two zones.
Option C is correct because a regional persistent disk synchronously replicates data across two zones, and when attached to a managed instance group (MIG) spanning those zones, it provides automatic failover with minimal data loss. This design ensures that if one zone fails, the MIG can detach the disk from the failed instance and attach it to a healthy instance in the surviving zone, preserving state with near-zero RPO.
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 a standard persistent disk and configure a global load balancer to failover.
Why it's wrong here
A standard disk is zonal and cannot be attached to an instance in another zone.
✗
Create a snapshot schedule and restore the snapshot to a new instance in another zone on failure.
Why it's wrong here
Snapshots are not real-time and may lose data between snapshots.
✓
Use a regional persistent disk attached to a managed instance group across two zones.
Why this is correct
Regional persistent disks replicate synchronously across zones, enabling fast failover.
Related concept
Read the scenario before looking for a memorised answer.
✗
Migrate to Cloud Filestore for shared file storage across zones.
Why it's wrong here
Filestore is file storage, not block storage; it may introduce latency and cost.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse high availability with backup strategies (snapshots) or assume that a load balancer alone can handle storage failover, failing to recognize that stateful applications require synchronous data replication across zones to achieve minimal data loss.
Detailed technical explanation
How to think about this question
Regional persistent disks use synchronous replication at the block level across two zones within the same region, ensuring that a write is acknowledged only after it is committed in both zones. When a MIG performs a rolling update or detects an instance failure, it can use the `regional-pd` operation to detach the disk from the failed instance and attach it to a new instance in the other zone, typically within seconds. This design achieves an RPO of effectively zero and an RTO limited by the time to spin up a new instance and attach the disk, making it ideal for stateful workloads that require high availability.
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 a regional persistent disk attached to a managed instance group across two zones. — Option C is correct because a regional persistent disk synchronously replicates data across two zones, and when attached to a managed instance group (MIG) spanning those zones, it provides automatic failover with minimal data loss. This design ensures that if one zone fails, the MIG can detach the disk from the failed instance and attach it to a healthy instance in the surviving zone, preserving state with near-zero RPO.
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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