The answer is a regional persistent disk attached to a single Compute Engine instance. This architecture is correct because regional persistent disks synchronously replicate data between two zones within a region, providing an effective RPO of zero—typically well under the required 5 seconds—and enabling a fast failover that meets a 1-minute RTO without complex replication overhead. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how synchronous replication differs from asynchronous snapshots or managed instance groups; a common trap is choosing a multi-instance setup with a shared disk, which adds failover latency. Remember the key distinction: regional disks handle the replication at the storage layer, not the application layer. For a memory tip, think “RPO = Replication, RTO = Recovery”—regional disks give you both without extra orchestration.
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 financial trading application on Compute Engine requires an RPO of 5 seconds and RTO of 1 minute for zone failures. 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 persistent disk attached to a single instance
Regional persistent disks provide synchronous replication of data between two zones within a region, ensuring an RPO of effectively zero (typically under 5 seconds) and enabling rapid failover to a secondary zone. By attaching the regional disk to a single Compute Engine instance, the application can quickly resume operations in the other zone upon failure, meeting the 1-minute RTO without data loss or complex replication overhead.
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.
✗
Persistent disk with periodic snapshots to a different zone
Why it's wrong here
Snapshots are not frequent enough to achieve 5-second RPO.
✗
Managed instance group with autoscaling and health checks
Why it's wrong here
This handles compute but does not replicate stateful data across zones.
✓
Regional persistent disk attached to a single instance
Why this is correct
Regional persistent disk synchronously replicates data across zones, allowing fast failover within RPO and RTO.
Related concept
Read the scenario before looking for a memorised answer.
✗
Two instances in different zones with data replicated via rsync
Why it's wrong here
rsync-based replication is asynchronous and unlikely to meet 5-second RPO.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that asynchronous replication methods (like snapshots or rsync) can meet strict RPO requirements, but only synchronous replication (as with regional persistent disks) guarantees sub-second data consistency across zones.
Detailed technical explanation
How to think about this question
Regional persistent disks use synchronous writes across two zones via Google's internal network, ensuring that every write is committed to both replicas before acknowledging the write to the application. This design achieves an RPO of near zero (typically under 5 seconds) because data is replicated in real-time, and failover involves simply reattaching the disk to a new instance in the paired zone, which can be automated to meet a 1-minute RTO. In practice, this architecture is ideal for stateful workloads like financial trading where even seconds of data loss are unacceptable, and it avoids the complexity of multi-instance replication schemes.
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 persistent disk attached to a single instance — Regional persistent disks provide synchronous replication of data between two zones within a region, ensuring an RPO of effectively zero (typically under 5 seconds) and enabling rapid failover to a secondary zone. By attaching the regional disk to a single Compute Engine instance, the application can quickly resume operations in the other zone upon failure, meeting the 1-minute RTO without data loss or complex replication overhead.
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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