The answer is to place a multi-region Spanner instance close to users and enable leader-optimized routing. Leader-optimized routing is the key technical mechanism here: it directs read requests to the nearest region that holds the leader replica for the specific data, drastically reducing inter-continental latency while still reading from the leader to guarantee strong consistency. Pairing this with a multi-region instance placed geographically near your user base minimizes network round-trip time, and Spanner’s synchronous replication across those regions maintains strong consistency even under high read/write throughput. On the Google Professional Cloud Developer exam, this question tests your understanding that strong consistency in Spanner requires reading from the leader, but you can optimize performance by placing leaders closer to traffic—a common trap is assuming read-only replicas can serve strongly consistent reads, which they cannot. Remember the mnemonic: “Leader first, then nearest” to recall that leader-optimized routing keeps consistency while cutting latency.
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 company is designing a globally distributed application using Cloud Spanner. The application requires strong consistency and the ability to handle high read/write throughput. The team is concerned about inter-continental latency. Which two design choices would optimize performance while maintaining strong consistency? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable leader-optimized routing to direct reads to the nearest leader region.
A is correct because leader-optimized routing directs read requests to the nearest region that contains the leader replica for the requested data, reducing inter-continental latency while still reading from the leader to ensure strong consistency. C is correct because placing a multi-region Spanner instance in geographic locations close to users minimizes network round-trip time, and Spanner's synchronous replication across regions maintains strong consistency even with high read/write throughput.
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.
✓
Enable leader-optimized routing to direct reads to the nearest leader region.
Why this is correct
Leader-optimized routing reduces read latency while maintaining strong consistency.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use read-only replicas in each continent to serve reads locally.
Why it's wrong here
Read-only replicas may not provide strong consistency.
✓
Place a multi-region Spanner instance in geographic locations close to users.
Why this is correct
Multi-region configurations reduce latency by providing local leaders.
Related concept
Read the scenario before looking for a memorised answer.
✗
Implement client-side caching with a short TTL for frequently accessed data.
Why it's wrong here
Caching can serve stale data, violating strong consistency.
✗
Increase the number of nodes in the Spanner instance to improve throughput.
Why it's wrong here
More nodes increase throughput but not necessarily latency; also adds cost.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that read-only replicas or caching can provide strong consistency, but in Spanner, only leader replicas guarantee strong consistency, and any form of caching or stale replica reads breaks that guarantee.
Detailed technical explanation
How to think about this question
Cloud Spanner uses the TrueTime API (based on GPS and atomic clocks) to enforce external consistency across global replicas. Leader-optimized routing leverages Spanner's partitioned leader distribution, where each split has its own leader, allowing reads to be served from the nearest leader region without sacrificing consistency. In a multi-region configuration, Spanner automatically handles synchronous replication via Paxos, ensuring that all writes are committed to a quorum of replicas before acknowledging the client, which is why placing instances close to users reduces latency without weakening consistency.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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: Enable leader-optimized routing to direct reads to the nearest leader region. — A is correct because leader-optimized routing directs read requests to the nearest region that contains the leader replica for the requested data, reducing inter-continental latency while still reading from the leader to ensure strong consistency. C is correct because placing a multi-region Spanner instance in geographic locations close to users minimizes network round-trip time, and Spanner's synchronous replication across regions maintains strong consistency even with high read/write throughput.
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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