The answer is Cloud Spanner because it is the only Google Cloud database that combines horizontal scaling with strong consistency and high write throughput. For a real-time leaderboard with strong consistency, Spanner’s TrueTime API synchronizes clocks across regions to provide external consistency, meaning every score update is immediately visible to all reads—a requirement Cloud SQL cannot meet at 100,000 writes per second due to its single-node write bottleneck. On the Google Professional Cloud Developer exam, this scenario tests your understanding of when to choose Spanner over Cloud SQL; the common trap is assuming Cloud SQL’s read replicas can handle the write load, but replicas only help reads, not writes. Remember the key trade-off: Cloud SQL scales up, Spanner scales out. Memory tip: “Spanner spans writes without waiting”—it horizontally distributes writes while keeping reads strongly consistent, unlike Cloud SQL which would need to sacrifice consistency or latency at this scale.
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 real-time leaderboard for a mobile gaming application. The leaderboard must support millions of concurrent users updating their scores and querying rankings with low latency (under 100ms). Scores change frequently and require strong consistency for reads. The development team is evaluating Cloud SQL and Cloud Spanner. They estimate they need to handle 100,000 writes per second. Which database should they choose and why?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Cloud Spanner because it provides horizontal scaling, strong consistency, and high write throughput.
Cloud Spanner is the correct choice because it provides horizontal scaling with strong consistency and can handle 100,000 writes per second while maintaining ACID transactions and low-latency reads. Unlike Cloud SQL, Spanner scales horizontally across nodes without sacrificing consistency, making it ideal for a real-time leaderboard with millions of concurrent users.
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.
✗
Cloud Firestore because it offers real-time synchronization and is serverless.
Why it's wrong here
Firestore has lower write throughput limits and is not designed for such high volumes.
✗
Cloud Bigtable because it's optimized for high write throughput and time-series data.
Why it's wrong here
Bigtable does not provide strong consistency for reads.
✗
Cloud SQL with read replicas because it's cost-effective and supports ACID transactions.
Why it's wrong here
Cloud SQL cannot scale to 100K writes per second without manual sharding.
✓
Cloud Spanner because it provides horizontal scaling, strong consistency, and high write throughput.
Why this is correct
Spanner is built for high-throughput, strongly consistent global workloads.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that Cloud SQL can scale writes via read replicas, but read replicas only offload read traffic, not write throughput, and Cloud SQL's single-primary architecture cannot handle 100,000 writes per second.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a globally distributed, synchronous replication model with TrueTime (Google's globally synchronized clock) to provide external consistency (strong consistency) across regions. It supports up to 2,000 write nodes per instance, each capable of thousands of writes per second, enabling aggregate throughput of 100,000 writes per second. Under the hood, Spanner uses Paxos-based consensus for writes, ensuring that all replicas agree on the order of transactions, which is critical for a real-time leaderboard where score updates must be immediately visible.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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: Cloud Spanner because it provides horizontal scaling, strong consistency, and high write throughput. — Cloud Spanner is the correct choice because it provides horizontal scaling with strong consistency and can handle 100,000 writes per second while maintaining ACID transactions and low-latency reads. Unlike Cloud SQL, Spanner scales horizontally across nodes without sacrificing consistency, making it ideal for a real-time leaderboard with millions of concurrent users.
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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