- A
Use a monotonically increasing integer as the primary key
Why wrong: Monotonically increasing keys cause all writes to go to the last tablet, creating a hotspot.
- B
Use a composite primary key with a leading hash prefix of the UUID
A hash prefix (e.g., first 4 bytes of SHA256) distributes writes evenly across splits, reducing hotspotting.
- C
Switch to regional Spanner instance instead of multi-region
Why wrong: Regional vs multi-region does not solve the hotspotting issue caused by key design.
- D
Use a secondary index on the UUID column and keep the primary key as a UUID
Why wrong: Secondary indexes can also hotspot; the issue is at the primary key level.
Question 175 of 999
PCD Practice Question: Design Scalable and Highly Available Cloud Database Solutions
This PCD practice question tests your understanding of design scalable and highly available cloud database solutions. 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.
You are designing a Cloud Spanner database for a global user application that must enforce strong consistency across regions. The primary key of the main table is a UUID. You notice that write latency is high and suspect hotspotting. Which design change is MOST likely to reduce hotspotting?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Clue:
"primary"Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a composite primary key with a leading hash prefix of the UUID
Option B is correct because using a composite primary key with a leading hash prefix distributes writes evenly across all Cloud Spanner splits, preventing hotspotting. A UUID primary key alone can still cause hotspots if the UUID generation is not perfectly random or if the application uses sequential UUIDs; a hash prefix ensures uniform distribution regardless of the UUID's characteristics.
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 monotonically increasing integer as the primary key
Why it's wrong here
Monotonically increasing keys cause all writes to go to the last tablet, creating a hotspot.
- ✓
Use a composite primary key with a leading hash prefix of the UUID
Why this is correct
A hash prefix (e.g., first 4 bytes of SHA256) distributes writes evenly across splits, reducing hotspotting.
Clue confirmation
The clue words "most likely", "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Switch to regional Spanner instance instead of multi-region
Why it's wrong here
Regional vs multi-region does not solve the hotspotting issue caused by key design.
- ✗
Use a secondary index on the UUID column and keep the primary key as a UUID
Why it's wrong here
Secondary indexes can also hotspot; the issue is at the primary key level.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume UUIDs are always perfectly random and thus immune to hotspotting, but Cloud Spanner's split-by-key-range design means that any sequential or clustered key pattern—including certain UUID implementations—can cause hotspots, and a hash prefix is the standard solution.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed storage system where data is split into tablets based on the primary key range. A hash prefix in the primary key ensures that writes are uniformly distributed across all tablets, as the hash function randomizes the leading bytes. In practice, even with UUIDs, if the UUID generation is not cryptographically random (e.g., time-based UUIDs), the leading bytes can be sequential, causing hotspotting; a hash prefix eliminates this risk.
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.
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FAQ
Questions learners often ask
What does this PCD question test?
Design Scalable and Highly Available Cloud Database Solutions — This question tests Design Scalable and Highly Available Cloud Database Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a composite primary key with a leading hash prefix of the UUID — Option B is correct because using a composite primary key with a leading hash prefix distributes writes evenly across all Cloud Spanner splits, preventing hotspotting. A UUID primary key alone can still cause hotspots if the UUID generation is not perfectly random or if the application uses sequential UUIDs; a hash prefix ensures uniform distribution regardless of the UUID's characteristics.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely", "primary". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jul 4, 2026
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