- A
Switch to a monotonically increasing integer key
Why wrong: Monotonically increasing keys cause hotspots on the last split, worsening latency.
- B
Use a composite primary key with a shard prefix (e.g., hash of UUID)
Correct: salting with a hash prefix distributes writes evenly across splits.
- C
Add a secondary index on the UUID column
Why wrong: Indexes do not resolve primary key hotspotting; they add write overhead.
- D
Enable interleaved tables
Why wrong: Interleaving stores child rows with parent, but does not solve hotspotting on the parent key.
Question 799 of 980
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. 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 Cloud Spanner table uses a UUID as the primary key. The application is experiencing high write latency and hotspotting. Which design change would BEST resolve the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
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 shard prefix (e.g., hash of UUID)
Option B is correct because adding a shard prefix (e.g., a hash of the UUID) as the first part of a composite primary key distributes writes evenly across Cloud Spanner's splits, eliminating hotspotting. UUIDs alone cause hotspotting because they are randomly distributed but still lead to a single split being overwhelmed if the table is small or the UUIDs are inserted in order; a hash prefix ensures that consecutive writes target different splits, reducing write latency.
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.
- ✗
Switch to a monotonically increasing integer key
Why it's wrong here
Monotonically increasing keys cause hotspots on the last split, worsening latency.
- ✓
Use a composite primary key with a shard prefix (e.g., hash of UUID)
Why this is correct
Correct: salting with a hash prefix distributes writes evenly across splits.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Add a secondary index on the UUID column
Why it's wrong here
Indexes do not resolve primary key hotspotting; they add write overhead.
- ✗
Enable interleaved tables
Why it's wrong here
Interleaving stores child rows with parent, but does not solve hotspotting on the parent key.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse UUIDs as inherently good for distribution, but in Cloud Spanner, random UUIDs still cause hotspotting because they are inserted into a single split until the table grows large enough to split, whereas a hash prefix proactively distributes writes.
Detailed technical explanation
How to think about this question
Cloud Spanner uses split-based architecture where each split is a contiguous range of primary key values; without a hash prefix, all UUIDs (or monotonically increasing keys) fall into a single split until it splits, causing high write latency. A hash prefix (e.g., using SHA-256 or a simple modulo of the UUID) creates a uniform distribution across splits from the start, leveraging Spanner's automatic split management. In real-world scenarios, this pattern is critical for high-throughput ingestion workloads like IoT or event logging, where even a brief hotspot can degrade performance.
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
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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 shard prefix (e.g., hash of UUID) — Option B is correct because adding a shard prefix (e.g., a hash of the UUID) as the first part of a composite primary key distributes writes evenly across Cloud Spanner's splits, eliminating hotspotting. UUIDs alone cause hotspotting because they are randomly distributed but still lead to a single split being overwhelmed if the table is small or the UUIDs are inserted in order; a hash prefix ensures that consecutive writes target different splits, reducing write latency.
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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Last reviewed: Jul 4, 2026
This PCD practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCD exam.
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