- A
Use a composite primary key with a hash prefix of the UUID
This spreads writes across splits, eliminating hotspotting.
- B
Enable interleaved tables to store child rows with parent
Why wrong: Interleaving improves locality but does not solve write hotspotting on the primary key.
- C
Change the primary key to a monotonically increasing timestamp
Why wrong: Monotonically increasing keys cause writes to concentrate on one split, worsening hotspotting.
- D
Add a secondary index on the UUID column
Why wrong: Secondary indexes do not affect the distribution of primary key writes.
Question 51 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. 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.
You are designing a Cloud Spanner schema for a global user database. Users are identified by a UUID. You notice hotspotting on a specific node during writes. What is the most effective way to resolve this?
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 hash prefix of the UUID
Hotspotting in Cloud Spanner occurs when writes are concentrated on a single node due to a monotonically increasing primary key (like a UUID without a hash prefix). By using a composite primary key with a hash prefix of the UUID, you distribute writes evenly across all nodes because the hash prefix ensures that successive writes land on different splits. This prevents a single node from becoming a bottleneck, thereby resolving the hotspotting issue.
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 composite primary key with a hash prefix of the UUID
Why this is correct
This spreads writes across splits, eliminating hotspotting.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Enable interleaved tables to store child rows with parent
Why it's wrong here
Interleaving improves locality but does not solve write hotspotting on the primary key.
- ✗
Change the primary key to a monotonically increasing timestamp
Why it's wrong here
Monotonically increasing keys cause writes to concentrate on one split, worsening hotspotting.
- ✗
Add a secondary index on the UUID column
Why it's wrong here
Secondary indexes do not affect the distribution of primary key writes.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common misconception is that secondary indexes or interleaved tables can fix write distribution issues, but the root cause is the primary key design, and only modifying the primary key (e.g., with a hash prefix) directly addresses hotspotting in Cloud Spanner.
Detailed technical explanation
How to think about this question
Cloud Spanner automatically splits data based on the primary key range; a hash prefix (e.g., using SHA256 or a simple modulo) creates a uniform distribution of writes across splits, each served by different nodes. Under the hood, Spanner uses Paxos-based replication and splits are balanced dynamically, but the initial key design dictates how evenly load is spread. In real-world scenarios, using a hash prefix of the UUID (e.g., first 4 bytes as a shard ID) is a common pattern to avoid hot keys in global user databases.
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
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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 hash prefix of the UUID — Hotspotting in Cloud Spanner occurs when writes are concentrated on a single node due to a monotonically increasing primary key (like a UUID without a hash prefix). By using a composite primary key with a hash prefix of the UUID, you distribute writes evenly across all nodes because the hash prefix ensures that successive writes land on different splits. This prevents a single node from becoming a bottleneck, thereby resolving the hotspotting issue.
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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Last reviewed: Jul 4, 2026
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