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 developer is using Cloud Spanner for a global application. They need to design a schema to avoid hotspots. Which practice should they follow?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a hash prefix on the primary key
Option D is correct because Cloud Spanner uses a distributed architecture that splits data across splits based on the primary key. A monotonically increasing or timestamp-based key causes all new writes to hit a single split, creating a hotspot. By using a hash prefix on the primary key, writes are distributed uniformly across all splits, avoiding hotspots and maximizing 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.
✗
Use a UUID primary key
Why it's wrong here
UUIDs are random but may still cause hotspots if not distributed properly; a hash prefix is more reliable.
✗
Use a monotonically increasing primary key
Why it's wrong here
Monotonically increasing keys cause all writes to a single tablet, creating a hotspot.
✗
Use a composite primary key with a timestamp
Why it's wrong here
Timestamp-based keys also lead to hotspots as writes are sequential.
✓
Use a hash prefix on the primary key
Why this is correct
A hash prefix distributes write load evenly across nodes, avoiding hotspots.
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 any random key (like UUID) automatically avoids hotspots, but in Cloud Spanner, the key distribution must be explicitly designed to avoid sequential patterns, and a hash prefix is the recommended pattern.
Detailed technical explanation
How to think about this question
Cloud Spanner splits data based on the primary key range; each split is a Paxos group. A hash prefix (e.g., a 4-byte hash of a business key) ensures that writes are spread across all splits, achieving linear scalability. In practice, you might use a hash of a user ID or order ID as the first part of the primary key, then append a timestamp for ordering within the same hash bucket.
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: Use a hash prefix on the primary key — Option D is correct because Cloud Spanner uses a distributed architecture that splits data across splits based on the primary key. A monotonically increasing or timestamp-based key causes all new writes to hit a single split, creating a hotspot. By using a hash prefix on the primary key, writes are distributed uniformly across all splits, avoiding hotspots and maximizing 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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