- A
Use a composite primary key with customer_id as the first column
Queries filtering by customer_id become local index scans, reducing latency and avoiding cross-split reads.
- B
Enable point-in-time recovery to reduce storage costs
Why wrong: Point-in-time recovery is for data protection, not performance or hotspot avoidance.
- C
Replace UUID primary key with a composite key starting with a hash prefix of the UUID
A hash prefix (e.g., first byte of SHA-256) distributes writes across splits, avoiding hotspots.
- D
Create a secondary index on customer_id
Why wrong: A secondary index on customer_id is not enough; the primary key design is the main driver of write distribution and query efficiency. A global index on customer_id can help but the scenario asks for design changes to avoid hotspots and ensure fast queries — the primary key redesign is essential.
- E
Use interleaved tables for related data such as transactions
Interleaved tables store child rows co-located with parent rows, reducing join costs and improving locality.
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.
A financial services company is designing a globally distributed ledger system using Cloud Spanner. They need to support high write throughput, avoid hotspots, and ensure fast queries on a customer_id column. The primary key is currently a UUID. Which THREE design changes should they implement? (Choose THREE.)
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 customer_id as the first column
Option A is correct because placing `customer_id` as the first column in a composite primary key enables Spanner to distribute writes across splits based on the leading key column, avoiding hotspots that occur when using a monotonically increasing UUID as the sole primary key. This design also allows fast point-lookups and range scans on `customer_id` without requiring a secondary index, as Spanner can directly locate the row via the primary key.
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 customer_id as the first column
Why this is correct
Queries filtering by customer_id become local index scans, reducing latency and avoiding cross-split reads.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Enable point-in-time recovery to reduce storage costs
Why it's wrong here
Point-in-time recovery is for data protection, not performance or hotspot avoidance.
- ✓
Replace UUID primary key with a composite key starting with a hash prefix of the UUID
Why this is correct
A hash prefix (e.g., first byte of SHA-256) distributes writes across splits, avoiding hotspots.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create a secondary index on customer_id
Why it's wrong here
A secondary index on customer_id is not enough; the primary key design is the main driver of write distribution and query efficiency. A global index on customer_id can help but the scenario asks for design changes to avoid hotspots and ensure fast queries — the primary key redesign is essential.
- ✓
Use interleaved tables for related data such as transactions
Why this is correct
Interleaved tables store child rows co-located with parent rows, reducing join costs and improving locality.
Clue confirmation
The clue word "primary" in the question point toward this answer.
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 a secondary index on a high-cardinality column like `customer_id` is sufficient for high write throughput, but the trap is that the index itself becomes a hotspot if its key is monotonically increasing, whereas a hash-prefixed composite primary key distributes writes across splits.
Trap categories for this question
Scenario analysis trap
A secondary index on customer_id is not enough; the primary key design is the main driver of write distribution and query efficiency. A global index on customer_id can help but the scenario asks for design changes to avoid hotspots and ensure fast queries — the primary key redesign is essential.
Detailed technical explanation
How to think about this question
Spanner uses a distributed key-value store where rows are sorted by primary key and split into tablets based on key ranges. A UUID primary key leads to random inserts across all splits, causing high write contention and split management overhead. By using a composite key with a hash prefix (e.g., `SHA256(UUID) % N`), writes are distributed uniformly across splits, while `customer_id` as the second column enables efficient queries via interleaved tables or secondary indexes. Interleaved tables physically colocate parent and child rows on the same split, reducing latency for joins and transactions on related data like transactions.
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
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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 customer_id as the first column — Option A is correct because placing `customer_id` as the first column in a composite primary key enables Spanner to distribute writes across splits based on the leading key column, avoiding hotspots that occur when using a monotonically increasing UUID as the sole primary key. This design also allows fast point-lookups and range scans on `customer_id` without requiring a secondary index, as Spanner can directly locate the row via the primary key.
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
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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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