- A
Add a salt prefix to the row key (e.g., hash of deviceID)
Salting distributes writes across tablets to avoid hotspots.
- B
Reverse the timestamp so that recent data appears first
This allows efficient scans for recent data from a device.
- C
Store data in multiple tables per device
Why wrong: Multiple tables do not solve hotspotting and complicate management.
- D
Use a single column family for all metrics
Why wrong: Column families do not affect row key distribution.
- E
Promote the device type to the start of the row key
Why wrong: This does not help with write distribution or scan performance for a device.
Question 26 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. 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 Bigtable schema for a time-series application that stores metrics from millions of devices. The row key is currently deviceID#timestamp. You want to avoid hotspotting on writes and optimize scan performance for reading all data from a specific device within a time range. Which two row key design strategies should you apply? (Choose 2)
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
✓
Add a salt prefix to the row key (e.g., hash of deviceID)
Option A is correct because adding a salt prefix (e.g., a hash of the deviceID) distributes write load across multiple tablet servers, preventing hotspotting on a single node when many devices write concurrently. This ensures that sequential timestamps for the same device are not all written to the same tablet, which would otherwise cause a bottleneck.
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.
- ✓
Add a salt prefix to the row key (e.g., hash of deviceID)
Why this is correct
Salting distributes writes across tablets to avoid hotspots.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Reverse the timestamp so that recent data appears first
Why this is correct
This allows efficient scans for recent data from a device.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Store data in multiple tables per device
Why it's wrong here
Multiple tables do not solve hotspotting and complicate management.
- ✗
Use a single column family for all metrics
Why it's wrong here
Column families do not affect row key distribution.
- ✗
Promote the device type to the start of the row key
Why it's wrong here
This does not help with write distribution or scan performance for a device.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that reversing the timestamp alone solves hotspotting, but it only optimizes scan order; the salt is required to distribute write load, and candidates may overlook that both strategies are needed together.
Detailed technical explanation
How to think about this question
Bigtable automatically splits tablets at row key boundaries, so a monotonically increasing row key like deviceID#timestamp can cause all writes to land on the last tablet, creating a hotspot. Salting with a hash prefix (e.g., MD5 or CRC32 of deviceID modulo a small number) spreads writes across tablets, while reversing the timestamp (e.g., deviceID#[MAX_TIMESTAMP - timestamp]) ensures that recent data is stored at the beginning of the tablet, making scans for the latest data faster without needing to traverse the entire tablet.
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.
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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: Add a salt prefix to the row key (e.g., hash of deviceID) — Option A is correct because adding a salt prefix (e.g., a hash of the deviceID) distributes write load across multiple tablet servers, preventing hotspotting on a single node when many devices write concurrently. This ensures that sequential timestamps for the same device are not all written to the same tablet, which would otherwise cause a bottleneck.
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.
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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