- A
Reverse the timestamp in the row key (e.g., deviceID#MAXTIME - timestamp)
This places recent data near each other, making range scans faster.
- B
Separate frequently accessed data into a different column family
Why wrong: Column families affect storage and access patterns but do not change row key ordering.
- C
Use a single table with a composite key including device type
Why wrong: Adding device type does not cluster recent data from specific devices.
- D
Add a salt prefix to the row key
Why wrong: Salting spreads writes but does not optimize read locality for recent data.
Question 119 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. 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.
Your Bigtable cluster is experiencing high latency for a table that stores IoT sensor data. The row key format is deviceID#timestamp. You discover that most reads query the last hour of data for a few devices. How can you optimize row key design to improve read performance?
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
✓
Reverse the timestamp in the row key (e.g., deviceID#MAXTIME - timestamp)
Option A is correct because reversing the timestamp (e.g., deviceID#MAXTIME - timestamp) converts the row key from monotonically increasing to more evenly distributed. In Bigtable, rows are sorted lexicographically by row key; with deviceID#timestamp, all writes for a device go to the same tablet server, causing a hotspot. By reversing the timestamp, recent data for a device is spread across multiple tablets, reducing write contention and improving read latency for the last hour of data.
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.
- ✓
Reverse the timestamp in the row key (e.g., deviceID#MAXTIME - timestamp)
Why this is correct
This places recent data near each other, making range scans faster.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Separate frequently accessed data into a different column family
Why it's wrong here
Column families affect storage and access patterns but do not change row key ordering.
- ✗
Use a single table with a composite key including device type
Why it's wrong here
Adding device type does not cluster recent data from specific devices.
- ✗
Add a salt prefix to the row key
Why it's wrong here
Salting spreads writes but does not optimize read locality for recent data.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that adding a salt prefix is always the best solution for hotspotting, but in this scenario, the salt would break the ability to efficiently query recent data for a specific device, making timestamp reversal the correct optimization.
Detailed technical explanation
How to think about this question
Bigtable uses a distributed, sorted map where each row key is stored in a tablet, and tablets are split at row key boundaries. Monotonically increasing row keys (like deviceID#timestamp) cause all new writes to land on the same tablet, creating a hotspot. Reversing the timestamp (e.g., using MAXTIME - timestamp) ensures that recent timestamps produce smaller values, distributing writes across tablets. For reads, the row key prefix (deviceID) remains constant, so queries for the last hour can use a narrow row range scan (e.g., deviceID#<recent_reversed_timestamp>) without scanning all data.
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: Reverse the timestamp in the row key (e.g., deviceID#MAXTIME - timestamp) — Option A is correct because reversing the timestamp (e.g., deviceID#MAXTIME - timestamp) converts the row key from monotonically increasing to more evenly distributed. In Bigtable, rows are sorted lexicographically by row key; with deviceID#timestamp, all writes for a device go to the same tablet server, causing a hotspot. By reversing the timestamp, recent data for a device is spread across multiple tablets, reducing write contention and improving read latency for the last hour of data.
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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