- A
Sensor ID followed by reversed timestamp
Spreading writes across sensor IDs and using reversed timestamp avoids hotspots and enables efficient scans per sensor.
- B
Only sensor ID
Why wrong: Only sensor ID may cause hotspot if many writes go to same sensor, and does not handle time-range queries well.
- C
Timestamp followed by sensor ID
Why wrong: Starting with timestamp causes sequential writes to hotspot a single tablet server.
- D
Random salt appended to timestamp
Why wrong: While better than timestamp first, it's less common; hashed prefix is recommended.
Question 519 of 1,000
PDE Storing the Data Practice Question
This PDE practice question tests your understanding of storing the data. 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 data engineer is designing a Bigtable row key for a time-series dataset where each row represents a sensor reading. The team expects high write throughput and wants to avoid hotspots. Which row key design is BEST?
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
✓
Sensor ID followed by reversed timestamp
Option A is correct because prepending the sensor ID distributes writes across multiple tablet servers, while reversing the timestamp ensures that recent data (which is written most frequently) is spread across different rows rather than creating a hotspot at the end of the table. This design leverages Bigtable's lexicographic ordering to balance write load.
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.
- ✓
Sensor ID followed by reversed timestamp
Why this is correct
Spreading writes across sensor IDs and using reversed timestamp avoids hotspots and enables efficient scans per sensor.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Only sensor ID
Why it's wrong here
Only sensor ID may cause hotspot if many writes go to same sensor, and does not handle time-range queries well.
- ✗
Timestamp followed by sensor ID
Why it's wrong here
Starting with timestamp causes sequential writes to hotspot a single tablet server.
- ✗
Random salt appended to timestamp
Why it's wrong here
While better than timestamp first, it's less common; hashed prefix is recommended.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a random salt alone can solve hotspots, but the trap here is that the salt must be placed at the beginning of the row key to actually distribute writes; appending it after a timestamp does not prevent the timestamp from creating a sequential write pattern.
Detailed technical explanation
How to think about this question
Bigtable stores rows in sorted order by row key, so a monotonically increasing key (like a raw timestamp) causes all new writes to be appended to the same tablet until it splits, creating a hotspot. Reversing the timestamp (e.g., Long.MAX_VALUE - timestamp) spreads recent writes across multiple tablets because the leading sensor ID already distributes the load, and the reversed timestamp ensures that older data is not adjacent to newer data for the same sensor. In practice, this design also improves read patterns for queries that retrieve the most recent readings for a specific sensor.
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 PDE question test?
Storing the Data — This question tests Storing the Data — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Sensor ID followed by reversed timestamp — Option A is correct because prepending the sensor ID distributes writes across multiple tablet servers, while reversing the timestamp ensures that recent data (which is written most frequently) is spread across different rows rather than creating a hotspot at the end of the table. This design leverages Bigtable's lexicographic ordering to balance write load.
What should I do if I get this PDE 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
This PDE 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 PDE exam.
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