Design innovative, scalable, and highly available cloud database solutions →easyMultiple ChoiceObjective-mapped
PCDE Practice Question: Design innovative, scalable, and highly available cloud database solutions
You are designing a Cloud Bigtable schema for a time-series application that stores temperature readings from sensors. Each reading has a sensor ID (string), a timestamp (microseconds), and a temperature value. Queries always filter by sensor ID and a time range. Which row key design is optimal?
⚠ Common exam trap
A common misconception is that a simple sensor_id prefix is sufficient for time-series data, ignoring the need for write distribution via salting to avoid hot-spotting in high-throughput scenarios in Cloud Bigtable.
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
✓
[salted_hash]#[sensor_id]#[reversed_timestamp]
Optimal because it uses a salted hash to distribute writes across Bigtable tablets, avoiding hot-spotting on a single node for high-write sensors. The sensor_id ensures all data for a sensor is co-located for efficient range scans, and the reversed timestamp allows queries for the most recent data to be served from the start of the row range, leveraging Bigtable's lexicographic ordering.
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]#[timestamp]
Why it's wrong here
Without salting, writes for the same sensor may hotspot on a single tablet server if sensor IDs are sequential.
- ✓
[salted_hash]#[sensor_id]#[reversed_timestamp]
Why this is correct
Salting distributes writes, sensor ID groups data, reversed timestamp enables efficient time-range queries.
- ✗
[timestamp]#[sensor_id]
Why it's wrong here
This leads to all writes hitting the same region for the current timestamp, causing hotspots.
- ✗
[sensor_id]#[reversed_timestamp]
Why it's wrong here
Lacks salting; if sensor IDs are sequential, writes may hotspot.
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